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Technical Manual – Signaling System HUAWEI M800 CDMA Mobile Switching Center
SS7 Signaling SystemTable of Contents
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Table of Contents
Chapter 1 Introduction to CDMA SS7.......................................................................................... 1-1 1.1 Concepts of SS7................................................................................................................ 1-1
1.1.1 Common Channel Signaling System ...................................................................... 1-1 1.1.2 SS7 Signaling Network ........................................................................................... 1-2 1.1.3 Signaling Transfer Mode ......................................................................................... 1-5
1.2 Architecture and Functions of SS7 .................................................................................... 1-5 1.2.1 Layered Architecture ............................................................................................... 1-6 1.2.2 Introduction to Functional Layers ............................................................................ 1-6
Chapter 2 Message Transfer Part ................................................................................................ 2-1 2.1 Introduction to MTP ........................................................................................................... 2-1 2.2 MTP Functions................................................................................................................... 2-1
2.2.1 Signaling Data Link Functions................................................................................. 2-2 2.2.2 Signaling Link Functions ......................................................................................... 2-2 2.2.3 Signaling Network Functions................................................................................... 2-4
2.3 MTP Messages.................................................................................................................. 2-7 2.3.1 Format of Signal Units............................................................................................. 2-7 2.3.2 Functions and Codes of Signal Unit Fields ............................................................. 2-8
Chapter 3 Telephone User Part.................................................................................................... 3-1 3.1 Introduction to TUP............................................................................................................ 3-1 3.2 TUP Functions ................................................................................................................... 3-1 3.3 TUP Messages .................................................................................................................. 3-1
3.3.1 Format of TUP Messages ....................................................................................... 3-1 3.3.2 Encoding of TUP Messages.................................................................................... 3-2 3.3.3 Example of TUP Messages..................................................................................... 3-4
Chapter 4 Signaling Connection Control Part............................................................................ 4-1 4.1 Introduction to SCCP ......................................................................................................... 4-1
4.1.1 TUP Signaling Transfer Based on MTP.................................................................. 4-1 4.1.2 SCCP Signaling Transfer Based on MTP ............................................................... 4-2
4.2 Services Provided by SCCP.............................................................................................. 4-3 4.2.1 SCCP Service Classes ........................................................................................... 4-4 4.2.2 Connectionless Services......................................................................................... 4-4 4.2.3 Connection-Oriented Services ................................................................................ 4-5 4.2.4 SCCP Service Procedure........................................................................................ 4-6
4.3 SCCP Addressing and Routing ......................................................................................... 4-8 4.4 SCCP Primitives ................................................................................................................ 4-9
4.4.1 Definition ................................................................................................................. 4-9 4.4.2 Structure................................................................................................................ 4-11
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4.4.3 SCCP User Primitives ........................................................................................... 4-11 4.4.4 MTP Service Primitives ......................................................................................... 4-13
4.5 SCCP Messages.............................................................................................................. 4-14 4.5.1 Format of SCCP Messages .................................................................................. 4-14 4.5.2 Encoding of SCCP Messages............................................................................... 4-17 4.5.3 Example of SCCP Messages................................................................................ 4-25
Chapter 5 ISDN User Part ............................................................................................................. 5-1 5.1 Introduction to ISUP........................................................................................................... 5-1 5.2 ISUP Functions .................................................................................................................. 5-2
5.2.1 Bearer Services....................................................................................................... 5-2 5.2.2 User Terminal Services........................................................................................... 5-2 5.2.3 Supplementary Services ......................................................................................... 5-2
5.3 ISUP Messages ................................................................................................................. 5-3 5.3.1 Format of ISUP Messages ...................................................................................... 5-3 5.3.2 Encoding of ISUP Messages .................................................................................. 5-4 5.3.3 Example of ISUP Messages ................................................................................... 5-8
Chapter 6 Transaction Capabilities Application Part ................................................................ 6-1 6.1 Introduction to TCAP ......................................................................................................... 6-1 6.2 TCAP Structure.................................................................................................................. 6-2
6.2.1 Transaction Sublayer .............................................................................................. 6-3 6.2.2 Component Sublayer .............................................................................................. 6-3
6.3 TCAP Messages................................................................................................................ 6-4 6.3.1 Encoding of TCAP Messages ................................................................................. 6-4 6.3.2 Format of TCAP Messages..................................................................................... 6-7 6.3.3 Transaction Portion ................................................................................................. 6-7 6.3.4 Dialog Portion........................................................................................................ 6-11 6.3.5 Component Portion ............................................................................................... 6-11 6.3.6 Example of TCAP Messages ................................................................................ 6-13
Chapter 7 Mobile Application Part............................................................................................... 7-1 7.1 Introduction to MAP ........................................................................................................... 7-1 7.2 MAP Functions................................................................................................................... 7-2
7.2.1 MAP Management Functions.................................................................................. 7-2 7.2.2 MAP Operations...................................................................................................... 7-4
7.3 MAP Messages.................................................................................................................. 7-7 7.3.1 Format of MAP Messages....................................................................................... 7-7 7.3.2 Encoding of MAP Messages ................................................................................... 7-7 7.3.3 Example of MAP Messages .................................................................................... 7-7
7.4 Common MAP Procedures .............................................................................................. 7-10 7.4.1 Location Registration ............................................................................................ 7-10 7.4.2 Inter-Office Call ..................................................................................................... 7-11 7.4.3 Handoff Forward.................................................................................................... 7-11
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Chapter 8 Base Station Application Part .................................................................................... 8-1 8.1 Introduction to BSAP ......................................................................................................... 8-1
8.1.1 About the A Interface .............................................................................................. 8-1 8.1.2 BSAP Functions ...................................................................................................... 8-1
8.2 BSAP Messages................................................................................................................ 8-2 8.2.1 Format of BSAP Messages..................................................................................... 8-2 8.2.2 Encoding of BSAP Messages ................................................................................. 8-3 8.2.3 Example of BSAP Messages .................................................................................. 8-6
8.3 BSAP Procedures............................................................................................................ 8-15 8.3.1 Location Update .................................................................................................... 8-15 8.3.2 Mobile Origination ................................................................................................. 8-16 8.3.3 Mobile Termination................................................................................................ 8-17 8.3.4 Call Clearing.......................................................................................................... 8-18 8.3.5 Circuit Block/Unblock ............................................................................................ 8-19 8.3.6 Circuit Reset.......................................................................................................... 8-21
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List of Figures
Figure 1-1 Architecture of a CCS system............................................................................... 1-1
Figure 1-2 A 3-layer signaling network................................................................................... 1-3
Figure 1-3 Associated transfer of signaling messages .......................................................... 1-5
Figure 1-4 Quasi-associated transfer of signaling messages................................................ 1-5
Figure 1-5 Functional blocks of SS7...................................................................................... 1-5
Figure 1-6 Layered architecture of SS7................................................................................. 1-6
Figure 2-1 Position of the MTP in SS7................................................................................... 2-1
Figure 2-2 Function levels of the MTP................................................................................... 2-2
Figure 2-3 Signaling message handling................................................................................. 2-5
Figure 2-4 Message routing ................................................................................................... 2-6
Figure 2-5 Structure of FISU.................................................................................................. 2-7
Figure 2-6 Structure of LSSU................................................................................................. 2-8
Figure 2-7 Structure of MSU .................................................................................................. 2-8
Figure 2-8 SIO structure ........................................................................................................ 2-9
Figure 2-9 Structure of SIF................................................................................................... 2-10
Figure 3-1 Position of TUP in SS7 ......................................................................................... 3-1
Figure 3-2 TUP message structure........................................................................................ 3-2
Figure 3-3 TUP label structure ............................................................................................... 3-2
Figure 3-4 IAI format .............................................................................................................. 3-4
Figure 3-5 Calling line identity field ........................................................................................ 3-8
Figure 3-6 IAM format ............................................................................................................ 3-9
Figure 4-1 The SCCP in the SS7In the signaling network..................................................... 4-3
Figure 4-2 Connectionless transfer of signaling messages................................................... 4-4
Figure 4-3 Connection-oriented transfer with a middle node................................................. 4-5
Figure 4-4 Connectionless service in GT addressing ............................................................ 4-6
Figure 4-5 Connection-oriented service signaling flow .......................................................... 4-7
Figure 4-6 Primitives and messages of MTP and SCCP..................................................... 4-10
Figure 4-7 Structure of a primitive........................................................................................ 4-11
Figure 4-8 Structure of SCCP messages............................................................................. 4-15
Figure 4-9 SCCP message .................................................................................................. 4-25
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Figure 5-1 ISUP position in SS7 ............................................................................................ 5-1
Figure 5-2 ISUP message structure....................................................................................... 5-4
Figure 5-3 Format of routing label in the ISUP message....................................................... 5-5
Figure 5-4 Format of the CIC in a ISUP message ................................................................. 5-5
Figure 5-5 IAM message...................................................................................................... 5-12
Figure 6-1 Position of TCAP in the SS7 network ................................................................... 6-2
Figure 6-2 Structure of TCAP................................................................................................. 6-2
Figure 6-3 Structure of TCAP IE ............................................................................................ 6-4
Figure 6-4 The format of a tag containing one octet .............................................................. 6-5
Figure 6-5 The format of a tag containing more than one octets........................................... 6-5
Figure 6-6 Length of contents -- the short form .....................................................................6-6
Figure 6-7 Length of contents – the long form.......................................................................6-6
Figure 6-8 TCAP message structure ..................................................................................... 6-7
Figure 6-9 RUIDIR message................................................................................................ 6-13
Figure 7-1 CDMA network architecture.................................................................................. 7-1
Figure 7-2 Structural relation between MAP and MTP messages ......................................... 7-7
Figure 7-3 Registration Notification message traced on a SS7 link ...................................... 7-7
Figure 7-4 Location registration procedure.......................................................................... 7-10
Figure 7-5 Inter-office call procedure ................................................................................... 7-11
Figure 7-6 Handoff forward procedure................................................................................. 7-12
Figure 8-1 Reference model of A interface protocol stack ..................................................... 8-2
Figure 8-2 BSAP message structure ..................................................................................... 8-3
Figure 8-3 Example of CM service request ........................................................................... 8-8
Figure 8-4 Location update procedure................................................................................. 8-16
Figure 8-5 Mobile origination procedure .............................................................................. 8-16
Figure 8-6 Mobile termination procedure............................................................................. 8-17
Figure 8-7 Call clearing initiated by BSC ............................................................................. 8-19
Figure 8-8 Circuit block procedure....................................................................................... 8-20
Figure 8-9 Circuit unblock procedure................................................................................... 8-20
Figure 8-10 BSC-initiated circuit reset ................................................................................. 8-21
Figure 8-11 MSC-initiated circuit reset................................................................................. 8-22
Figure 8-12 MSC-initiated circuit reset failure...................................................................... 8-22
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List of Tables
Table 2-1 SI codes allocation ................................................................................................. 2-9
Table 2-2 SSF codes allocation............................................................................................ 2-10
Table 3-1 Coding format of H0 ............................................................................................... 3-3
Table 3-2 Calling party category............................................................................................. 3-4
Table 3-3 Message indicators ................................................................................................ 3-6
Table 3-4 Encoding of the address signals ............................................................................ 3-7
Table 3-5 Encoding of the first indicator octet ........................................................................ 3-7
Table 3-6 Address indicators .................................................................................................. 3-8
Table 3-7 Calling line identity codes....................................................................................... 3-9
Table 4-1 SCCP user primitives ........................................................................................... 4-11
Table 4-2 MTP service primitive ........................................................................................... 4-13
Table 4-3 SCCP message type and code ............................................................................ 4-16
Table 4-4 SCCP message parameters................................................................................. 4-17
Table 4-5 Structure of address indicator............................................................................... 4-18
Table 4-6 Correspondence between GT type codes and GT types ..................................... 4-19
Table 4-7 Allocation of SCCP SSNs..................................................................................... 4-19
Table 4-8 Type 1 GT............................................................................................................. 4-20
Table 4-9 Type 2 GT............................................................................................................. 4-20
Table 4-10 Type 3 GT........................................................................................................... 4-20
Table 4-11 Type 4 GT ........................................................................................................... 4-21
Table 4-12 Protocol classes ................................................................................................. 4-22
Table 4-13 Handling of messages in case of transfer failure ............................................... 4-22
Table 4-14 Coding of release causes................................................................................... 4-23
Table 4-15 Coding of return causes ..................................................................................... 4-24
Table 5-1 Encoding of ISUP messages.................................................................................. 5-5
Table 5-2 Parameters of IAM ................................................................................................. 5-8
Table 5-3 Code of the nature of connection indicators........................................................... 5-9
Table 5-4 Codes of forward call indicator ............................................................................. 5-10
Table 5-5 Codes of the calling party category ...................................................................... 5-11
Table 6-1 Structure of TCAP message tag............................................................................. 6-4
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Table 6-2 TCAP package type identifier ................................................................................. 6-8
Table 6-3 Correspondence between TCAP package type and cell........................................ 6-9
Table 6-4 P-Abort causes..................................................................................................... 6-10
Table 6-5 IEs contained in the dialog portion ....................................................................... 6-11
Table 6-6 Correspondence between component types and component type identifiers ..... 6-11
Table 6-7 Correspondence between component types and IEs .......................................... 6-12
Table 7-1 MAP operations ...................................................................................................... 7-5
Table 8-1 Type 1 IE structure ................................................................................................. 8-4
Table 8-2 Type 2 IE structure ................................................................................................. 8-4
Table 8-3 Type 3 IE structure of type 3 (example 1) .............................................................. 8-5
Table 8-4 Type 3 IE structure (example 2) ............................................................................. 8-5
Table 8-5 Type 4 IE structure (example 1) ............................................................................. 8-6
Table 8-6 Type 4 IE structure o (example 2) .......................................................................... 8-6
Table 8-7 Complete layer 3 message..................................................................................... 8-7
Table 8-8 CM service request message................................................................................. 8-7
Table 8-9 Paging request message...................................................................................... 8-12
Table 8-10 Paging request message.................................................................................... 8-13
Table 8-11 Connect message............................................................................................... 8-14
Table 8-12 Assignment request message ............................................................................ 8-14
Table 8-13 Assignment complete messages........................................................................ 8-14
Table 8-14 SCCP messages used by BSAP........................................................................ 8-15
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Chapter 1 Introduction to CDMA SS7
This chapter describes the concepts, architecture, and functions of signaling system No.7 (SS7) for the CDMA system.
1.1 Concepts of SS7
This section introduces some important concepts related to SS7.
Before introducing the major concepts, this section explains the following basic terms:
Information: the content carried in a message Message: carrier of information Signal unit: entity made up of a message and some signaling information fields
necessary for applications and transferred on signaling links Signaling: information set used to interconnect different entities in
communication networks
1.1.1 Common Channel Signaling System
The signaling system helps network entities to cooperate with each other to implement particular tasks.
There are two types of signaling systems:
Common channel signaling (CCS) system Channel associated signaling (CAS) system
SS7 is a common channel signaling system.
I. Definition of CCS System
In a CCS system, signaling channels and traffic channels are separate. Signaling is transferred on common data links (signaling channels in this case) in the form of messages.
Figure 1-1 shows the architecture of a CCS system.
Switchingnetwork
Switchingnetwork
Signalingequipment
Signalingequipment
Publiccontrol
Publiccontrol
Switch A Switch BTraffic channel
Data link
Figure 1-1 Architecture of a CCS system
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II. Transfer of SS7 Signaling Messages
SS7 messages are data packets exchanged between processors of various nodes (such as exchanges) in a telecommunication network. They are transmitted in packet switching mode on signaling links.
Therefore, the SS7 network is essentially a data communication network (a special packet switching network) independent of the service switching system.
One timeslot (excluding TS 0) of each digital trunk line in a 2-Mbit/s primary group is used as the signaling channel, also called the signaling link. The transmission rate of a signaling link is 64 kbit/s.
Most of the timeslots are used as traffic channels. For example, a speech channel transmitting voice information is also a traffic channel.
SS7 messages can also be transferred on analog transmission lines. In this case, the SS7 messages are sent by using a modulator-demodulator (Modem). Typical transmission rates are 2.4 kbit/s and 4.8 kbit/s.
III. Advantages of CCS System
Compared with the CAS system, the CCS system has the following advantages:
High channel utilization High signaling transmission speed Large signaling capacity Wide applications in the integrated services digital network (ISDN) , mobile
communications network and intelligent network Easy maintenance and management due to the separation of signaling network
and communication network Adaptive to new signaling protocols for new service provisioning
To realize the above features, the CCS system must:
Maintain high reliability of signaling links. Adopt advanced signaling network functions and security measures. Possess the function of speech channel continuity check (to ensure high
performance of speech channels).
1.1.2 SS7 Signaling Network
A signaling network is dedicated to the transmission of signaling messages. It is logically independent of the communication network.
Because channel associated signaling messages are transmitted together with traffic data, the concept of signaling network applies only to the common channel signaling system.
A signaling network consists of
Signaling points (SPs)
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Signaling transfer points (STPs) Signaling links
The SS7 signaling network is a support network. It is physically integrated with the communication network.
Figure 1-2 is an example of a 3-layer signaling network.
HSTP
LSTP
SP SP: Signaling point LSTP: High-level signaling transfer point LSTP: Low level signaling transfer point
Figure 1-2 A 3-layer signaling network
I. Signaling Point
An SP is where the signaling messages are processed or exchanged. It functions as either the origination or the destination in signaling message transmission.
Generally, an SP can be an exchange, an operation and maintenance center, or a network database.
Usually, an SP corresponds to one physical node. It is represented by the symbol "O" in a network topology diagram.
In some cases, however, two logically separate signaling points are configured for one physical node. This often happens on gateway offices, for example, an international incoming and outgoing office is an SP of the national signaling network and an SP of the international signaling network.
An SP is identified by a signaling point code (SPC).
An originating SP (OSP) is identified with an originating point code (OPC); A destination SP (DSP) is identified with a destination point code (DPC).
There are two types of SPCs:
14-bit SPC 24-bit SPC
II. Signaling Transfer Point
An STP transfers the signaling messages it receives from one signaling link to another. It is presented by a " " in a network topology diagram.
An STP is identified by the SPC.
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There are two types of STPs:
Independent STP Integrated STP
An independent STP only functions to transfer signaling. It is not regarded as an SP.
An integrated STP not only to transfers signaling but also serves as an SP where signaling is originated or terminated.
As shown in Figure 1-2, in a 3-level signaling network, STPs are classified into two types:
Low-level signaling transfer point (LSTP) High-level signaling transfer point (HSTP)
III. Signaling Link
Signaling links are the physical channels that connect SPs and STPs and transmit signaling messages.
IV. Signaling Link Set
A collection of signaling links with the same attributes is called a link set.
The links to the same office may belong to one link or several link sets. The links between two adjacent SPs, however, must be configured in one link set.
V. Signaling Link Code
Each signaling link is uniquely identified within an office with a signaling link code (SLC) .
All signaling links between two adjacent SPs are uniquely numbered in the same way. The SLCs of these links must be consistent at the two SPs.
For signaling links to different offices, the SLCs may be identical.
VI. Signaling Route
A signaling route is a path along which signaling messages are transmitted from an OSP to a DSP.
The selection of signaling route depends on the signaling relations and the transfer mode.
VII. Signaling Route Set
All signaling routes that correspond to a signaling relation form a route set.
A given signaling message is transmitted along a specific route in normal cases. When this route becomes faulty, an alternative route is taken.
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1.1.3 Signaling Transfer Mode
There are two signaling transfer modes:
Associated transfer Quasi-associated transfer
I. Associated Transfer
In associated transfer, the signaling messages between two SPs are transmitted on direct signaling links. In this case, the traffic channels and signaling links are parallel.
Figure 1-3 shows the associated transfer of signaling messages.
Traffic channel
Signaling link
SP SP
Figure 1-3 Associated transfer of signaling messages
II. Quasi-Associated Transfer
In quasi-associated transfer, signaling messages between two SPs are transmitted on indirect signaling links. These signaling links are designated by data configuration for signaling transmission.
Figure 1-4 shows the quasi-associated transfer of signaling messages.
STP
SP SP
Signaling link
Traffic channel
Designated Signaling links
Figure 1-4 Quasi-associated transfer of signaling messages
1.2 Architecture and Functions of SS7
SS7 consists of several functional blocks, including a message transfer part (MTP) and a number of user parts (UPs), as shown in Figure 1-5.
User Part MTP User Part
Figure 1-5 Functional blocks of SS7
MTP is responsible for signaling message transfer.
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UPs are responsible for the generating, grammar checking, semantic analysis and processing control of signaling messages. All UPs function with the support of MTP.
1.2.1 Layered Architecture
Figure 1-6 shows the architecture of SS7.
INAP OMAP MAP ISUP TUP
ISP
SCCP
MTP-3
MTP-2
MTP-1
L 2
BSAPMAP ISUP TUP
TCAP
ISP
SCCP
MTP-3
MTP-2
MTP-1
HLRVLR
BSAP
L 1
L 7
L 4 - L 6
L 3
Figure 1-6 Layered architecture of SS7
There is a correspondence between the SS7 and the open system interconnection (OSI) network model (seven layers).
In the layered architecture, an upper layer is the user of its lower layer, and is served by the lower layer.
1.2.2 Introduction to Functional Layers
The functional layers of SS7 follow the hierarchy of data transfer strictly. Signaling messages in SS7 are transferred transparently in a layer that is not responsible for the processing of the messages. Signaling messages are transferred between the corresponding functional levels at both sides.
This section gives a brief introduction to the functions of each functional layer of the SS7.
I. Message Transfer Part
MTP serves as a transport system providing reliable transfer of the signaling messages.
It is divided into the three levels:
MTP-1: Signaling data link function MTP-2: Signaling link function MTP-3: Signaling network function
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II. Signaling Connection Control Part
Signaling connection control part (SCCP) is based on MTP and provides additional functions to MTP.
It provides connectionless and connection-oriented network services.
The connectionless network service means that UP transfers signaling messages without establishing signal connection in advance. With connectionless network service, the data of one UP can be transferred to another UP on the signaling network. For example, authentication of subscribers in mobile networks, and account inquiry in intelligent networks are all transferred by this means.
The connection-oriented network service means that a message transport channel between the two nodes (UPs) is established after exchange of requests and responses between the UPs prior to data transfer.
III. Telephone User Part
Telephone user part (TUP) handles call-related signaling messages, such as those related to the setup, monitoring and release of calls.
TUP messages are classified into several message groups, such as forward and backward call setup messages, call monitoring messages, circuit and circuit group monitoring messages and network management messages.
TUP messages are transferred in signal units on signaling links.
IV. ISDN User Part
ISDN user part (ISUP) provides signaling functions to support ISDN basic services and supplementary services.
ISUP has all functions of the TUP. Therefore, it can also function as the TUP.
V. Transaction Capabilities Application Part
Transaction capabilities application part (TCAP) provides interfaces for various communication network services, such as mobile services and intelligent services.
TCAP provides the dialog capabilities to support information request and response for the applications of network services.
TCAP is a public protocol and does not involve specific applications. The specific applications implement the message transfer on the interfaces provided by the TCAP. For example, the MAP implements the location of roaming subscribers with the support of the TCAP. The intelligent application part (INAP) implements the service control point (SCP) database registration and data query with the support of the TCAP.
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VI. Intermediate Service Part
Intermediate service part (ISP) corresponds to layer 4 to layer 6 of the OSI model. It is not defined yet.
Together with the TCAP, it is referred to as the transaction capabilities (TC).
VII. Mobile Application Part
Mobile application part (MAP) is a functional unit used for interconnection within the public land mobile network (PLMN) and between the PLMN and other networks.
VIII. Base Station Application Part
Base station application part (BSAP) BSAP is an application part based on A interface protocols. It fulfills the functions of A1 interface between MSC and BSC.
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Chapter 2 Message Transfer Part
This chapter introduces the concepts related to message transfer part (MTP), as well as its functions and position in the SS7.
2.1 Introduction to MTP
MTP constitutes the bottom layers (LI, L2 and L3) of SS7, providing physical links for signaling transmission to ensure reliable message transfer. MTP also provides signaling route management and signaling network management functions.
Figure 2-1 shows the position of the MTP in SS7.
INAP OMAP MAP ISUP TUP
TCAP
ISP
SCCP
MTP-3
MTP-2
MTP-1
HLR VLR
L 2
BSAPINAP OMAP MAP ISUP TUP
TCAP
ISP
SCCP
MTP-3
MTP-2
MTP-1
HLR VLR
BSAP
L 1
L 7
L 4 - L 6
L 3
Figure 2-1 Position of the MTP in SS7
2.2 MTP Functions
MTP functions are classified to three levels:
Signaling data link functions Signaling link functions Signaling network functions
These three levels correspond to the MTP-1, MTP-2, and MTP-3 shown in Figure 2-2.
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Signaling Network Fuctions
Signaling Link functions
Signaling Data Link
User Part
MTP-1
MTP-2
MTP-3
Figure 2-2 Function levels of the MTP
2.2.1 Signaling Data Link Functions
Signaling data link functions are performed by the MTP at Level 1, defining the physical and electrical features and the connection mode of signaling data links.
A signaling data link is a bi-directional transmission path for signaling. It assumes a timeslot of the pulse code modulation (PCM) system with a bit rate of 64 kbit/s. An analogue signaling data link with modems may also be adopted with a bit rate typically at 2.4 kbit/s or 4.8 kbit/s.
As signaling transmission is bi-directional, a signaling point (SP) receives signaling from and delivers signaling to another SP. Therefore, full duplex operation over a 4-wire transmission link is adopted.
The operational signaling data link shall be exclusively dedicated to the use of an SS7 signaling link.
A signaling link is transparent, that is, bit integrity of the transmitted data stream must be ensured. Equipment such as echo suppressors, digital pads, or A/u law converters attached to the transmission link must be disabled.
2.2.2 Signaling Link Functions
The signaling link functions are performed by the MTP at Level 2.
Together with a signaling data link as a bearer, the signaling link functions provide a signaling link for reliable transfer of signaling messages between two directly connected SPs.
Errors may occur to the data link between directly connected SPs after long distance transmission, which are not allowed in SS7. In the case of data link errors, the signaling link functions can ensure the reliable transmission of signaling messages.
The signaling link functions comprise:
Signal unit delimitation Signal unit alignment Error detection
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Initial alignment Signaling link error monitoring Flow control Processor error control
I. Signal Unit Delimitation
The beginning and end of a signal unit are indicated by a unique 8-bit pattern "01111110", called flag.
The end flag of one signal unit is usually the start flag of the following signal unit.
Measures are taken to ensure that the pattern cannot be imitated elsewhere in the unit.
The transmitting signaling link terminal inserts a 0 after every sequence of five consecutive 1s before attaching the flags. At the receiving signaling link terminal, the 0s that directly follow a sequence of five consecutive 1s will be deleted.
II. Signal Unit Alignment
The alignment here does not refer to initial alignment. It refers to the alignment of signaling link in the delimitation procedure.
Normally, the length of a signal unit is a multiple of eight bits.
Loss of alignment occurs when a bit pattern disallowed by the delimitation procedure (more than six consecutive 1s) is received, or when a certain maximum length of signal unit is exceeded. In such cases, the received signal unit is discarded and the signal unit error rate monitor or alignment error rate monitor is incremented.
III. Error Detection
The error detection function is performed by means of 16 check bits provided at the end of each signal unit.
Two forms of error correction are provided, the basic method and the preventive cyclic retransmission method.
The basic method applies for signaling links where the one-way propagation delay is less than 15 ms.
The preventive cyclic retransmission method applies for signaling links where the one-way propagation delay is greater than or equal to 15 ms.
IV. Initial Alignment
The initial alignment procedure is appropriate to both first time initialization and alignment in association with restoration after a link failure.
There are five phases of alignment procedure. They are:
Idle Not aligned
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Aligned Proving period Aligned ready
The initial alignment involves the following four link statuses:
Status indication out of service (SIOS) Status indication out of alignment (SIO) Status indication normal (SIN) Status indication emergency (SIE)
V. Signaling Link Error Monitoring
Two signaling link error rate monitors are provided:
Signal unit error rate monitor Alignment error rate monitor
The signal unit error rate monitor is employed while a signaling link is in service.
The alignment error rate monitor is employed while a link is in the proving state of the initial alignment procedure (first time initialization and alignment in association with restoration after a link failure).
VI. Flow Control
Flow control is initiated when congestion is detected at the receiving end of the signaling link.
The congested receiving end of the link notifies the remote transmitting end of the condition by means of an appropriate link status signal unit.
The remote transmitting end indicates the link as failed if the congestion continues too long.
VII. Process Error Control
This function marks the faulty state of a processor.
2.2.3 Signaling Network Functions
The signaling network functions are performed by the MTP at Level 3 to ensure reliable transmission of signaling messages through control of signaling network route and performance.
The signaling network functions can be divided into two basic categories:
Signaling message handling Signaling network management
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I. Signaling Message Handling
Signaling message handling is to ensure that signaling messages originated by a particular UP at an SP (originating SP) are delivered to the same UP at the destination SP indicated by the sending UP.
As illustrated in Figure 2-3, signaling message handling functions are classified into:
Message discrimination Message distribution Message routing
Messagedistribution
Messagediscrimination
Messagerouting
To/From L2To/From L4
Figure 2-3 Signaling message handling
Message Discrimination
The message discrimination function is used at an SP to determine whether or not a received message is destined to the point itself.
A message destined to the SP itself is transferred to the message distribution function. Otherwise, the message is transferred to the message routing function.
Message Distribution
The message distribution function is used at each SP to deliver the received messages (destined to the SP itself) to the appropriate UP.
Message Routing
The message routing function is used at each SP to determine the outgoing signaling link on which a message (originated or transferred from the message discrimination) is sent towards its destination SP.
Figure 2-4 illustrates the message routing function.
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Messagerouting
Signalingnetworkfunction
Signaling linkfunction
SP 1
SP 2
SP N
﹕
﹕
﹕
Link 1
Link 2
Link N
Figure 2-4 Message routing
II. Signaling Network Management
Signaling network management is to provide reconfiguration of the signaling network in case of failures and to control traffic in case of congestion.
The signaling network management functions are divided into three categories:
Signaling traffic management Signaling link management Signaling route management
Signaling Traffic Management
When message security and correct transmission can be assured, the signaling traffic management modifies signaling routing to transfer messages from unavailable signaling links to available links.
In the case of congestion at SPs, the signaling traffic management may need to slow down signaling traffic on certain routes.
Signaling Link Management
In the case of a signaling link failure, the signaling link management function tests the link and restores it.
Signaling Route Management
When an SP or link fails to transmit messages as a result of fault, the signaling route management function notifies the SP of the condition and allocates another route for the messages to ensure reliable transmission.
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2.3 MTP Messages
Signaling messages are transferred in the SS7 in different lengths.
A signaling message is a set of information defined by the UP of the MTP. Some signaling network management and test maintenance messages can be defined by Level 3 functions of the MTP.
For the sake of reliable transmission, some signaling information fields are attached to each message to constitute the single unit (SU) actually transmitted in the signaling link.
The length of a signal unit is a multiple of eight bits. The length of signal units is described in octets. An octet consists of eight bits.
2.3.1 Format of Signal Units
In SS7, there are three types of signal units:
Fill-in signal unit (FISU) Link status signal unit (LSSU) Message signal unit (MSU)
I. FISU
An FISU contains no information. It is a null signal transferred between network nodes when the link is idle. The purpose of transferring FISUs is to ensure that the signaling link is available and the local end can receive messages from the peer.
Figure 2-5 shows the structure of an FISU.
8 16
F CK
First bit transmitted8
LIFIB
FSN
BIB
BSN
F
6 1 7 1 72 Figure 2-5 Structure of FISU
II. LSSU
An LSSU carries the information of network link status that is indicated by an SF field.
Figure 2-6 shows the structure of an LSSU.
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8 16
F CK
8/16 First bit transmitted8
LIFIB
FSN
BIB
BSN
F
6 1 7 1 7
SF
2
Figure 2-6 Structure of LSSU
III. MSU
An MSU conveys the real information to be transmitted. The information is encapsulated in a signaling information field (SIF) and a service information octet (SIO) . Figure 2-7 shows the structure of an MSU.
8 16
F CK
8n, n>2
SIF
First bit trasmitted8
LIFIB
FSN
BIB
BSN
F
6 1 7 1 72
SIO
8
Figure 2-7 Structure of MSU
2.3.2 Functions and Codes of Signal Unit Fields
A signal unit comprises a number of fields, namely:
Signal unit delimitation flag (F) Check bit (CK) Length indicator (LI) Service information octet (SIO) Signaling information field (SIF) Sequence numbering and indicator bits
I. Signal Unit Delimitation Flag
The bit pattern for the signal unit delimitation flag is 01111110.
The opening F of one signal unit is normally the closing F of the preceding signal unit. For example, in Figure 2-5, the opening F is on the right, and the closing F is on the left.
Other fields in the middle can be inserted randomly to lower system processing load in case of overload.
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II. Check Bit
Every signal unit has 16 check bits for error detection.
Cyclic redundancy codes (CRC) are assumed for check bits.
III. Length Indicator
The length indicator (LI) is used to indicate the number of octets following the length indicator octet and preceding the check bits. The unit of LI is octet.
The length indicator differentiates between the three types of signal units as follows:
LI = 0: FISU LI = 1 or 2: LSSU LI > 2: MSU
IV. Service Information Octet
The SIO is present only in MSUs to indicate the type of messages.
MTP Level 3 functions distribute messages to corresponding function modules according to their SIO and indicate whether the message is from an international network or a national network.
The service information octet is divided into the service indicator (SI) and the sub-service field (SSF), each taking four bits as shown in Figure 2-8.
D C B A D C B A
SSF SI
Transmit direction
Figure 2-8 SIO structure
The codes of SI and SSF are allocated as described in Table 2-1 and Table 2-2. Bits A and B are spare bits.
Table 2-1 SI codes allocation
D C B A Meaning
0 0 0 0 Signaling network management messages
0 0 0 1 Signaling network testing and maintenance messages
0 0 1 0 Spare
0 0 1 1 Signaling connection control part (SCCP)
0 1 0 0 Telephone user part (TUP)
0 1 0 1 ISDN user part (ISUP)
0 1 1 0 Data user part (DUP) (call and circuit-related messages)
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D C B A Meaning
0 1 1 1 Data user part (DUP) (facility registration and cancellation messages)
1 0 0 0 to 1 1 1 1 Spare
Table 2-2 SSF codes allocation
D C Network indicator
00 International network
01 Spare (for international use only)
10 National network
11 Reserved for national use
V. Signaling Information Field
The SIF, the message intended to be delivered, consists of an integral number of octets, greater than or equal to 2 and less than or equal to 272.
In view of shortening signaling transmission delay, the maximum length of SIF used to be 62 octets, and 63 octets plus SIO. The LI value was set to 6 bits, spanning from 0 to 63.
With the development of ISDN services, larger capacity of signaling messages is demanded. As the processing capacity of processors is elevated, the maximum length of SIF can ascend and has been increased to 272 octets.
To maintain the former signal unit format, the LI code is not changed. The LI value of the signal unit with SIF of 63 or more octets is set to 63.
An SIF is composed of two parts, signal information (SI) and label, as shown in Figure 2-9.
SICIC DPC OPC
Label
Figure 2-9 Structure of SIF
SI is encoded depending on user part and its message types.
MTP Level 3 functions select a signaling route according to the label. The label contains three fields:
Originating point code (OPC)
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Destination point code (DPC) Circuit identification code (CIC)
The least sign four bits in the CIC form the signaling link selection (SLS) code.
VI. Sequence Numbering and Indicator Bits
The forward sequence number (FSN) is the sequence number of the signal unit in which it is carried.
The backward sequence number (BSN) is the sequence number of a signal unit being acknowledged.
The forward (retransmission) indictor bit (FIB) indicates the current transmitted signal unit. The value is "0" or "1". When the value of FIB is inverted, the signaling message will be retransmitted.
The backward (retransmission) indicator bit (BIB) indicates the acknowledgements of the received signal unit. When the BIB is inverted, the signaling receiving terminal will notify the transmitting terminal to retransmit messages from the BSN +1.
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Chapter 3 Telephone User Part
This chapter introduces the concepts related to telephone user part (TUP), as well as its functions and position in the SS7.
3.1 Introduction to TUP
The TUP is at layer 4 of the SS7. It is one part of the user part.
Huawei CDMA system employs the TUP compliant with the ITU-T Recommendation of Blue Book Fascicle V1.8 (1988).
Figure 3-1 shows the position of TUP in the SS7.
INAP OMAP MAP
TCAP
ISP
SCCP
MTP-3
MTP-2
MTP-1
HLR VLR
BSAPINAP OMAP MAP
TCAP
ISP
SCCP
MTP-3
MTP-2
MTP-1
HLR VLR
BSAP
L 1
L 2
L 3
L 4 - L6
L 7
ISUP TUP
Figure 3-1 Position of TUP in SS7
3.2 TUP Functions
The TUP defines the circuit signaling function necessary in the call control signaling of SS7 (signaling messages transmitted between MSCs).
3.3 TUP Messages
This section introduces the format and encoding of TUP messages and provides some TUP message examples.
3.3.1 Format of TUP Messages
In SS7, TUP messages are carried on the signaling data link by means of MSUs.
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The signaling information of each message constitutes the SIF of the corresponding signal unit.
It basically contains
The label The heading code (H0, H1) One or more signals and/or indications
The length of the message is changeable. Figure 3-2 shows the structure of TUP messages.
F CK SIF SIO LI FIB FSN BIB BSN F
8 16 8n 8 2 6 1 7 1 7 8 n>=2
First bit transmitted
Signaling Information (SI) H1 H2 Label 8n 4 4 64
F: Delimitation flag BSN: Backward sequence number BIB: Backward indicator bit FSN: Forward sequence number FIB: Forward indicator bit LI: Length indicator SIO: Service information octet SIF: Signaling information field CK: Check bit
Figure 3-2 TUP message structure
3.3.2 Encoding of TUP Messages
TUP messages are encoded by means of label and heading code.
I. Label
The label is an item of information that forms part of every signaling message. It is used by the message routing function at MTP Level 3 to select the appropriate signaling route. The UP identifies the particular transaction (e.g. the call) to which the message pertains. The length of a label must be a multiple of 8 bits.
Figure 3-3 shows the label structure.
CIC OPC DPC
First bit transmitted 4 12 24 24
Figure 3-3 TUP label structure
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The label contains the following fields:
DPC OPC CIC
II. Heading Code
All telephone signal messages contain a heading consisting of two parts, heading code H0 and heading code H1 that identify each telephone signal.
Heading Code H0
The heading code H0 occupies 4-bit field following the label to identify up to 16 message groups. It is coded as in Table 3-1.
Table 3-1 Coding format of H0
DCBA Meaning
0000 Spare, reserved for national use
0001 Forward address messages (FAM)
0010 Forward set-up messages (FSM)
0011 Backward set-up request messages (BSM)
0100 Successful backward set-up information messages (SBM)
0101 Unsuccessful backward set-up information messages (UBM)
0110 Call supervision messages (CSM)
0111 Circuit supervision messages (CCM)
1000 Circuit group supervision messages (GRM)
1001 Spare, reserved for international use
1010 Circuit network management messages (CNM)
1011 Reserved for international and basic national use
1100 Successful national backward set-up information messages (NSB)
1101 National call supervision messages (NCB)
1110 Unsuccessful national backward set-up information messages (NAM) (out of service now)
Heading Code H1
The heading code H1 occupies 4 bits. It either contains a signal code or in case of more complex messages, identifies the format of these messages, when several signal codes and message indicators are contained. A message group identified by an H0 contains a maximum of 16 messages.
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3.3.3 Example of TUP Messages
There are 65 kinds of TUP messages in SS7. Each kind of message has its own function and format. The following cites the initial address message with additional information (IAI) as an example to describe the format and encoding of TUP messages.
I. IAI Message
In a mobile network, an MSC connects with another MSC or a transit exchange using IAIs and forwards initial address messages (IAM) to the local office.
Figure 3-4 shows the basic format of the initial address message with additional information. The IAM message comes before the first indicator octet.
Figure 3-4 IAI format
The following codes are used in IAI in the MSC:
Heading Code H0
Coded 0001, indicating the FAM
Heading Code H1
Coded 0010
Calling Party Category
The calling party category occupies 6 bits containing 64 kinds of calling party categories. It is coded as described in Table 3-2.
Table 3-2 Calling party category
EFDCBA
000000 - 001000 Spare
001001 Operator (no interrupting function)
001010 Ordinary calling subscriber, used between the mobile office and the local office (transit exchange)
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EFDCBA
001011 Calling subscriber with priority , used between mobile offices
001100 Data call
001101 Test call
001110 Spare
001111 Spare
010000 Ordinary calling subscriber, free of charge, used between the mobile office and the remote office
010001 Ordinary calling subscriber, periodic, for inter-office use (including international offices)
010010 Ordinary calling subscriber, user table, immediate (received from the local office only)
010011 Ordinary calling subscriber, printer, immediate (receiving only)
010100 Calling subscriber with priority, free following charge, used between the mobile office and the remote office
010101 Calling subscriber with priority, periodic, for inter-office use (including international offices)
010110 Spare
010111 Spare
011000 Ordinary calling subscriber, received from the local office only
011001 - 111111 Spare
Message Indicators
Table 3-3 lists the message indicators.
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Table 3-3 Message indicators
Nature of address indicator
00 Local subscriber number
01 spare
10 national (significant) number
BA
11 international number
Nature-of-circuit indicator
00 No satellite circuit in the connection
01 One satellite circuit in the connection
10 Spare
DC
11 Spare
Continuity-check indicator
00 Continuity-check not required
01 Continuity-check required on this circuit
10 Continuity-check performed on a previous circuit
FE
11 Spare
Outgoing echo-suppressor indicator
0 Outgoing half echo suppressor not included
G
1 Outgoing half echo suppressor included
Incoming international call indicator
0 Call other than international incoming
H
1 Incoming international call
Redirected call indicator (related to call forwarding)
0 Not a redirected call
I
1 Redirected call
All-digital-path-required indicator (related to ISDN services)
0 Ordinary call
J
1 Digital path required
Signaling path indicator
0 Any path
k
1 All SS7 path
L Spare
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Number of Address Signals
A code expressing in pure binary representation the number of address signals contained in the IAM.
Address Signals
Table 3-4 describes the encoding of the address signals.
Table 3-4 Encoding of the address signals
0000–001 Digits 0–9
1010 Spare
1101 Spare
1110 Spare
1011 Code 11 (*), used in international network connection
1100 Code 12 (#), used in international network connection
1111 ST
Filler
In case of an odd number of address signals, the filler code 0000 is inserted after the last address signal. This ensures that the variable-length field that contains the address signals consists of an integral number of octets.
First Indicator Octet (related to additional information)
The encoding of the first indicator octet various according to whether or not additional information is attached.
Table 3-5 describes the encoding of the first indicator octet.
Table 3-5 Encoding of the first indicator octet
Network capability or user facility information indicator (not in current use, set as 0)
0 Network capability or user facility information not included
A
1 Network capability or user facility information included
Closed user group information indicator
0 Closed user group information not included B
1 Closed user group information included
Additional calling party information indicator (unavailable)
0 Additional calling party information included C
1 Additional calling party information not included
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Additional routing information indicator (unavailable)
0 Additional routing information not included D
1 Additional routing information included
Calling line identity indicator
0 Calling line identity not included E
1 Calling line identity included
Original called address indicator
0 Original called address not included F
1 Original called address included
Charging information indicator (unavailable)
0 Charging information not included G
1 Charging information included
H Spare
Calling Line Identity
As shown in Figure 3-5, the calling line identity consists of three parts: 4-bit address indicator, 4-bit number of address signals, and calling line identity.
Calling line
identity
Number of
address
signals
Address
indicator
First bit transmitted 8n 4 4
D C B A D C B A
Figure 3-5 Calling line identity field
Address indicators
Table 3-6 lists the address indicators.
Table 3-6 Address indicators
Nature of address indicator
00 Local subscriber number
01 Spare, reserved for national use
10 National significant number
BA
11 International number
C Calling line identity presentation indicator
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0 Calling line identity presentation not restricted
1 Calling line identity presentation restricted
Incomplete calling line identity indicator
0 No indication D
1 Incomplete calling line identity
Number of address signals
A code expressing in pure binary representation the number of address signals.
Calling line identity
Table 3-7 lists the calling line identity codes.
Table 3-7 Calling line identity codes
0000-1001 Digits 0-9
1010 Spare
1011 Code 11 (*), used in international network connection
1100 Code 12 (#), used in international network connection
1101 Spare
1110 Spare
1111 ST
Original Called Address
The original called address is the same as the calling line identity except that the bits D and C of the address indicator are spare.
II. IAM Message
Figure 3-6 gives an example of IAM message.
Figure 3-6 IAM format
The meaning of the values in the message is as follows:
84 Service Indication Octet
10------ Network Indication: National network
--00---- Spare bit: Reserved
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----0100 Service Indication: TUP
21 Initial address message with additional information (IAI)
00 12 Circuit identity code: 00 12
00 Signaling link selection: 00
36 66
00------ Spare: 00
--110110 OPC: 36 66
11 E3
00------ Spare: 00
010001 DPC: 11 E3
00 00 Maintenance station reserve 2 bytes: 00 00
TUP message
0F Calling party category
00------ Spare: Reserved
--001111 Calling party category: Payphone
Message indicator
00
0------- International incoming call indicator: Call other than international incoming
-0------ Echo suppressor indicator: Outgoing half echo suppressor not included
--00---- Continuity-check indicator: Continuity check not required
----00— Circuit nature indicator: No satellite circuit in the connection
------00 Address nature indicator: Subscriber number
B4
1011---- Address signal number: 0B
----0--- Collect call indicator: Not collect call
-----1— Signal path indicator: All signaling system No.7 path
------0- All-digital-path-required indicator: Ordinary call
-------0 Redirected call indicator: Not a redirected call
97 20 69 11 03 02 Address signal: 79029611302
02 of it
0000---- Filler: 0
----0010 2
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10 First indicator octet
0------- Spare: Reserved
-0------ Charging information indicator: Charging information not included
--0----- Original called address indicator: Original called address not included
---1---- Calling party subscriber line indicator: Calling line identity included
----0--- Additional routing information indicator: Additional routing information not included
-----0-- Additional calling party information indicator: Additional calling party information not included
------0- Closed user group information indicator: Closed user group information not included
-------0 Network capability or user facility information indicator: Network capability or user facility information not included
Calling party subscriber line identity
72 Calling party subscriber line identity head
0111---- Number of address signal: 07
Address indicator
----0--- Incomplete calling line identity indicator: No indication
-----0— calling line identity presentation indicator: Calling line identity presentation not restricted
------10 Nature of address indicator: National number
55 05 00 F1 Address signal: 5550001
F1 of it
1111---- Filler: F
----0001
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Chapter 4 Signaling Connection Control Part
This chapter introduces the concepts related to signaling connection control part (SCCP) , as well as its functions and position in the SS7.
4.1 Introduction to SCCP
This section introduces the features of:
TUP signaling transfer based on MTP SCCP signaling transfer based on MTP
4.1.1 TUP Signaling Transfer Based on MTP
There are two types of signaling messages on the telecommunication networks:
Circuit-related message Non-circuit-related message
The following details the advantage and disadvantage of TUP when transferring these two types of signaling messages based on MTP.
I. Circuit-Related Messages
The signaling messages transferred on telephone networks are circuit-related messages. That means all signaling messages are related to call circuits, and the paths for these messages are usually in one-to-one correspondence with call connection paths.
Circuit-related messages feature real-time but small-capacity transfer.
The 4-layer signaling architecture with TUP on MTP enables efficient transfer of various call control and connection control messages. It is ideal for telephone network, especially digital switching telephone network.
II. Non-Circuit-Related Messages
Non-circuit-related messages are also called node-to-node messages. They are irrelevant to calls or call circuits.
Non-circuit-related messages feature large-capacity transfer with certain time delay.
III. Disadvantages of TUP Signaling Transfer Based on MTP
The following describes the disadvantages of TUP signaling transfer of call-related and non-call-related messages.
Transfer of Call-Related Messages
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TUP is used to set up call connection paths between two exchanges. It transfers messages along the established call connection paths segment by segment. This increases resources waste and transfer delay.
Transfer of Non-Call-Related Messages
MTP selects a route and determines the terminal user according to the DPC and SI. This addressing method has the following disadvantages:
SPC is defined by the network to which the SP belongs. As a result, the code format may be different, resulting in inter-network addressing failure.
According to CCITT specifications, a SPC is composed of 14 bits. Therefore, the maximum number of SPs on a signaling network is only 16,384 due to code limitation.
An SI is composed of four bits. That is, MTP can allocate messages to a maximum of 16 user parts. This capacity is being challenged by ever-increasing service demands.
MTP allows connectionless transfer only. It does not support non-real-time connection-oriented transfer of large amount of messages between network nodes.
4.1.2 SCCP Signaling Transfer Based on MTP
SCCP and MTP together realize the functions of an OSI network. They allow transparent transfer of signaling messages directly between any two SPs. The SCCP and the MTP in this case are referred to as the network service part (NSP).
SCCP provides connectionless and connection-oriented network services between exchanges and network centers to transfer signaling messages and other types of information.
Figure 4-1 shows the relation between SCCP and other functional elements in a signaling network.
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INAP OMAP MAP ISUP TUP
TCAP
ISP
SCCP
MTP-3
MTP-2
MTP-1
HLR VLR
BSAPINAP OMAP MAP ISUP TUP
TCAP
ISP
SCCP
MTP-3
MTP-2
MTP-1
HLR VLR
BSAP
L 1
L 2
L 3
L 4 - L6
L 7
Figure 4-1 The SCCP in the SS7In the signaling network
In SS7 hierarchical architecture, ISUP and TCAP are the users of SCCP.
ISUP, with the help of SCCP, realizes end-to-end message transfer and supports related ISDN supplementary services.
TCAP utilizes the complete network-layer services provided by SCCP and MTP to provide the following functions:
Realizes long-distance transfer of non-circuit-related messages. Supports new services and functions of wireless networks, intelligent networks,
telecom administration networks, and so on.
Operation and maintenance application part (OMAP) , mobile application part (MAP), home location register (HLR) , and visitor location register (VLR) are all referred to as SCCP subsystems. SCCP performs management on these subsystems with the support of the TCAP. When there is signaling interaction between two of these subsystems, the signaling messages to be transferred are encoded for routing in the SCCP before they are transferred to the peer subsystem. These two subsystems can be of the same SP or of different SPs. However, the signaling messages transferred between two subsystems of the same SP will not go through MTP.
The SCCP provides the following functions:
Transfers non-circuit-related signaling messages. Performs enhanced addressing and routing to realize direct signaling transfer
between different SS7 networks worldwide. Provides connectionless and connection-oriented network services.
4.2 Services Provided by SCCP
SCCP provides various connectionless and connection-oriented network services to meet different demands for data transfer quality.
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4.2.1 SCCP Service Classes
The SCCP provides four classes of service:
Class 0: Basic connectionless class Class 1: In-sequence delivery connectionless class Class 2: Basic connection-oriented class Class 3: Flow control connection-oriented class
4.2.2 Connectionless Services
Connectionless services are performed similar to the transfer of datagram in packet switching. No message transfer channel needs to be set up before message transfer. Signaling data are transferred on the signaling network. Therefore, routing function is provided by the SCCP.
In signaling message transfer, SCCP converts a called address into a signaling point code that can be recognized by MTP.
In connection services, messages are transferred as a whole (as unit data UDT) instead of being segmented.
Figure 4-2 shows the transfer procedure of UDT.
SCCP1 SCCP2 SCCP1 SCCP3 SCCP4 SCCP2
MTP1 MTP3 MTP4 MTP2
UDTUDTUDT
Node 1 Node 2 Node 1 Node 3 Node 4 Node 2
(a) Logical transfer path (b) Actual transfer path
Figure 4-2 Connectionless transfer of signaling messages
Class 0 and Class 1 are connectionless services.
Class 0: Basic connectionless service
Signaling messages are transferred independent of one another. Therefore, there is no guaranteed in-sequence delivery of signaling messages to the destination signaling point.
Class 1: In-sequence delivery connectionless service
The data from the same information flow are attached with a signaling link selection code (SLS) . Data packets with the same SLS are transferred on the same signaling link. Therefore, it is guaranteed that messages can be delivered to the destination signaling point in accordance with the transfer sequence.
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4.2.3 Connection-Oriented Services
Connection-oriented services are performed similar to the packet switching over virtual circuits.
Before signaling message transmission, a message transfer channel (logical connection or virtual connection) is established between the originating node and the destination node.
The connection-oriented service is suitable for transferring a large amount of data.
SCCP1 SCCP1 SCCP1
CR CR
CC CC
DATA
RLSD
RLC
DATA
DATA DATA
RLSD
RLC
Node 1 Middle node Node 2
CR: Connection request CC: Release confirm RLSD: Released RLC: Release complete
Figure 4-3 Connection-oriented transfer with a middle node
Class 2 and Class 3 are connection-oriented services.
Class 2: Basic connection-oriented class
This type of service can guarantee that signaling messages are sent in the same sequence as they are received. Therefore, a long message can be transferred in segments, and reassembled after it is received.
Class 3: Flow control connection-oriented class
The features of class 2 are complemented by the inclusion of flow control, expedited data transfer, and detection of message loss or mis-sequencing.
Connection-oriented services are classified into temporary signaling connections and permanent signaling connections.
Temporary signaling connections are always under control (for example, during establishment, data transfer, and release). They are released after the transfer is finished. It is similar to a call connection.
Permanent signaling connections are similar to permanent virtual circuits in packet data switching. They are established0 and controlled by the operation
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and maintenance system of the local or a remote MSC or managed by connected nodes. Mobile subscribers cannot control these connections.
The signaling transfer procedures of temporary and permanent signaling connections are the same.
4.2.4 SCCP Service Procedure
This section describes the signaling procedures of connectionless and connection-oriented services provided by the SCCP.
I. Connectionless Services
Figure 4-4 shows the transfer of connectionless service messages in GT addressing.
USERA SCCPA SCCPC SCCPB USERB
N-UNITDATA Request
N-UNITDATA Indication
UDT
UDT
Figure 4-4 Connectionless service in GT addressing
The following explains the procedure:
1) USERA sends an N-UNITDATA Request to SCCPA to request for connectionless service data transfer.
The N-UNITDATA Request primitive contains:
Called address: GT (implicit DPC=B, SSN=USERB) Calling address: DPC=A, SSN=USERA In-sequence control: Sequence Return selection: Error return User data
2) After SCCPA receives the primitive, it analyzes the called address. After it determines that the called address is a GT, it transfers the UDT to SCCPC.
3) After SCCPC receives the UDT, SCCP translates the GT of the called address into the following parameters:
DPC=B SSN=USERB
Then It performs addressing according to the DPC and the SSN. If SCCPC finds that the DPC and SSN of the destination SCCP are available, SCCPC transfers the UDT message to the destination SCCPB.
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4) After SCCPB receives the UDT, it sends an N-UNITDATA Indication to USERB, and transfers user data to USERB.
In this example, GT translation is completed by SCCPC. In actual networks, GT translation may be done by different nodes on the network, or centralized in one node.
II. Connection-Oriented Services
Figure 4-5 shows the signaling procedure of connection-oriented service provided by the SCCP.
USERA SCCPA SCCPC SCCPB USERB
N-CONNECT Request
N-CONNECT Indication
CR
CR
N-CONNECT Confirmation
N-CONNECT Response
CC
CC
Figure 4-5 Connection-oriented service signaling flow
The following explains the procedure:
1) USERA (calling party) sends an N-CONNECT Request to SCCPA to request the signaling connection with USERB (called party).
2) After SCCPA receives the N-CONNECT Request, it checks whether resources are available. If yes, SCCPA allocates a source local reference and a SLS for the connection section.
Then SCCPA establishes a correspondence between the called address and the connection section, and determines protocol class and credit. Finally it selects the route for the CR message and transfers it to SCCPC.
3) When SCCPC receives the CR message, it checks whether the called address is a local SCCP user through the routing and diagnosis function. If the SCCP user is not a local SCCP user, the SCCP requires setting up a connection section.
SCCPC checks whether the resource is available. If yes, SCCPA allocates the received source local reference and SLS for the connection section. Then it establishes a correspondence between the input connection section and output connection section and determines the protocol class and credit.
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SCCPC forwards the CR message to SCCPB through the routing function without changing the addressing contents of CR message.
4) After SCCPB receives the CR message, it checks whether the called address is a local user through the routing and diagnosis function, and whether the destination monitoring has available resources to establish the connection section. If the results are positive, SCCPB allocates the received source local reference and SLS to the input connection section (between the middle node and the destination).
Then SCCPB determines protocol class and credit, and, through an N-CONNECT Indication, notifies USERB to establish a connection.
5) If the connection is approved, USERB sends an N-CONNECT Response to the SCCPB.
6) After SCCPB receives N-CONNECT Response, it allocates a protocol class and credit, and determines the local reference of input connection section. Then it sends a CC message to the source SCCPC of the connection section by using SCCP routing function.
7) After SCCPC receives the CC message, it allocates the source local reference of the CC to the output connection section. Then SCCPC determines the protocol class and credit and the local reference for the corresponding output connection section. Finally it transfers the CC message to the source SCCPA corresponding to the input connection section by using the SCCP routing function.
8) After SCCPA receives the CC message, it allocates the protocol class and credit, and allocates the source local reference in the received CC to the connection section. Finally it sends an N-CONNNECT Confirmation to USERA, notifying that USERA signaling connection is successful.
After that, the user can transfer data through the signaling connection. The connection is released after data transfer is finished.
4.3 SCCP Addressing and Routing
There are three types of SCCP address:
Signaling point code (SPC) Subsystem number (SSN) Global title (GT)
SPC refers to the address of MTP. It is valid in a SS7 network only.
The MTP designated by SPC identifies a destination SP with the received DPC and performs routing to the destination SP. In addition, it identifies the user of the destination SP according to Service Indicator (SI).
SSN is the local addressing information used by SCCP to identify SCCP users in the same node.
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For example, SSNs can be used to represent TCAP, ISUP, MAP, and so on. It serves as a supplement to MTP messages in defining subscriber address. With the SSN, the local addressing range of SI is extended and thus an SS7 network can meet the requirements for future new services development.
GT applies when the destination network address is unknown to the originating node.
GT can be used to identify any signaling point and subsystem worldwide.
MTP, however, cannot perform routing according to GT. The SCCP must first translate the GT of the called party into DPC or DPC+SSN. In addition, when SCCP sends the DPC or DPC+SSN to MTP, it needs to specify the numbering plan of the GT.
The calling address and called address in a SCCP message may be any or the combination of SPC, SSN, and GT.
SCCP can perform addressing and routing according to the following two types of addresses:
DPC+SSN GT
If SCCP sends GT+DPC+SSN to MAP, it must specify whether the routing for message transfer is performed in accordance with GT or DPC+SSN.
4.4 SCCP Primitives
This section gives the definition and structure of SCCP primitives, followed by a detailed description of SCCP user and service primitives
4.4.1 Definition
In SS7 layered architecture, any layer can be regarded as Layer N except for the top and the bottom layers. The upper layer and the lower layer next to layer N is referred to as Layer N+1 and Layer N-1. Layer N+1 is the user of Layer N, and Layer N is in turn the user of Layer N-1. Services are provided by Layer N-1 to Layer N and by Layer N to Layer N+1.
To realize the information exchange between Layer N+1 and its peer end, Layer N+1 requests that Layer N communicates with the peer Layer N on the basis of the connection provided by Layer N-1 with peer Layer N-1. In this process, Layer N provides services to Layer N+1.
When Layer N+1 requests services from Layer N or Layer N provides services to Layer N+1, the service user shall interact with the service provider. The signaling data transferred between these two layers in this process are referred to as primitives.
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Four types of primitives are available:
Request: A primitive issued by a service user to invoke a service element. Indication: A primitive issued by a service provider to indicate that a service
element has been invoked by the service user. Response: A primitive issued by the peer service user in response to the
request. Confirmation: A primitive issued by a service provider acknowledge the reception
of the response.
Note:
Not every primitive has all of the four types (Request, Indication, Response and Confirmation). It is subject to specific service protocol procedure.
Service interfaces from SCCP to upper layer and MTP are defined by primitives and parameters.
The lower layer of SCCP is MTP, and the corresponding primitive is the MTP-primitives. The upper layer is SCCP user, the primitive between the SCCP and its user is N-primitives (also called SCCP user primitives).
Inter-SCCP communication is performed with SCCP messages, and inter-MTP communication, with the MTP messages.
Figure 4-6 shows the primitives and messages of the SCCP and MTP.
USER
SCCP
MTP
USER
MTPMTP Message
SCCP MessageSCCP
Request Confirmation Response Indication
Request Confirmation Response Indication
Figure 4-6 Primitives and messages of MTP and SCCP
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4.4.2 Structure
A complete primitive consists of three parts: generic name, specific name, and parameters, as shown in Figure 4-7.
×Genericname Specific name Parameters
Figure 4-7 Structure of a primitive
%: Stands for the functional block providing the services (M represents MTP, and N represents SCCP).
Generic name: Primitive name, indicating the service provided and the task shall be completed in addressing layer.
Specific name: Primitive type, indicating the direction of primitive flow. Parameter: Data needed for implementing this service.
For example, a signaling message is transferred to the destination in the form of UDT. When the destination SCCP transfers this data to its user, the indication primitive of the UDT is:
N-UNITDATA indication (CDA, CGA and UD)
Where:
"N" represents the network layer (the SCCP primitive). "UNIDATA" is the generic name. "CDA, CGA, and UD" are the primitive parameters, representing the called
address, calling address and subscriber data.
4.4.3 SCCP User Primitives
Table 4-1 lists the name, protocol type, and parameters of SCCP user primitives.
Table 4-1 SCCP user primitives
Protocol type Generic
name Specific
name 0 1 2 3
Parameters
N-UNITDATA Request Indication
ª ª
Calling address Called address Sequence control Return option User data
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Protocol type Generic
name Specific
name 0 1 2 3
Parameters
N-NOTICE Indication ª ª
Called address Calling address Reason for return User data
N-CONNECT
Request Indication Response Confirm
ª ª
Calling address Called address Responding address Receiving response selection Expedited data selection Quality of service parameter set User data Connection identification
N-DISCONNECT
Request Indication
ª ª
Originator User data Responding address Connection identification Reason
N-DATA Request Indication
ª ª
Response request User data Connection identification
N-EXPEDITED DATA
Request Indication
ªUser data Connection identification
N-RESET
Request Indication Response Confirmation
ª
Originator Reason Connection identification
N-INFORM Request Indication
ª ª
Reason Connection identification QOS parameter set
The following describes these SCCP user primitives:
N-UNITDATA: UDT primitive, used to transfer the UDT in connectionless service.
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N-NOTICE: Notice primitive, used to notify the originating end, destination or transfer point that the SCCP cannot transfer the message in case of the connectionless service.
N-CONNECT: Connection primitive, used to establish a connection. N-DISCONNECT: Disconnection primitive, used for disconnection. N-DATA: Data primitive, used to transfer data in connection-oriented service. N-EXPEDITED DATA: Expedited data primitive, used for Class 3 to transfer
expedited data. N-RESET: Reset primitive, used to transfer reset messages in Class 3 protocol
and to restart the flow control procedure from initial state. N-INFORM: Inform primitive, used to transfer relevant network or user
information during data transfer phase in connection-oriented service.
4.4.4 MTP Service Primitives
SCCP layer communicates with MTP by exchanging MTP service primitives.
Table 4-2 lists the MTP service primitives.
Table 4-2 MTP service primitive
Generic name Specific name Parameters
MTP-TRANSFER Request Indication
SCCP message
MTP-RESUME Indication Affected signaling point
MTP-PAUSE Indication Affected signaling point
MTP-STATUS Indication Affected signaling point
MTP-UPU Indication Affected signaling point
The following describes these MTP service primitives:
MTP-TRANSFER Request: Used by SCCP to access MTP signaling message handling function.
MTP-TRANSFER Indication: MTP message handling function, transferring signaling messages to SCCP.
MTP-PAUSE Indication: Sent by MTP to indicate that it fails to transfer messages to the designated destination.
MTP-RESUME Indication: Sent by MTP to notify the user that MTP is capable of providing MTP service to the designated destination.
MTP-STATUS Indication: Sent by MTP to notify the user that MTP is partially capable of providing MTP service to the designated destination. This primitive is also used to notify the user of the cause of peer user available or unavailable.
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4.5 SCCP Messages
On receiving the primitive request or response from a user, SCCP encapsulates user data and necessary control and routing information into SCCP messages.
4.5.1 Format of SCCP Messages
SCCP messages encapsulated in the MSUs in MTP before transfer. For a MSU, SCCP message is its signaling information field (SIF).
I. Message Structure
Figure 4-8 shows the structure of SCCP messages.
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F CK SIF SIO LI FIB FSN BIB BSN F
Routing label
Message type
Mandatory fixed part(F)
Mandatory variablepart (V)
Optional part (O)
Mandatory parameter A
¡-¡-
Mandatory parameter I
¡-¡-
Parameter M pointer
Parameter P pointer
Start pointer of optional part
Parameter M length
Parameter M
¡-¡-
Parameter P length
Parameter P
Parameter name X
Parameter X length
Parameter A
¡-¡-
Paraemter name Z
Paraemter Z length
Paraemter Z
End of optional parameter
Figure 4-8 Structure of SCCP messages
The following describes the components of SCCP messages:
Routing label: The structure is DPC+OPC+SLS. It provides routing information for the transfer of SCCP messages.
Message type: It identifies different SCCP messages. It is a mandatory byte for all messages, and determines the function and format of a message.
Mandatory fixed part (F): It includes all mandatory parameters with fixed length in this message.
Mandatory variable part (V): It includes all mandatory parameters with variable length in this message.
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Optional part (O): It includes all optional parameters in this message.
II. Message Type
Table 4-3 shows the message types and their codes of various messages.
Table 4-3 SCCP message type and code
Protocol class Message type
0 1 2 3 Message type
code
Connection request (CR) √ √ 0000 0001
Connection confirm (CC) √ √ 0000 0010
Connection refused (CREF) √ √ 0000 0011
Released (RLSD) √ √ 0000 0100
Release complete (RLC) √ √ 0000 0101
Data form 1 (1DT1) √ 0000 0110
Data form 2 (2DT2) √ 0000 0111
Data acknowledgement (AK) √ 0000 1000
Unit data (UDT) √ √ 0000 1001
Unit data service (UDTS) √ √ 0000 1010
Protocol data unit error (ERR) √ √ 0000 1111
Inactivity test (IT) √ √ 0001 0000
Functions of SCCP message types are as follows:
CR and CC are used to establish a connection. CREF is sent to the originating node when the middle node or the destination
node lacks sufficient resources. DT1, DT2 and ED are messages used to transfer data after successful
connection. RLSD and RLC are release signals after data transfer. ERR is sent when protocol error is detected. IT is used to detect whether the two ends of the connection can work normally. UDT, UDTS, XUDT, and XUDTS are connectionless service messages. UDT
and XUDT are used to transfer connectionless service data and segmented data of over-length messages. UDTS and XUDTS are sent to the originating point to specify the cause if UDT or XUDT fails to reach the destination.
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4.5.2 Encoding of SCCP Messages
A SCCP message contains 17 parameters in total.
Table 4-4 lists all parameter names and their codes.
In the table, M stands for "mandatory" and O stands for "optional".
Table 4-4 SCCP message parameters
Field Message Code
UDT
UDTS
CR
CC
CREF
RLSD
RLC
DT1
DT2
AK
ED
EA
RSR
RSC
ERR
IT
Message type M M M M M M M M M M M M M M M M
Destination local reference M M M M M M M M M M M M M 00000001
Source local reference M M M M M M M 00000010
Called party address M M M O O 00000011
Calling party address M M O 00000100
Protocol class M M M M 00000101
Segmenting/reassembling M 00000110
Receive sequence number
M 00000111
Sequencing/segmenting M M 00001000
Credit O O M M 00001001
Release cause M 00001010
Return cause M O O O 00001011
Reset cause M 00001100
Error cause M 00001101
Data M M O O O O M M M 00001111
Refusal cause M 00001110
End of optional parameters O O O O O O 00000000
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The following describes these parameters:
I. Destination Local Reference and Source Local Reference
Destination local reference and source local reference apply to connection-oriented services. They are the internal numbers used by the destination of a signaling connection section and the source SCCP to identify this connection section. They are allocated by SCCP at both ends of a connection when the connection is set up. These two references identify the path for later data transfer.
The parameter is a three-octet field. The "all 1s" codes are reserved for future use.
II. Calling Address and Called Address
Calling address and called address identify the originating and destination signaling point and the user part.
The calling and called addresses in a connectionless service message represent the origination and destination of the SCCP message. Those in a connection-oriented service message represent the source and destination of a signaling connection (not signaling connection section) and are used for connection setup and connection acknowledgement messages.
Calling/called address codes consist of the following units in order:
Address indicator Signaling code SSN GT
The following details address indicator, SSN, and GT.
Address Indicator
Address indicator indicates the type of address information contained in the address field. The address consists of one or the combination of signaling point code, GT, and SSN.
Table 4-5 shows the structure of an address indicator.
Table 4-5 Structure of address indicator
7 6 5 4 3 2 1 0
Reserved for national use
Routing indicator GT indicator SSN
indicator Signaling point code indicator
Signaling point code indicator
0: Signaling point code not included.
1: Signaling point code included.
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Subsystem indicator
0: SSN not included.
1: SSN included.
GT indicator
There are four types of GT.
Table 4-6 shows the correspondence between GT indicator codes (bits) and GT types.
Table 4-6 Correspondence between GT type codes and GT types
GT type code GT type
0000 GT not included.
0001 GT includes nature of address indicator only
0010 GT includes translation type only
0011 GT includes translation type, numbering plan and encoding scheme
0100 GT includes translation type, numbering plan, encoding scheme and nature of address indicator
0101~1110 Spare international
1110~1111 Reserved for extension
Routing indicator
0: Select the route according to the GT in the address.
1: Select the route according to the DPC and the SSN in the called address.
SSN
SSN is a one-octet code. It identifies an SCCP user function.
Table 4-7 explains the meanings of SSNs.
Table 4-7 Allocation of SCCP SSNs
SSN SCCP user
0000 0001 SCCP management
0000 0101 Reserved for compatibility
0000 0110 Home Location Register (HLR)
0000 0111 Visitor Location Register (VLR)
0000 1000 Mobile Switching Center (MSC)
0000 1010 Authentication Center (AC)
1110 1110 Message Center (MC)
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SSN SCCP user
1110 1111 SCP
1111 1100 BSMAP
GT
There are four types of GT. Table 4-8, Table 4-9, Table 4-10, and Table 4-11 show the structures of these GTs.
Table 4-8 Type 1 GT
8 7 6 5 4 3 2 1
O/E Nature of address indicator
The 2nd address signal The 1st address signal
…
Filler (if necessary) The Nth address signal
Table 4-9 Type 2 GT
8 7 6 5 4 3 2 1
Translation type
The 2nd address signal The 1st address signal
…
Filler (if necessary) The Nth address signal
Table 4-10 Type 3 GT
8 7 6 5 4 3 2 1
Translation type
Numbering plan Encoding scheme
The 2nd address signal The 1st address signal
…
Filler (if necessary) The Nth address signal
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Table 4-11 Type 4 GT
8 7 6 5 4 3 2 1
Translation type
Numbering plan Encoding scheme
Spare Nature of address indicator
The 2nd address signal The 1st address signal
…
Filler (if necessary) The Nth address signal
The following describes the fields in a GT:
Nature of address indicator: Nature of the GT
0 0 0 0 0 0 1 — Subscriber number
0 0 0 0 0 1 0 — Reserved for national use
0 0 0 0 0 1 1 — National significant number
0 0 0 0 1 0 0 — International number
Numbering plan is as follows:
0 0 0 0 — Unknown
0 0 0 1 — ISDN/telephony numbering plan (Recommendations E.163 and E.164)
0 0 1 0 — Generic numbering plan
0 0 1 1 — Data numbering plan (Recommendation X.121)
0 1 0 0 — Telex numbering plan (Recommendation F.69)
0 1 0 1 — Maritime mobile numbering plan (Recommendations E.210, E.211)
0 1 1 0 — Land mobile numbering plan (Recommendation E.212)
0 1 1 1 — ISDN/mobile numbering plan (Recommendation E.214)
Encoding scheme is as follows
0 0 0 0 — Unknown
0 0 0 1 — Binary-Coded Data (BCD), odd number of digits
0 0 1 0 — BCD, even number of digits
Translation type: Coding and definition is to be specified. In present CDMA systems, all-zero codes are used.
The coding of internal interface in the network subsystem (NSS) uses type 4 GT. In the A interface SCCP message, the address information generally does not contain the GT.
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III. Protocol Class
Table 4-12 lists the protocol classes defined by bits 1–4.
Table 4-12 Protocol classes
Bits
Bit 4 Bit 3 Bit 2 Bit 1 Protocol Class
0 0 0 0 Class 0
0 0 0 1 Class 1
0 0 1 0 Class 2
0 0 1 1 Class 3
When bits 1–4 are coded to indicate a connection-oriented-protocol class (class 2, class 3), bits 5-8 are spare.
When bits 1–4 are coded to indicate a connectionless protocol class (class 0, class 1), bits 5–8 are used to specify message handling as described in Table 4-13:
Table 4-13 Handling of messages in case of transfer failure
Bits
Bit 8 Bit 7 Bit 6 Bit 5 Return message in case
of transfer failure
0 0 0 0 No
1 0 0 0 Yes
IV. Segmenting/Reassembling
The parameter Segmenting/Reassembling determines whether data in the DT1 is to be transferred in several segments and reassembled at the destination.
This parameter is a one-octet field.
Bits 2–8 are spare.
Bit 1 indicates whether more data is followed. It is called M bit. It is coded as follows:
M=0: no more data M=1: more data
V. Receive Sequence Number and Credit
These two parameters are used in data acknowledgement. They apply to protocol class 3 only.
Receive sequence number indicates the sequence number of the next expected message.
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Credit is used for flow control function. It contains a window size value coded in pure binary.
VI. Sequencing/Segmenting
This parameter consists of two octets. It is used for DT2.
This parameter has two functions:
Indicates the sequence number of a sent message and the number of next expected message to receive for flow control function.
Indicates whether the message is segmented.
VII. Release Cause
The Release Cause parameter is a one-octet field containing the cause for the release of a connection.
Table 4-14 shows the coding of this field.
Table 4-14 Coding of release causes
Bits
Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Release cause
0 0 0 0 0 0 0 0 End user originated
0 0 0 0 0 0 0 1 End user congestion
0 0 0 0 0 0 1 0 End user failure
0 0 0 0 0 0 1 1 SCCP user originated
0 0 0 0 0 1 0 0 Remote procedure error
0 0 0 0 0 1 0 1 Inconsistent connection data
0 0 0 0 0 1 1 0 Access failure
0 0 0 0 0 1 1 1 Access congestion
0 0 0 0 1 0 0 0 Subsystem failure
0 0 0 0 1 0 0 1 Subsystem congestion
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Bits
Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Release cause
0 0 0 0 1 0 1 0 MTP failure
0 0 0 0 1 0 1 1 Network congestion
0 0 0 0 1 1 0 0 Expiration of reset timer
0 0 0 0 1 1 0 1
Expiration of receive inactivity timer
0 0 0 0 1 1 1 0 Reserved
0 0 0 0 1 1 1 1 Unqualified
0 0 0 1 0 0 0 0 SCCP failure
0 0 0 1 0 0 0 1
… … … … … … … …
1 1 1 1 1 1 1 1
Spare
VIII. Return Cause
The Return Cause parameter, used in connectionless protocol UDTS, is a one-octet field containing the cause for message return.
The coding of the Return Cause field is as shown in Table 4-15
Table 4-15 Coding of return causes
Bits
Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Return cause
0 0 0 0 0 0 0 0
No translation for an address of such nature
0 0 0 0 0 0 0 1
No translation for this specific address
0 0 0 0 0 0 1 0 Subsystem congestion
0 0 0 0 0 0 1 1 Subsystem failure
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Bits
Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Return cause
0 0 0 0 0 1 0 0 Unequipped user
0 0 0 0 0 1 0 1
… … … … … … … …
1 1 1 1 1 1 1 1
Spare
IX. Reset Cause, Refusal Cause, and Error Cause
These three parameters indicate reset cause, refusal cause and error cause respectively.
X. User Data
This is a variable-length field containing SCCP user data to be transferred transparently between SCCP user functions.
4.5.3 Example of SCCP Messages
The following is an example of SCCP messages.
Figure 4-9 shows the SCCP message traced on an SS7 link.
Figure 4-9 SCCP message
The following explains the message:
83 Service indicator octet field
10⎯⎯ Network indicator: National
⎯00⎯⎯ Spare
⎯⎯0011 Service indicator: SCCP
00 Invalid
00 Circuit identifier: 00 00
00
0A Signaling link selection code: 0A
60 60 60 source code: 60 60 60
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00 0B 11 destination code: 00 0B 11
0A Message type: Connection confirm
******Length fixed mandatory parameter part******
9A 0E 05 destination local reference number: 9A 0E 05
02 5E 0F source local reference number: 02 5E 0F
******Protocol class part******
02
⎯⎯0010 Protocol class: Class 2
0000⎯⎯ Spare
01 Start pointer of optional part: 01
******Optional part******
Data part
0F Data
0E SCCP user data part length indicator: 14
00 BSSMAP message indication: 00
0C Data length: 0C
For the analysis of BSAP messages, see Chapter 8 "Base Station Application Part".
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Chapter 5 ISDN User Part
This chapter introduces the concepts related to ISDN user part (ISUP), as well as its functions and position in the SS7.
5.1 Introduction to ISUP
ISUP is in the 4th functional block of SS7 architecture, corresponding to layers 4–7 function in OSI reference model.
ISUP is added with non-speech bearer service protocol and supplementary service protocol based on the TUP.
ISUP supports basic bearer services and supplementary services of ISDN users, and realizes the functions of TUP and data user part (DUP).
Figure 5-1 shows the ISUP in the SS7 architecture.
INAP OMAP MAP TUP
TCAP
ISP
SCCP
MTP-3
MTP-2
MTP-1
HLR VLR
BSAPINAP OMAP MAP TUP
TCAP
ISP
SCCP
MTP-3
MTP-2
MTP-1
HLR VLR
BSAP
L 1
L 2
L 3
L 4 - L6
L 7
ISUP
Figure 5-1 ISUP position in SS7
As shown in Figure 5-1, ISUP needs the support of MTP and SCCP.
ISUP has more functions than TUP, but fewer message types.
The features of ISUP are as follows:
Complete message types: Information carried in the message is abundant. Variable message length: Multiple parameters can be carried. Simple signaling program. Powerful functions: Supports various speech, non-speech, and supplementary
services.
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5.2 ISUP Functions
ISUP provides bearer services, user terminal services, and supplementary services.
5.2.1 Bearer Services
Bearer service is a low-layer message transfer capability provided by a network. It only indicates ISDN communication capability, and is irrelevant to the type of terminals. Therefore, different terminals can use the same bearer capability.
ISUP supports the following bearer services:
64 kbit/s circuit switching unrestricted Speech 3.1 kHz audio 2 x 64 kbit/s 384 kbit/s unrestricted 1,920 kbit/s unrestricted
5.2.2 User Terminal Services
User terminal service is application oriented. It includes the communication capability provided by a network and that of a terminal.
For example, a videophone terminal requires that the minimum bearer capability is 2 x 64 kbit/s unrestricted. The properties of a user terminal service include low-layer, high-layer, and general properties.
Low-layer property indicates the necessary bearer capability of a network, and the property value may be identical with that of the bearer service.
High-layer property indicates the fixed capability of a terminal such as G4 facsimile machine and telephone.
General property indicates available supplementary service, QoS, and so on.
ISUP supports the following user terminal services:
Telephone Intelligent user telegraph G2 and G3 facsimile G4 facsimile Hybrid mode Videotext Videophone
5.2.3 Supplementary Services
Supplementary services are the extra functions provided by a network to complement bearer services and user terminal services.
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Supplementary services must be provided together with a bearer service or user terminal service.
ISUP provides the following supplementary services:
Call forwarding–unconditional (CFU) Call forwarding–busy (CFB) Call forwarding–no answer (CFNA) Call forwarding–default (CFD) Calling number identification presentation (CNIP) Calling number identification restriction (CNIR) Calling number identification restriction over (CNIR-Over) Call waiting (CW) Call transfer (CT) Three-way calling (3WC) Conference calling (CC) Remote feature control (RFC) Subscriber PIN access (SPINA) Subscriber PIN intercept (SPINI) Do not disturb (DND) Preferred language (PL) Call forwarding to voice mailbox Voice message retrieval (VMR) Message waiting notification (MWN) Subscriber lock Feature code service
5.3 ISUP Messages
ISUP messages are transferred in the form of MSU. The structure of ISUP messages is similar to that of SCCP messages.
The following details the structure and coding of ISUP messages.
5.3.1 Format of ISUP Messages
ISUP messages are transferred in a signaling link through MTP layer.
The SIF of ISUP messages is in an octet field stack, as shown in Figure 5-2.
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Routinglabel
Circuit Identification Code (CIC)
Message type code
Mandatory fixed part (F)
Mandatory fixed part Fn
End of optional parameter field (00)
12345678
Bit sending sequence
Pointer (Location of parameter V1)
Pointer (Location of parameter Vn)
Mandatory variable part V1 Parameter length indicatorParameter content
Pointer (Start location of optional parameter group)
Optional parameter O1
Mandatory variable part Vn Parameter length indicatorParameter content
Parameter contentParameter length indicator
Parameter name
Optional parameter OnParameter content
Parameter length indicatorParameter name
Comm
on pa
rtSp
ecial
part
Octet
send
ing se
quen
ce
Figure 5-2 ISUP message structure
A ISUP message consists of the following parts:
Routing label Circuit identification code (CIC) Message type code Mandatory fixed part Mandatory variable part Optional parameters
In message transmission, the system first transfers the routing label, and then the optional part. Each byte is transmitted starting from the least signification bit.
5.3.2 Encoding of ISUP Messages
Each ISUP message consists of a number of parameters. Each parameter is allocated a name and is encoded according to bytes.
The length of a parameter can be fixed or variable.
Each variable parameter contains a length indicator indicating the number of bytes in the parameter. A length indicator occupies a byte.
I. Routing Label
Figure 5-3 shows the format of the routing label in an ISUP message.
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SLS OPC DPC
Bits sent first 4 4 24 24
Figure 5-3 Format of routing label in the ISUP message
DPC: Destination signaling point code.
OPC: Originating signaling point code.
SLS: Signaling link selection code used for load sharing. At present, only the least significant four bits are used.
II. CIC
CIC is used for the connection between originating and destination signaling points. At present, the least significant 12 bits are used, and the remained 4 bits are spare (0000).
Figure 5-4 shows the structure of CIC.
CIC (Least significant bit)
Spare
12345678
CIC (Most significant bit)
1
2
Figure 5-4 Format of the CIC in a ISUP message
III. Message Type Code
Table 5-1 describes the encoding of ISUP messages.
A message code defines the function and format for an ISUP message.
Table 5-1 Encoding of ISUP messages
Message type Abbreviation Code
Address Complete ACM B00000110
Answer ANM B00001001
Blocking BLO B00010011
Blocking Acknowledgement BLA B00010101
Call Progress CPG B00101100
Circuit Group Blocking CGB B00011000
Circuit Group Blocking Acknowledgement CGBA B00011010
Circuit Group Query CQM B00101010
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Message type Abbreviation Code
Circuit Group Query Acknowledgement CQA B00101011
Circuit Group Reset GRS B00010111
Circuit Group Reset Acknowledgement GRA B00101001
Circuit Group Unblocking CGU B00011001
Circuit Group Unblocking Acknowledgement CGUA B00011011
Charge Information CRG* B00110001
Confusion CFN B00101111
Connect CON B00000111
Continuity COT B00000101
Continuity Check Request CCR B00010001
Facility FAC B00110011
Facility Accepted FAA B00100000
Facility Reject FRJ B00100001
Facility Request FAR B00011111
Forward Transfer FOT B00001000
Identification Request IDR B00110110
Identification Response IRS B00110111
Information INF B00000100
Information Request INR B00000011
Initial Address IAM B00000001
Loop Back Acknowledgement LPA B00100100
Network Resource Management NRM B00110010
Overload OLM B00110000
Pass Along PAM B00101000
Release REL B00001100
Release Complete RLC B00010000
Reset Circuit RSC B00010010
Resume RES B00001110
Segmentation SGM B00111000
Subsequent Address SAM B00000010
Suspend SUS B00001101
Unblocking UBL B00010100
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Message type Abbreviation Code
Unblocking Acknowledgement UBA B00010110
Unequipped Circuit Identification Code UCIC B00101110
User Part Available UPA B00110101
User Part Test UPT B00110100
User-to-User Information USR B00101101
Operator Information OPR B11111110
Metering Pulse Message MPM B11111101
Calling Party Clear Information CCL B11111100
Note:
The item marked with "*" is not used at present. The code Bxxxxxxxx indicates binary Xxxxxxxx.
IV. Mandatory Fixed Part (F)
This part contains those parameters that are mandatory and of fixed length.
The position, length and order of the parameters are uniquely defined by the message type; thus, the names of parameters and the length indicators are not included in the message.
V. Mandatory Variable Part (V)
This part contains mandatory parameters of variable length in a message.
Pointers are used to indicate the beginning of a parameter.
The name of a parameter and the order in which the pointers are sent is implicit in the message type. Therefore, the message type define both the number of parameters and the number of pointers.
VI. Optional Part (O)
The optional part consists of parameters that may or may not occur in a message.
This part may include fixed length and variable length parameters.
Each optional parameter includes the parameter name and a length indicator followed by parameter contents.
"End of optional parameters" octet contains all zeros.
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5.3.3 Example of ISUP Messages
Below is the description of ISUP messages by using the example of the Initial Address (IAM) .
I. Introduction to IAM
The IAM can contain at most 35 parameters, including 5 mandatory fixed parameters, 1 mandatory variable parameter, and 29 optional parameters.
The IAM contains called party address information, and other information related to call connection control.
Table 5-2 describes the parameters of the IAM.
Table 5-2 Parameters of IAM
Parameter name Type Length Remarks
Message Type F 1 B00000001
Nature of connection indicators F 1 Mandatory fixed
Forward call indicator F 2 Mandatory fixed
Calling party category F 1 Mandatory fixed
Transmission medium request F 1 Mandatory fixed
Called party number V 4-11 Mandatory optional
Transit network selection O 4-? Optional
Calling party number O 4-12 Optional
Optional forward call indicator O 3 Optional
Redirecting number O 4-12 Optional
Redirection information O 3-4 Optional
Closed user group interlock code O 6 Optional
Original called number O 4-12 Optional
User-to-user information O 3-131 Optional
Access transport O 3-? Optional
User service information O 4-13 Optional
User-to-user indicators O 3 Optional
Generic number O 5-13 Optional
Propagation delay counter O 4 Optional
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Parameter name Type Length Remarks
User service information O 4-13 Optional
Network dedicated performance O 4-? Optional
Generic digits O ? Optional
Originating International Switching Center (ISC) point code O 4 Optional
Future terminal service information O 7 Optional
Parameter compatibility information O 4-? Optional
Generic notification O 3 Repeated
Transmission medium requirement O 3 Optional
Location number O 5-12 Optional
End of optional parameters O 1 Optional
II. Encoding of IAM Parameters
Message Code
00000001
Nature of Connection Indicators
This is a mandatory fixed parameter with one octet field (A–F). Table 5-3 gives the codes of the parameter.
Table 5-3 Code of the nature of connection indicators
Satellite indicator
00 No satellite circuit in the connection
01 One section of satellite circuit in the connection
10 Two sections of satellite circuit in the connection
BA
11 Spare
Continuity check indicator
00 Continuity check not required
01 Continuity check required in the circuit
10 Continuity check completed in the previous circuit.
DC
11 Spare
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Echo control device indicator :
0 Outgoing half echo control device not included. E
1 Outgoing half echo control device included.
F–H Spare
Forward Call Indicator
This is a mandatory fixed parameter with the length of two octets (A–M). Table 5-4 gives the codes of the parameter.
Table 5-4 Codes of forward call indicator
National/international call indicator
0 Call to be treated as a national call A
1 Call to be treated as an international call
End-to-end indicator
00 None (only section by section forward method available) CB
01 Transfer method available
End-to-end method indicator
10 SCCP method available CB
11 Transfer mode and the SCCP method available
Interworking indicator
0 Interworking not encountered (SS7 signal in all directions) D
1 Interworking encountered
End-to-end information indicator
0 No end-to-end information available E
1 End-to-end information available
ISDN User Part indicator
0 ISDN user part not used in all directions F
1 ISDN user part used in all directions
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ISDN user part preference indicator:
00 ISDN user part preferred in all directions
01 ISDN user part not required in all directions
10 ISDN user part not required in all directions
HG
11 Spare
ISDN access indicator
0 Initial accessing non-ISDN I
1 Initial accessing ISDN
SCCP method indicator
00 No indicator
01 Connectionless method available
10 Connection-oriented method available KJ
11 Connectionless and connection-oriented methods available
L Spare
P–M Reserved for national use
Note:
Bits B–F and J–K constitute a protocol control indicator.
Calling Party Category
This is a mandatory fixed parameter. Different from the ISUP message of a fixed network, the calling party category is a one-octet field (H–A).
Table 5-5 gives the codes of the parameter.
Table 5-5 Codes of the calling party category
HGFEDCBA Description
00000000 Calling party category unknown (receive only)
00000001–00001000 Spare
00001001 Operator (no insertion function)
00001010 Ordinary subscriber, used between a mobile office and local office, and between a mobile office and tandem office
00001011 Preference subscriber, used between mobile offices
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HGFEDCBA Description
00001100 Data call
00001101 Test call
00001110–11101111 Spare
11110000 Ordinary, Free, used between a mobile office and toll office (including international office)
1110001 Ordinary, periodic, used between a mobile office and toll office (including international office)
11110010 Ordinary, subscriber table, immediate (receive only from the local office or tandem office)
11110011 Ordinary, printer, immediate (receive only from local office or tandem office)
11110100 Preference, free, used between a mobile office and toll office (including international office)
11110101 Preference, periodic, used between a mobile office and toll office (including international office)
11110110–11111111 Spare
Other than the above mandatory parameters (including the mandatory variable parameter Called Party Number), the IAM also includes 29 optional parameters.
Optional parameters are selected in accordance with the basic services and supplementary services supported by the IAM.
For example, if the call transfer service exists, the IAM parameters shall include redirecting number, redirection information, original called number, and generic notification.
III. Example of IAM
Figure 5-5 shows an IAM message traced.
Figure 5-5 IAM message
The following explains the message:
85 Service indicator octet (SIO)
10------ Network indicator: National network (NAT)
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--00---- Spare
----0101 Service indicator: ISUP
01 Message type: Initial Address (IAM)
00 01 Circuit identification code: 00 01
01 Signaling Link Selection: 01
11
00------ Spare
--010001 OPC: 11 E3
E3
36 66
00------ Spare
--110110 DPC: 36 66
00 00 Maintenance station reserve 2 bytes: 00 00
00 Nature of connection indicators
000----- Spare: 00
---0---- Echo control device indicator: outgoing half echo control device not included
----00- Continuity check indicator: continuity check not required
------00 Satellite indicator: no satellite circuit in the connection
Forward call indicators
20
00------ ISDN user part preference indicator: ISDN user part preferred in all directions
--1----- ISDN user part indicator: ISDN user part used in all directions
---0---- End-to-end information indicator: no end- to- end information available
----0--- Interworking indicator: no interworking encountered
-----00- End-to-end method indicator: no end- to- end method available
-------0 National/international call indicator: call to be treated as a national call
01
000----- Reserved for national use: 00
---0---- collect call indicator: not collect call
----0--- spare
-----00- SCCP method indicator: no indicator
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-------1 ISDN access indicator
0A Calling party’s category: ordinary calling subscriber
00 Transmission medium requirement: speech
02 Pointer to mandatory variable part: 02
06 Pointer to start of optional part: 06
******Mandatory Variable part******
Called party number
04 Length indicator of Called party number: 04
81
1------- Odd/even indicator: odd number of address signals
-0000001 Nature of address indicator: subscriber number
90
1------- Internal network number indicator (INN ind): routing to internal network number not allowed
-001---- Numbering plan indicator: ISDN (Telephony) numbering plan (Recommendation E.164)
----0000 Spare
Address signal, F indicates ST, address complete
23 32F
0F
0000---- Filler: 0
----1111 F
******Optional part******
08 Optional forward call indicator
01 Length indicator of Optional forward call indicators: 01
00 Optional forward call indicators
0------- Connected line identity request indicator: not request
-0000--- spare: 00
-----0-- Simple segmentation indicator: no additional information will be sent
------00 Closed user group call indicator: non−CUG call
0A Calling party number
07 Length indicator of calling party number: 07
03
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0------- Odd/even indicator: even number of address signals
-0000011 Nature of address indicator: national number
13
0------- Calling party number incomplete indicator (NI): complete
-001---- Numbering plan indicator: ISDN numbering plan
----00- Address presentation restricted indicator: presentation allowed
------11 Screening indicator: network provided
Address signal, F indicates ST, address complete
09 92 16 31 50 Calling party number 0929611305
1D User service information
03 Length indicator of User service information: 03
80
1------- Extension indicator: last octet
-00----- Coding standard: ITU-T standardized coding, as described below
---00000 Information transfer capability: speech
90
1------- Extension indicator: last octet
-00----- Transfer mode: circuit mode
---10000 Information transfer rate: 64kibit/s
A3 Other information: A3
31 Propagation delay counter
02 Length indicator of Propagation delay counter: 02
00 00 Propagation delay counter: 00 00
3F Location number
03 Length indicator of Location number: 03
83
1------- Odd/even indicator: odd number of address signals
-0000011 Nature of address indicator: national number
97
1-------- Internal network number indicator: routing to internal number not allowed
-001---- Numbering plan indicator: ISDN numbering plan
----01- Address presentation restricted indicator: presentation restricted.
------11 Screening indicator: network provided
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Address signal, F indicates ST, address complete
0F
0000---- Filler: 0
----1111 F
00 End of optional parameter
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Chapter 6 Transaction Capabilities Application Part
This chapter introduces the concepts related to transaction capabilities application part (TCAP), as well as its functions and position in the SS7.
6.1 Introduction to TCAP
With the development of telecommunication networks, more services are demanded. Such services include the intelligent services like freephone (FPH) and virtual private network (VPN) , as well as the operation, administration, maintenance and provision (OAM&P) and mobile application part (MAP).
These services and applications are irrelevant to call control. That is, message transfer functions are separated from call control functions. They are provided on the basis of the correlation between:
Exchanges Exchanges and network service centers Subscribers and network service centers
To address these demands, the transaction capability (TC) protocol is applied.
Transaction capabilities are functions that control non-circuit-related information transfer between two or more signaling nodes through the SS7. They serve as the interface between several applications and one particular service.
The TC protocol provides general standards for the applications as a whole instead of for a particular application.
The TC consists of transaction capability application part (TCAP) and intermediate service part (ISP).
The former corresponds to Layer 7 of the OSI model, and the latter, Layers 4–6.
In the CDMA system only TCAP is involved. That is, TCAP is directly involved in data transfer.
Figure 6-1 shows the position of TCAP in the SS7 network.
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INAP OMAP MAP ISUP TUP
TCAP
ISP
SCCP
MTP-3
MTP-2
MTP-1
HLR VLR
BSAPINAP OMAP MAP ISUP TUP
TCAP
ISP
SCCP
MTP-3
MTP-2
MTP-1
HLR VLR
BSAP
L 1
L 2
L 3
L 4 - L6
L 7
Figure 6-1 Position of TCAP in the SS7 network
Currently there are two TCAP standards:
TCAP defined by International Telecommunication Union - Telecommunication Standardization Sector (ITU-T)
TCAP defined by American National Standard Institute (ANSI)
CDMA system uses the latter standard.
6.2 TCAP Structure
TCAP is divided into
Component sublayer (CSL) : responsible for operation administration Transaction sublayer (TSL) : responsible for transaction administration
The CSL communicates with TC user over TC primitive interface and with the TSL over TR primitive interface.
Figure 6-2 shows the structure of TCAP.
CSL
TC User A
TCAP
TC User B
TCAP
SS7 NetWorks
TC-Primitive
TR- Primitive
N- Primitive
TSL
TC-Primitive
CSL
TSL
TR- Primitive
N- Primitive
Figure 6-2 Structure of TCAP
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6.2.1 Transaction Sublayer
TSL is responsible for the signaling exchange between TC users and the transaction management in this process. A user at this layer is referred to as a TR user. Currently the only TR user defined is the CSL.
Communication between the CSL of the same level (also the TC users of the same level) is referred to as a session.
A session is the process of TCAP message exchange between two TC users performed in the signaling exchange to realize the provisioning of a particular service.
The initiation and termination of message exchange and the sequence of messages exchanged are controlled and explained by TC users.
TSL is responsible for the management on the initialization, proceeding and termination of sessions, as well as the detection of errors and subsequent troubleshooting during sessions.
The protocol applied is also applicable to all application service related sessions (that is, transactions). Therefore, in ANSI TCAP, transaction and session are regarded as equivalent concepts.
In ANSI TCAP, two sorts of sessions are defined: layered and non-layered sessions. This definition is made from the point of view of session management, and does not involve actual applications.
I. Non-Layered Session
A non-layered session is similar to a connectionless transfer of SCCP. It contains only a unidirectional TCAP message sent by the local end and no reply is expected.
There is no division of initialization, proceeding, and termination phases for a session of this type. That is, no TCAP transaction is established.
II. Layered Session
A layered session contains three phases: initialization, proceeding (also TCAP message exchange), and termination.
Similar to connection-oriented data transfer, these three phases are initiated by TC users.
More than one session can proceed between two TC users, and each of these sessions is marked with a unique ID.
6.2.2 Component Sublayer
CSL consists of the dialog portion and component portion, respectively responsible for the control of sessions and the processing of components.
A TCAP message exchanged in a session contains one or several components, each of which reflects the request for the execution or results of a particular operation. In
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some cases such a message may contain no component and this type of messages are responsible for the control of the session only.
Each component is marked with a unique Invoke ID, and is invoked by this ID with other components.
The Invoke ID is defined only for CSL to recognize different components and perform monitoring and management on the components. That is, the Invoke ID of a component must be perceived differently from an operation code that is defined by a TC user.
The indication of an Invoke ID is determined by the actual application, and will not be analyzed or processed by TCAP.
6.3 TCAP Messages
The encoding of TCAP message complies with the specifications in abstract syntax notation one (ASN.1).
6.3.1 Encoding of TCAP Messages
All information elements (IE) are formed in the designated “Tag + Length + Contents” format, as shown in Figure 6-3.
IE
Tag
Length
Contents
Figure 6-3 Structure of TCAP IE
I. Tag
The tag identifies an IE and describes the contents of the IE.
A tag is composed of class, form, and tag code.
A tag may contain one or several octets. See Table 6-1 for the structure of a tag.
Table 6-1 Structure of TCAP message tag
BIT7 BIT6 BIT5 BIT4 BIT3 BIT2 BIT1 BIT0
Class Form Tag Code
Class
00: Universal
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01: Application wide
10: Context specific
11: Private use
Form
0: Primitive
1: Constructor
Tag Code
A tag code is formed by taking out the bits 0–4 in the tag when the tag contains only one octet.
When the tag has extension octets, the tag code is formed by taking out bits 0–4 from the first octet and adding it to the extension octets.
In a tag that contains only one octet, the range for the tag code is 00000–11110, as shown in Figure 6-4.
Class Form Tag Code(00000 - 11110)
Figure 6-4 The format of a tag containing one octet
In a tag that contains more than one octets, suppose bits 0–4 in the first 8 bits are “11111” (binary), that is, “0X9F” (hexadecimal).
If the first bit (bit 0) in the second octet is “1” (for example, this octet is “0X81”), it indicates that there is another octet that follows.
If the first bit (bit 0) in the second octet is “0” (for example, this octet is “0X02”), it indicates that this is the last octet of the tag.
Class Form Tag Code1 1 1 1 1
Ext1 MSB
Ext0 LSB
Figure 6-5 The format of a tag containing more than one octets
II. Length
The length field is coded to indicate the number of octets in the contents of an IE. That is, it does not include the tag field or the length field itself.
Short Form
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When the contents in an IE are shorter than 128 octets, the short form is then used. The length contains one octet, with bit 7 set to “0”. See Figure 6-6.
Length of contentsMSB LSB
0 Length of contentsMSB LSB
0
Figure 6-6 Length of contents -- the short form
Long Form
When the contents are longer than 127 octets, the long form is used.
Bit 7 of the first octet is coded “1”.
Bits 0 to 6 of this octet encode a number less than the size of the length whose most significant bit (MSB) and least significant bit (LSB) are bits 0 and 6 respectively.
The length of the contents is encoded from the second octet on, with the MSB as the bit 7 of the second octet and the LSB as the bit 0 of the last octet. See Figure 6-7.
1 Size of length - 1 MSB LSB
MSB
Length of contents
LSB
Figure 6-7 Length of contents – the long form
Indefinite Form
The indefinite form is one octet long. It has the fixed value 10000000, serving as the tag of the indefinite code instead of indicating its length.
Indefinite form is applicable to IEs of any length. The maximum length of an IE is determined by the maximum length of SCCP messages.
Indefinite form can be used to replace long or short form when the IE is a combination.
Examples
If the contents of the TCAP message are within 0x00 (hexadecimal) to 0x7F (hexadecimal) octets long, the length contains one octet.
If the contents of a TCAP message are 0x80 (hexadecimal) octets long, the length contains two octets, that is, 0x81 0x80.
If the contents of a TCAP message are 0x90 (hexadecimal) octets long, the length contains two octets, that is, 0x81 0x90.
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If the length is 0x82 0x01 0x00, the contents of the TCAP message is 0x01 00 (hexadecimal) octets long, that is, 256 (decimal) octets long.
Note:
In ANSI standards, an IE with the length “0” does not have any contents. While in ITU-T standards, an IE with the length “0” is one that does not actually exist.
III. Contents
Contents are the substance of an IE, containing the primary information the element is intended to convey.
IEs are classified into atomic IE and constructor IE.
The contents in an atomic IE are inseparable.
The contents in a constructor IE contain other IE who has a similar structure. The length of a constructor IE is the integer multiple of octet.
6.3.2 Format of TCAP Messages
A TCAP message includes three portions: transaction portion, dialog portion, and component portion, as shown in Figure 6-8.
Transaction Portion
Dialog Portion
Component Portion
Figure 6-8 TCAP message structure
The transaction portion is mandatory, and the dialog portion and component portion are optional. However, either the dialog portion or the component portion (or both) must be present in a TCAP message.
6.3.3 Transaction Portion
The following introduces the elements of transaction portion.
I. Package Type Identifier
The package type identifier is used to differentiate the TCAP package type in a TCAP message. One byte is used to indicate the package type identifier.
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Table 6-2 lists the TCAP package types and codes.
Table 6-2 TCAP package type identifier
Package Type Code Description
Unidirectional 0xE1This transaction package type sends information in one only direction without reply. No TCAP transaction is established.
Query with permission 0xE2
This transaction package type initiates a TCAP transaction and informs the destination node (that is, the node that receives the message) that it may end the TCAP transaction.
Query without permission 0xE3
This transaction package type initiates a TCAP transaction and it informs the destination node that it may not end the TCAP transaction.
Response 0xE4 This transaction package type ends the TCAP transaction.
Conversation with permission 0xE5
This transaction package type is the continuation of the TCAP transaction and it informs the destination node that it may end the TCAP transaction.
Conversation without permission
0xE6This transaction package type is the continuation of the TCAP transaction and it informs the destination node that it may not end the TCAP transaction.
Abort (P-Abort) 0xF6
This transaction package type informs the destination node that the source node has terminated the established TCAP transaction without sending any pending components that may be expected due to a prior message.
Abort (User-Abort) 0xF6
This transaction package type informs the destination node that the source node has terminated the established TCAP transaction without sending any pending components that may be expected due to a prior message.
The Unidirectional is adopted only by T-ANSWER and O-ANSWER in the provisioning of CDMA IN services.
Table 6-3 shows the correspondence between TCAP package type and IE.
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Table 6-3 Correspondence between TCAP package type and cell
Package type IE
IE name Type Tag
Unidirectiona
l
Query w
ith perm
ission
Query
without
permission
Response
Conversation
with
permission
Conversation
without
permission
Abort
(p-abort)
Abort
(user-abort)
Transaction ID
Constructor
0xE1/0xE2/0x E3/0x E4/0xE5/0xE6/0xF6
M M M M M M M M
P-Abort Cause
Primitive
0XD7
None None None No
ne None None M None
User-Abort Cause
Primitive
0XD8
None None None No
ne None None None M
Dialog Portion
Constructor 0XF9 O O O O O O Non
e O
Component Sequence
Constructor
0XE8 M O O O O O Non
e Non
e
Note:
The dialog portion and component portion do not belong to the transaction portion. This table only describes the IEs contained by each TCAP package type.
II. Total TCAP Message Length
This length field indicates total message length.
III. Transaction ID
The transaction ID includes the originating transaction ID and responding transaction ID. They are used to realize simultaneous interaction of several transactions between two entities. If they two appear together, the responding transaction ID is always presented after the originating transaction ID.
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IV. P-Abort Cause
This ID indicates the cause of a P-Abort message, as described in Table 6-4.
Table 6-4 P-Abort causes
Cause Cause description Code
Unrecognized Package Type
The package type tag is not included in Table 6-2. 0x01
Incorrect (Mistyped) Transaction Portion
The transaction portion is incorrectly tagged. 0x02
Badly Structured Transaction Portion
Basic errors such as length error occurred to the codes of the transaction portion.
0x03
Unassigned Responding Transaction ID
The transaction ID received is mismatched with the ongoing transaction.
0x04
Permission to Release Problem P-abort cause is not clear currently. 0x05
Resource Unavailable The resources at the TSL are insufficient for establishing transactions. 0x06
Unrecognized Dialog Portion ID The ID for the dialog portion is incorrect. 0x07
Badly Structured Dialog Portion
Some codes are lost for the dialog portion. 0x08
Missing Dialog Portion The mandatory dialog portion is lost. 0x09
Inconsistent Dialog PortionThe contents of the dialog portion are mismatched with the status of the transaction.
0x0A
V. User-Abort Cause
This IE indicates the cause of TC user aborting transaction.
VI. Dialog Portion
The dialog portion does not belong to the transaction portion and it is a part of component sublayer. For details, refer to section 6.3.4 "Dialog Portion".
VII. Component Sequence
Actually the component sequence is not a part of the transaction portion. It is a part of the component portion used to indicate the sequence of one or several components.
The components are processed in the sequence that they are received. ANSI-41 standard requires that each transaction correspond to an individual component only.
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6.3.4 Dialog Portion
Table 6-5 lists the IEs contained in the dialog portion.
Table 6-5 IEs contained in the dialog portion
Dialog Portion Optional/Mandatory
Protocol Version Optional
Integer Application Context Optional
User Information Identifier Optional
Integer Security Context Optional
Confidentiality Identifier Optional
6.3.5 Component Portion
The following introduces the elements of component portion.
I. Component Sequence Identifier
This filed identifies the component sequence and is coded “E8”.
II. Component Sequence Length
This filed encodes the total length in octet of the component sequence.
III. Component Type Identifier
This field encodes the type of the component.
Table 6-6 gives the correspondence between component types and component type identifiers.
Table 6-6 Correspondence between component types and component type identifiers
Component Types Component Type Identifier Remarks
Invoke (Last) 0xE9 Used to request for the invocation of remote application process.
Return Result (Last) 0xEA Used to request for the return of results for particular application.
Return Error 0xEB Used to report the invocation failure of particular application.
Reject 0xEC Used to report the status of a Component or Transaction of being accepted or rejected.
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Component Types Component Type Identifier Remarks
Invoke (Not Last) 0xED Used to request for the initiation of a remote application.
Return Result (Not Last) 0xEE Used to return the results for
particular application.
Table 6-7 lists the IEs contained in various types of components.
Table 6-7 Correspondence between component types and IEs
Component TypeIE
IE name IE type Tag
Invoke Return Result Return Error Reject
Component ID Primitive 0XCF M M M M
Operation Code Primitive 0XD0/0XD
1 M None None None
Error Code Primitive 0XD3/0XD4 None None M None
Problem Code Primitive 0XD5 None None None M
Parameter Constructor 0XF2 M M M M
IV. Component Length
This IE indicates the length of the component (excluding the fields of component type identifier and component length).
V. Component ID
The component ID identifies a particular component. It correlates the Invoke of an operation and the Response. It is mandatory only if an invoke ID is present in the corresponding invoke.
Invoke ID: Assigned by the initiator of an operation. Correlation ID: Received from the Invoke ID in the response to the initiation of an
operation.
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VI. Operation Code
The operation code indicates the operation to be initiated by a TC user. It is application specific and is unexamined by the TCAP. It can be National or Private. For more information, refer to ANSI-T1.114.
VII. Error Code
The error code provides the reason why a specific operation could not be completed. It is application specific and is unexamined by the TCAP. For more information, refer to ANSI-T1.114.
VIII. Problem Code
This field indicates the reason the component or transaction portion was rejected.
IX. Parameter
It indicates a particular parameter. It is application specific and is unexamined by the TCAP.
6.3.6 Example of TCAP Messages
Figure 6-9 gives an example of a TCAP message: the remote user interactive directive (RUIDIR) message.
Figure 6-9 RUIDIR message
The following describes the values in this message:
83 Network Indicator
10------ Network Indicator: National
--00---- Spare
----0011 Service Indicator: SCCP
00 Invalid Bit
00 00 Circuit Identification Code
01 Signaling Link Selection Code
77 77 77 Originating Signaling Point Code
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FE 55 55 Destination Signaling Point Code
09 Unit Data (UDT) Message Type
00 Return upon Absence of Class 0 Protocol (Non-Sequential Connectionless Service) Setting
03 Mandatory Variable Pointer 1
10 Mandatory Variable Pointer 2
1D Mandatory Variable Pointer 3
0D 12 08 00 61 04 64 00 03 59 5515 00 F0 Called Address 460030955551000
0D 12 06 00 60 04 64 00 03 19 11 01 00 F0 Calling Address 460030911110000
3A SCCP User Data Length Indicator (Length: 58 bytes).
E6 TCAP Message Package Type: CONVERSATION WITHOUT PERMISSION
38 TCAP Message Length
C7 Transaction ID
08 Transaction ID Length
59 05 01 38 9A 00 01 5C Transaction ID Contents
E8 Component Sequence Identifier
2C Component Sequence Length
E9 Component Type Identifier (Invoke Last)
2A Component Type Identifier Length
CF Component Type Identifier
01 Component Identifier Length
4D Contents
D1 Operation Code Identifier
02 Operation Code Length
09 0D Operation Code Contents: Remote User Interactive Directive (compliant with ANSI-41 protocol).
The rest of the parameters are MAP related. Refer to Chapter 7 "Mobile Application Part" for details.
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Chapter 7 Mobile Application Part
This chapter introduces the concepts related to mobile application part (MAP), as well as its functions and position in the SS7.
7.1 Introduction to MAP
MAP is a specialized functional entity for a public land mobile network (PLMN) to achieve intra-network and inter-network connections. MAP specifies intersystem data transfer between the network entities in a CDMA network to support mobile roaming. These network entities include:
Mobile switching center (MSC) Visitor location register (VLR) Home location register (HLR) Authentication center (AC) Message center (MC) Service control point (SCP)
Figure 7-1 presents the interfaces between these entities.
MS BSS MSC/VLR
MC
MSC/VLR
HLRSCP
A
T1C/D
E
Q
MS: Mobile station BSS: Base station subsystem
Figure 7-1 CDMA network architecture
Except for the A interface, all interfaces in the CDMA network can transmit MAP messages.
The following describes these interfaces.
A interface
The A interface is between the network subsystem and base station subsystem. This interface carries messages related to MS management, BTS management, mobility management, call processing, and so on.
B interface
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B interface is VLR to MSC interface. Through this interface, the MSC requests location information from the VLR and notifies the VLR to update the location information of a MS. This interface also carries supplementary services operation messages.
C interface
C interface is MSC to HLR interface. In a mobile terminated call, the GMSC obtains roaming number from the HLR through the C interface. In a mobile terminated short message service, the MC obtains, over this interface, the number of serving MSC from HLR through GMSC.
D interface
D interface is VLR to HLR interface. Over this interface, VLR and HLR exchange MS location and subscriber management information to ensure that the subscribers in the serving area can make and receive calls normally.
E interface
E interface is between two MSCs, controlling the handoff of MSs between to neighbor MSCs. E interface carries messages between the MSCs to initiate and implement handoff operations.
Note:
In actual implementation, MSC and VLR are usually integrated into one physical entity. As a result, the B interface becomes an internal interface. C interface and D interface may share physical links.
7.2 MAP Functions
MAP implements intra-PLMN and inter-PLMN interworking functions and operations. This section describes the MAP functions.
7.2.1 MAP Management Functions
MAP location and data management are the basic functions of CDMA network.
The functions include:
Realizes automatic roaming and roaming restrictions. Provides user data for other services. Maintains data consistency between VLR and HLR. Protects network resources from being accessed by illegal users.
MAP enables intersystem information transfer in the following procedures in a CDMA network.
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I. Location Management
MAP involves in the following operations:
Power-up registration: Used in user location registration, qualification authentication, and user data access
Power-down registration Default registration: Used in the registration in the case of no user data available
when a call is originated Location cancellation Qualification request: Obtains the subscriber service list and qualification period Qualification directive: Maintains consistency of subscriber service list between
HLR and MSC. Bulk deregistration Unreliable roaming
II. Authentication Management
The purpose of MAP authentication management is to prevent illegal subscribers from accessing the system. This includes the authentication for location registration, mobile originated calls, and mobile terminated calls. It also includes the subscriber authentication and periodic authentication performed by the authentication center (AC) using its algorithms.
III. Handoff Management
Handoff management function enables subscribers to move freely without affecting the connection quality. MAP handoff management complies with protocols, ensuring the interconnection between equipment of different suppliers and roaming in different MSCs.
Handoff management function includes:
Basic handoffs, namely, handoff forward, handoff backward, and handoff to a third party.
Transparent signaling transmission after handoff MAP circuit management
IV. Call Functions
MAP call functions include:
Origination request: Obtains calling subscriber data from the HLR or SCP. Location request: Obtains location information of the called party from the HLR. Forwarding request: Obtains the forwarding number.
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V. Supplementary Service Support
MAP supports various call-related and non-call-related supplementary services, such as conference calling. MAP can identify and support feature operations intended to the service control point (SCP) .
VI. Intelligent Services
MAP supports the following intelligent services:
Intelligent control
MAP applies for processing related to intelligent services and obtains related data such as Trig Type, TOD and TDO. The data is then forwarded by the MAP to the SCP. The processing result is transmitted from the SCP to the MAP and forwarded to the related call module.
MAP can also receive call control instructions initiated by the SCP and forward the instructions to the related call module. The processing result is passed to MAP and forwarded to SCP.
SCP-based forwarding services
Intelligent subscribers can subscribe to forwarding service at the SCP. Upon receiving instructions from the call module, the MAP sends a TBUSY or TNOANS message to the SCP to obtain the forwarding number and forwards the information to the call module to continue the forwarding operation.
Pre-paid charging (PPC)
In a PPC service, MAP restores the MSC or SCP in case of abnormality. When the MSC is recovered, the MAP initiates a recovery operation on the related SCP. It also transmits call records after the SCP sends an abnormality recovery request.
Intelligent announcement playback
MAP supports the intelligent peripherals module (IPM) in the MSC and a standalone intelligent peripheral (IP) .
7.2.2 MAP Operations
Implementation of each MAP function contains several operations. Each operation is defined by a set of elements including:
Operation name, code, and type Invoke parameter Success parameter Failure code and parameter Linked operations allowed
Table 7-1 lists the MAP operations.
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Table 7-1 MAP operations
Operation code Abbreviation Label
Handoff Measurement HANDMREQ 01
Facilities Directive FACDIR 02
Mobile On Channel MSONCH 03
Facilities Release FACREL 05
Qualification Request QUALREQ 06
Qualification Directive QUALDIR 07
Blocking BLOCKING 08
Unblocking UNBLOCKING 09
Reset Circuit RESETCKT 0A
Trunk Test TTEST 0B
Trunk Test Disconnect TTESTDISC 0C
Registration Notification REGNOT 0D
Registration Cancellation REGCANC 0E
Location Request LOCREQ 0F
Routing Request ROUTREQ 10
Feature Request FEATREQ 11
Unreliable Roamer Data Directive UNRELDIR 14
MS Inactive MSINACT 16
Transfer To Number Request TRANUMREQ 17
Redirection Request REDREQ 18
Flash Request FLASHREQ 1A
Authentication Directive AUTHDIR 1B
Authentication Request AUTHREQ 1C
Base Station Challenge BSCHALL 1D
Authentication Failure Report AFREPORT 1E
Count Request COUNTREG 1F
Bulk Deregistration BULKDEREG 22
Handoff Measurement Request HANDMREQ 23
Handoff Back HANDBACK 25
Handoff To Third HANDTHIRD 26
Authentication Directive Forward AUTHDIRFWD 27
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Operation code Abbreviation Label
Authentication Status Report ASREPORT 28
Information Directive INFODIR 2A
Information Forward INFOFWD 2B
Inter System Answer ISANSWER 2C
Origination Request ORREQ 2F
Random Variable Request RANDREQ 30
Remote User Interaction Directive RUIDIR 32
SMS Delivery Backward SMDBACK 33
SMS Delivery Forward SMDFWD 34
SMS Delivery Point To Point SMDPP 35
SMS Notification SMSNOT 36
SMS Request SMSREQ 37
MAP operations are classified into four categories:
Category 1 operations: Report is required regardless of the operation result. In the case of a successful operation, the result is reported; in the case of an unsuccessful operation, the error is reported.
Category 2 operations: Report is required only in the case of operation failure. Category 3 operations: Report is required only in the case of operation success. Category 4 operations: Report is not required.
For the sake of security, when MAP originates a remote operation, the operation time limit must be specified. If no report is received in the time limit, processing is as follows:
For categories 1 and 3, it is considered operation failure For categories 2 and 4, it is considered success.
Currently, category 4 only contains OANSWER and TANSWER operations. Other operations are all classified to category 1.
Note:
The above operation codes do not cover the newly added intelligent service operations.
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7.3 MAP Messages
MAP messages are transferred based on the services provided by MTP, SCCP, and TCAP.
7.3.1 Format of MAP Messages
In the SS7, MAP messages are transmitted as part of TCAP messages. Figure 7-2 shows the structural relation between MAP and MTP messages.
MAP messageTCAP messageSCCPmessage
MTPmessage
Figure 7-2 Structural relation between MAP and MTP messages
MAP messages are coded in ASN.1 format. The message type is in one to one correspondence with the operation code in the TCAP component.
In message transmission, one MAP message corresponds to one invoke ID. The invoke ID is the unique identifier of a MAP message. Thus, a TCAP component can be translated to a MAP message based on the invoke ID.
7.3.2 Encoding of MAP Messages
MAP messages are encoded in the same way as TCAP messages. For detailed information, refer to section 6.3 "TCAP Message".
7.3.3 Example of MAP Messages
Take the Registration Notification message as an example. Figure 7-3 shows the message traced on a SS7 link of the MSC.
Figure 7-3 Registration Notification message traced on a SS7 link
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The following explains the message:
83
10------ Network indicator: national network
--00---- Spare
----0011 Service indicator: SCCP
00 Invalid bits resulting from signaling link tracing
00 00 Circuit identification
05 Signaling link selection
FE 55 55 Originating signaling point code (OPC)
77 77 77 Destination point code (DPC)
09 Unit data message type (UDT)
80 Class 0 protocol (Non-sequential connectionless service), report required.
03 Mandatory variable pointer 1
10 Mandatory variable pointer 2
1D Mandatory variable pointer 3
0D 52 06 00 61 04 64 00 03 59 53 03 00 01 Called address 460030953530001
0D 12 07 00 61 04 64 00 03 59 5515 00 00 Calling address 460030955551000
74 SCCP user data part length indication, 116 bits
E2 TCAP Package Type: QUERY WITH PERMISSION
72 Total TCAP Message length
C7 Transaction ID identifier
04 Transaction ID length
98 00 01 5C Transaction IDs
E8 Component Sequence identifier
6A Component Sequence length
E9 Component Type identifier (Invoke Last)
68 Component length
CF Component ID identifier
01 Component ID length
4C Component IDs
D1 Operation Code
02 Operation Code length
09 0D Operation Code: Registration Notification (employing ANSI-41 standard)
F2 Parameter Set
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5F Parameter Set length
89 ESN
04 ESN Length
53 53 00 01 ESN content
88 MIN
05 MIN length
90 35 35 00 10 MIN content
95 MSCID
03 MSCID length
64 05 01 MSCID content
91 Qualification Information Code
01 Qualification Information Code length
03 Qualification Information Code content: Require approval from the roaming subscriber and service list.
96 System My Type
01 System My Type length
00 System My Type content
9F 20 PC_SSN
05 PC_SSN Length
05 55 55 FE 07 9F PC_SSN Content
67 Sender Identification Number
0C Sender Identification Number length
00 31 61 0F 64 00 03 59 55 15 10 00 00 Sender Identification Number content
9F 22 System Access Type
01 System Access Type length
03 System Access Type content
9F 31 System Capacity
01 System Capacity length
0F System Capacity content
9F 2F Terminal Type
01 Terminal Type length
20 Terminal Type content
9F 7B Transmission Capacity
02 Transmission Capacity length
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1F 30 Transmission Capacity content
9F 68 Message Address
0C Message Address length
00 31 61 0F 64 00 03 59 55 15 00 00: Message Address content
BF 82 18 WIN Capabilities
0C WIN Capabilities length
9F 82 15 Trigger Capability
03 Trigger Capability length
FF FF 1F Trigger Capability content
9F 82 19 WIN Operation Capability
01 WIN Operation Capability length
03 WIN Operation Capability content
7.4 Common MAP Procedures
This section details the flow of MAP messages in various procedures.
7.4.1 Location Registration
Figure 7-4 shows the transfer of MAP messages in location registration procedure.
BSS MSC/VLR A HLR/AC MSC/VLR B
CC
LA_UPDATE_REQ
REGNOT
REGCANC
regcanc
regnot
LA_UPDATE_ACC
CLEAR COMMAND
CLEAR COMPLETE
a
b
c
d
e
f
h
i
g
Figure 7-4 Location registration procedure
The following describes the MAP messages only.
REGNOT: The originating MSC (MSC/VLR A in the figure) sends a location registration request to the HLR through interfaces C and D.
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REGCANC: The HLR sends a registration cancellation message to the original VLR (VLR B).
regcanc: The original VLR sends response to the HLR after registration cancellation.
regnot: The HLR notifies the VLR A of the registration success.
7.4.2 Inter-Office Call
Figure 7-5 shows the transfer of MAP messages in an inter-office call.
BSS MSC HLR MSC BSS
CM SERV REQCC
LOCREQ
routreq(TLDN)
locreq
ASSIGNMENT
ASSIGN COMP
ASSIGNMENT
ASSIGN COMP
ANC
a
b
c d
f
g
p
q
CONNECT
ACMo
n
m
CC l
PAGING RSP k
PAGING REQ j
IAI (TLDN) i
h
e
ROUTREQ
Figure 7-5 Inter-office call procedure
The following describes the MAP messages only.
LOCREQ: The originating MSC sends a called party location request to the HLR. ROUTREQ: The HLR sends a routing request to the serving MSC. routreq: The serving MSC returns a routing response containing the temporary
mobile directory number (TLDN). locreq: The HLR sends a location response to the originating MSC.
7.4.3 Handoff Forward
Figure 7-5 shows the transfer of MAP messages in handoff forward procedure.
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Anchor&ServingMSC Target MSC BSC
HANDMREQ
handmreq
FACREQ
facreq
Handoff Order
MS arrives on new channel
MSONCH
Handoff Complete
a
b
c
d
e
i
Call in progress
f
g
h
Figure 7-6 Handoff forward procedure
The following describes the MAP messages only.
HANDMREQ: The serving MSC determines whether handoff to an adjacent MSC is required using the internal algorithm. It sends a Handoff Measurement Request (HANDMREQ) to adjacent MSCs to request a signal quality measurement.
handmreq: The adjacent MSC measures signal quality and reports the result to the serving MSC.
FACREQ: The serving MSC determines that handoff to the adjacent MSC (the target MSC) is required. It sends a FACREQ message to the target MSC to initiate the handoff forward.
facreq: If a free traffic channel is available in the designated cell, the target MSC adds "1" to the segment counter of the BillingID. The new BillingID will be employed for later billing. Notification that the request was accepted is reported to the serving MSC to start the handoff forward.
MSONCH: The target MSC sets up a traffic channel and the connection of trunk circuit to the serving MSC. It then notifies the serving MSC the arrival of the MS on the new channel and the completion of related processes. The serving MSC then connects the call to the inter-MSC trunk circuit to complete the handoff.
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Chapter 8 Base Station Application Part
This chapter introduces the concepts related to base station application part (BSAP) , as well as its functions and position in the SS7.
8.1 Introduction to BSAP
BSAP is an application part based on A interface protocols. It fulfills the functions of A1 interface between MSC and BSC.
8.1.1 About the A Interface
The A interface, between an MSC and a BSC, consists of:
Signaling channel A1 interface: Transfers common channel signaling. User traffic channel A2 interface: Transfers voice, data, or unrestricted digital
information processed with pulse code modulation (PCM) . Duplex data channel A5 interface between the interworking function (IWF) and
selection/distribution unit (SDU): Transfers fax service and asynchronous data service traffic.
In the signaling system, the A interface generally refers to A1 interface. Established on the basis of SS7, the A interface is divided into three layers:
L1: Physical interface between neighboring nodes L2: Transmission layer. It consists of the MTP and SCCP, and ensures effective
transmission of data at application layer. L3: BSAP above the SCCP.
8.1.2 BSAP Functions
BSAP accomplishes the functions of the MSC to BSC interface. It consists of two parts:
BS management application part (BSMAP) Direct transfer application part (DTAP)
I. BSMAP
BSMAP supports all radio resource management and facility management procedures between the MSC and BSC.
BSMAP messages are not passed to a MS. They are used only to perform functions at the MSC or the BSC. Only one type of BSMAP messages (called complete L3 information) is used together with a DTAP message to establish a connection
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between a MS, BSC, and MSC. For detailed description of the complete L3 information, refer to 3GPP2 specifications.
II. DTAP
DTAP messages are used to transfer call processing and mobility management messages between the BSC and MSC. BSC does not use DTAP messages, but converts the messages into appropriate messages to be transmitted on air interface.
Figure 8-1 shows the A interface protocol model.
A InterfaceBS side MSC side
Physical Layer
TransportProtocol(s)
BSAP
DTAP BSMAP
BSAP
DTAP BSMAP
TransportProtocol(s)
BS Base station BSAP Base station application part
BSMAP Base station management application part DTAP Direct transfer application part
MSC Mobile switching center
Figure 8-1 Reference model of A interface protocol stack
8.2 BSAP Messages
This section introduces the format and encoding of BSAP messages. An example is given to explain BSAM messages.
8.2.1 Format of BSAP Messages
One or two bytes in the message head (call message discrimination flag) is used to distinguish between DTAP messages and BSMAP messages. The subsequent bytes contain the length indicator and complete L3 information.
Figure 8-2 shows the structure of BSMAP and DTAP messages.
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Message Discrimination
DLCI (Always set to 0)
Message Discrimination
DTAP Message Header BSMAP Message Header
octet 1 octet 1
octet 2
Length Indicator Length Indicatoroctet 3 octet 2
octets 3 to koctets 4 tok+1
BSAPMessageHeader
Layer 3MessageLength
Layer 3Message
APPLICATION
MESSAGE
APPLICATION
MESSAGE
Figure 8-2 BSAP message structure
BSMAP message
The message discrimination flag of a BSMAP message contains only the message discrimination parameter that is coded with one byte. If the message discrimination parameter is set to 0, the message is a BSMAP message.
The length indicator is of one byte, indicating the length of subsequent data.
DTAP message
The message discrimination flag of a DTAP message is of two bytes: message discrimination parameter and the data link connection identifier (DLCI) . If the message discrimination parameter is set to 1, the message is a DTAP message. The DLCI indicates the type and treatment of messages transmitted between the BSC and MSC. The DLCI is set to 0 for A1 interface.
The length indicator is of one byte, indicating the length of subsequent data.
8.2.2 Encoding of BSAP Messages
Each A interface message consists of a series of IES. In the following description, the nature of IEs are indicated as follows:
M: mandatory IEs O: optional IEs
Among optional IEs, R denotes required IEs and C denotes conditional IEs.
In the following description, it is conventional to adopt the following sequence to denote the bits and bytes. The bits in one byte are denoted with 0-7. Bit 0 is the least significant bit (LSB) and is transmitted first. Bit 7 is the most significant bit (MSB).
Bytes (or octets) are identified with numbers. Byte 1 is sent first, followed by byte 2, byte3, and so on. A variable-length IE contains one length indicator byte, indicating the length of subsequent IEs.
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The IE includes two types: fixed-length IE and variable-length IE. The bytes contained in a fixed-length IE are predefined, and the length varies with the IE. For variable-length IEs, a length indicator follows the IE indicator. If the IE indicator is omitted (for mandatory IEs it can be omitted), the length byte is the first byte of the variable-length IE.
Four IE types are defined:
IEs with 1/2 octets of content (Type 1) IEs with 0 octets of content (Type 2) IEs with fixed length and at least one octet of content (Type 3) IEs with variable length (Type 4)
I. Type 1
Type 1 IE is a fixed-length IE with one byte. Table 8-1 shows the structure of a type 1 IE.
Table 8-1 Type 1 IE structure
7 6 5 4 3 2 1 0 Octet
1 IEI CIE 1
Bits 0, 1, 2, and 3 (that is, ½ byte) are content of information element (CIE).
Bits 4, 5, and 6 are the information element identification (IEI), except that 010 is used for type 2 IE.
Bit 7 is coded 1.
Type 1 IE can be the optional or a mandatory IE in a BSMAP or DTAP message.
II. Type 2
Type 2 IE is a fixed-length IE. It is of one byte without the CIE. Table 8-2 shows the structure of a type 2 IE.
Table 8-2 Type 2 IE structure
7 6 5 4 3 2 1 0 Octet
1 0 1 0 IEI 1
Bits 0, 1, 2, and 3 are the IEI.
Bits 4, 5, and 6 are coded 010, indicating that the IE is of type 2.
Bit 7 is coded 1.
Type 2 IE cannot be the mandatory IE in a DTAP message.
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III. Type 3
Type 3 IE contains the content of fixed length, followed by the CIE. It can be used as the mandatory IE in a DTAP message.
Optional IE
When the type 3 IE is optional, the IE contains the IEI. Table 8-3 shows the IE structure.
Table 8-3 Type 3 IE structure of type 3 (example 1)
7 6 5 4 3 2 1 0 Octet
0 IEI 1
LI 2
CIE 3
... ...
CIE n
Bits 0–6 in the first byte are the IEI.
Bit 7 in the first byte is 0.
Bytes 2–n denote are the CIE.
Mandatory IE
When the type 3 IE is mandatory, the IEI is omitted. Table 8-4 shows the IE structure.
All bytes denote the CIE.
Type 3 mandatory IE is used in a DTAP message.
Table 8-4 Type 3 IE structure (example 2)
7 6 5 4 3 2 1 0 Octet
CIE 1
CIE 2
... ...
CIE n
IV. Type 4
Type 4 IE contains the variable-length CIE.
Optional IE
When type 4 IE is optional, the IEI is contained. Table 8-5 shows the IE structure.
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Table 8-5 Type 4 IE structure (example 1)
7 6 5 4 3 2 1 0 Octet
0 IEI 1
2
CIE 3
... ...
CIE n
Bits 0–6 in the first byte denotes the IEI.
Bit 7 in the first byte is coded 0.
The second byte is the length indicator, indicating the length of CIE (byte number).
Bytes 3–n denote the CIE.
Mandatory IE
When type 4 IE is mandatory, the IEI is omitted. Table 8-6 shows the IE structure.
Table 8-6 Type 4 IE structure o (example 2)
7 6 5 4 3 2 1 0 Octet
LI 1
CIE 2
... ...
CIE n
The first byte is the length indicator, indicating the length of CIE (byte number).
Bytes 2–n denote the CIE.
Type 4 mandatory IE is used in DTAP messages.
The A interface messages are constructed by the four types of IE described above according to a certain sequence. Generally, mandatory IEs are placed before optional IEs, and different IEs are aligned according to predefined sequence in different messages.
8.2.3 Example of BSAP Messages
The following gives examples of various types BSAP messages.
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I. Complete L3 Information
Complete layer 3 information is sent from the BSC to the MSC upon receipt of the first message from a MS. This message contains a CM Service Request, a Paging Response, or a Location Updating Request message.
A complete layer 3 message contains message category, cell identity, and layer 3 message, as described in Table 8-7.
Table 8-7 Complete layer 3 message
IE Direction Type
Message category BS -> MSC M
Cell identity BS -> MSC M
Layer 3 information BS -> MSC M
II. CM Service Request
CM service request is sent from the BSC to the MSC to request for connection-oriented services (such as voice calls). This message contains such information as radio channel information, subscriber ID, requested service type, MS location, called number, and authentication parameters.
Table 8-8 lists the IEs in the message
Table 8-8 CM service request message
IE Element direction Type
Protocol discriminator BS -> MSC M
Reserved (octet) BS -> MSC M
Message type BS -> MSC M
CM service type BS -> MSC M
Classmark information type 2 BS -> MSC M
Mobile identity (IMSI) BS -> MSC M
Called party BCD number BS -> MSC O
Mobile identity (ESN) BS -> MSC O
Slot cycle index BS -> MSC O
Authentication response parameter (AUTHR) BS -> MSC O
Authentication confirmation parameter (RandC) BS -> MSC O
Authentication parameter COUNT BS -> MSC O
Authentication challenge parameter (RAND) MSC -> BS O
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IE Element direction Type
Service option BS -> MSC O
Voice privacy request (unnecessary in phase 1) BS -> MSC O
Called party ASCII number BS -> MSC O
Authentication event (the RAND and RANDC are mismatched at the BTS side) BS -> MSC O
Authentication data BS -> MSC O
Figure 8-3 gives a CM service request traced on a SS7 link.
Figure 8-3 Example of CM service request
The following describes the traced message. The following IEs are aligned in indent format, and not all IEs are contained. From the top down, they are:
1) MTP message header 2) SCCP message header 3) BSAP message header 4) BSAP message content
For the definition of bits in each IE, refer to 3GPP2 specifications.
83
10------ Network indicator: national network (2)
--00---- Spare1: (0)
----0011 Service indicator: SCCP (3)
00 00 00 Destination point code: (0)
00 00 00 Originating point code: (0)
00
0000---- Signaling link code: (0)
----0000 Spare2: (0)
01 SCCP connection request
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00 00 00 Source local references: (0)
02
0000---- Spare: (0)
----0010 Protocol class: Class 2 (2)
02 Offset 1 : 2
07 Offset 2 : 7
05 Called party address
43
0------- National/International Indicator: no (0)
-1------ Routing Indicator: yes (1)
--0000-- Global Title Indicator: no Global Title included (0)
------1- Subsystem Number Indicator: yes (1)
-------1 Point Code Indicator: yes (1)
00 00 00 Signaling Point Code: (0)
00 Subsystem Number: Not known (0)
0F Data
41 Data length
00 BSMAP
3F BSMAP length
57 Complete L3 Information
05 Cell Identifier
03 Cell Identifier length
02 Cell Identifier discriminator: CI (2)
00 00 Cell: (0)
17 Layer 3 message
37 Layer 3 message length
03
0000---- Reserved4: 0
----0011 Protocol discriminator: call processing call related SS (3)
00 Reserved8: (0)
24 CM service request
91
1001---- CM service type
----0001 Mobile originating call establishment (1)
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0C Classmark information type 2 (length)
00
000----- mobile p rev: (0)
---0---- reserved1a: (0)
----0--- see list of entries: (0)
-----000 RF power capability: class 1 vehicle and portable (0)
00 Reserved8a: 0
40
0------- nar a cap: (0)
-1------ is 95: (1)
--0----- slotted: (0)
---00--- reserved2: (0)
-----0-- dtx: (0)
------0- mobile term1: (0)
-------0 reserved1b: (0)
00 Reserved8b: (0)
00
000000-- reserved6: (0)
------0- mobile term2: (0)
-------0 psi: (0)
00 SCM length: (0)
00 Station class mark: (0)
01 Count of band class entries: (1)
03 Band class entry length: (3)
00
000----- reserved3a: (0)
---00000 band class n: (0)
00
000----- reserved3b: 0
---00000 band class n air interfaces supported: 0
00 Band class n MS protocol level: 0
01 Mobile identity IMSI
0E
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00001--- identity digit: 0
-----110 type of identity: IMSI (6)
5E Called party BCD number
02 Called party BCD number length
80
1------- Ext:1
-000---- Type of number: unknown (0)
----0000 Numbering plan identification: unknown (0)
F0 Digit: 0F
0D Mobile identity ESN
05 Mobile identity ESN length
05
0000---- identity digit1: 0
----0--- odd even indicator: 0
-----101 type of identity: ESN (5)
00 00 00 00 ESN value: 0
42 Authentication response parameter AUTHR
04 Authentication response parameter AUTHR length
01
0000---- reserved4: 0
----0001 auth signature type: AUTHR (1)
00
000000-- reserved6: 00
------00
00 00 Authentication signature value: 0
28 Authentication confirmation parameter RANDC
00 00
40 Authentication parameter count
00
00------ reserved2:0
--000000 count: 00
41 Authentication challenge parameter
05 Authentication challenge parameter length
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01
0000---- reserved4:0
----0001 random number type: RAND (1)
00 00 00 00 Random value: (0)
03 Service option
80 00 Speech 13k (32768)
1D Radio environment and resources
00
0------- reserved1: 0)
-0------ include priority: 0
--00---- forward: not reported (0)
----00-- reverse: not reported (0)
------0- alloc: resources are not allocated (0)
-------0 avail: resources are not available (0)
4A T
01 Authentication event
01 Parameters not received (1)
00 EOP:00
III. Paging Request
Paging request is sent from the MSC to the BSC. It contains sufficient information to locate the cell serving the MS.
This message contains the location area identity (LAI) , Mobile identity, service type, slot cycle index, and so on. Table 8-9 describes the IEs in a paging request.
Table 8-9 Paging request message
IE Element direction Type
Message type MSC -> BS M
Mobile identity (IMSI/ESN) MSC -> BS M
Tag MSC -> BS O
Cell identifier list MSC -> BS O
Slot cycle index MSC -> BS O
Service option MSC -> BS O
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IV. Paging Response
Paging response is sent from the BSC to the MSC when the BSC receives a page response from the MS.
The message contains the mobile identity, radio channel information, slot cycle index, service type, authentication parameter, and so on. Table 8-10 describes the IEs in a paging response.
Table 8-10 Paging request message
IE Element direction Type
Protocol discriminator BS -> MSC M
Reserved (octet) BS -> MSC M
Message type BS -> MSC M
Classmark information type 2 BS -> MSC M
Mobile identity (IMSI) BS -> MSC M
Tag BS -> MSC O
Mobile identity (ESN) BS -> MSC O
Slot cycle index BS -> MSC O
Authentication response parameter (AUTHR) BS -> MSC O
Authentication confirmation parameter (RANDC) BS -> MSC O
Authentication parameter COUNT BS -> MSC O
Authentication challenge parameter (RAND) MSC -> BS O
Service option BS -> MSC O
Voice privacy request (unnecessary in phase 1) BS -> MSC O
Authentication event (when the RAND and RANDC are mismatched at the BTS side) BS -> MSC O
V. Connect
This Connect message is sent by the BSC to the MSC to indicate that the called mobile subscriber has accepted the call.
Table 8-11 lists the IEs of the message.
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Table 8-11 Connect message
IE Element direction Type
Protocol discriminator BS <-> MSC M
Reserved (octet) BS <-> MSC M
Message type BS <-> MSC M
VI. Assignment Request
Assignment request is sent from the MSC to the BSC to request the later to assign radio resources. The message may include the terrestrial circuit to be used if one is needed for the call.
The message contains the circuit identification code, possible calling number, service option, emergency call identifier, and so on. Table 8-12 lists the IEs of the message.
Table 8-12 Assignment request message
IE Element direction Type
Message type MSC -> BS M
Channel type MSC -> BS M
Circuit identification code MSC -> BS O
Encryption information (unnecessary in phase 1) MSC -> BS O
Service option MSC -> BS O
Signal MSC->BS O
Calling party ASCII number MSC->BS O
VII. Assignment Complete
This BSMAP message is sent from the BSC to the MSC, indicating that the requested assignment is completed.
Table 8-13 lists the contents in the message.
Table 8-13 Assignment complete messages
IE Element direction Type
Message type BS -> MSC M
Channel number BS -> MSC M
Encryption information (unnecessary in phase 1) BS -> MSC O
Service option BS -> MSC O
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8.3 BSAP Procedures
BSAP protocol specifies the message format and procedures to support the wireless service functions between the BSC and MSC. Major A interface procedures include:
Mobile origination Mobile termination Call clearing Circuit management
This section introduces these procedures. For handoff procedure, refer to Technical Manual – System Function.
In the protocol stack, BSAP is above SCCP layer. BSAP uses two types of service provided by SCCP: connection-oriented service and connectionless service. Therefore, BSAP messages are carried by SCCP messages.
Table 8-14 lists the SCCP messages used by BSAP.
Table 8-14 SCCP messages used by BSAP
Service SCCP frame User data field (BSMAP/DTAP)
SCCP Connection Request (CR) Optional
SCCP Connection Confirm (CC) Optional
SCCP Connection Refused (CREF) Optional
SCCP Released (RLSD) Optional
SCCP Release Complete (RLC) Not applicable
Connection Oriented (CO) Protocol Class 2
SCCP Data Transfer 1 (DT1) Mandatory
Connectionless (CL) Protocol Class 0 SCCP Unit Data (UDT) Mandatory
For detailed description of the SCCP messages used by BSAP, refer to 3GPP2 specifications.
8.3.1 Location Update
A MS informs the MSC of current location (or parameter) change through the location update procedure as shown in Figure 8-4.
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Figure 8-4 Location update procedure
The procedure is as follows:
1) On receiving the update message from a MS, the BSC constructs a Location Updating Request, places it in the complete layer 3 information, and sends it to the MSC. The BSC then starts timer T3210.
2) The MSC sends a Location Updating Accept message to the BSC to indicate that the request has been processed. Upon receipt of the message, the BSC stops timer T3210.
8.3.2 Mobile Origination
When a MS sends a mobile origination service request, the BSC initiates the mobile origination procedure shown in Figure 8-5.
Figure 8-5 Mobile origination procedure
The procedure is as follows:
1) The BSC constructs a CM Service Request, places it in the Complete Layer 3 Information, and sends the message to the MSC. At the same time it starts timer
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T303. For circuit switched calls, the BSC may request the MSC to allocate a preferred terrestrial circuit.
2) If global challenge is used, the MSC will continue the call setup process while waiting for an authentication confirmation (If an authentication failure indication is received at the MSC, it may clear the call).
The MSC sends an Assignment Request to the BSC to request assignment of radio resources. This message includes information of terrestrial circuit, if a terrestrial circuit is to be used between the MSC and BSC. The MSC then starts timer T10.
If the BSC requests a preferred terrestrial circuit in the CM Service Request and the MSC supports that terrestrial circuit, the MSC will use the same terrestrial circuit. Upon receipt of the Assignment Request message from the MSC, the BSC stops timer T303.
3) After the radio channel and terrestrial circuit are set up, the BSC sends an Assignment Complete message to the MSC. This indicates that the calling party is in conversational state and hears the ringback tone or other prompt tones. The MSC stops timer T10 upon receipt of the Assignment Complete message.
8.3.3 Mobile Termination
Figure 8-6 shows the mobile termination procedure.
Figure 8-6 Mobile termination procedure
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The procedure is as follows:
1) The MSC finds that an incoming call terminates to a MS in its serving area and sends a Paging Request to the BSC to initiate a mobile terminated call setup scenario. The MSC then starts timer T3113.
2) The BSC constructs a Paging Response message, places it in the Complete L3 Information message, and sends the message to the MSC. After that, it starts timer T303.
The BSC may request the MSC to allocate a preferred terrestrial circuit in this message.
3) The MSC stops timer T3113 upon receipt of the Paging Response from the BSC.
The MSC sends an Assignment Request to the BSC to request assignment of radio resources. This message also includes the information of a terrestrial circuit. The MSC then starts timer T10.
If the BSC requested a preferred terrestrial circuit in the Service Request, the MSC will use the same terrestrial circuit. The MSC may also assign a different terrestrial circuit.
Upon receipt of the Assignment Request from the MSC, the BSC stops timer T303.
4) After the radio traffic channel and terrestrial circuit are established, the BSC sends an Assignment Complete message to the MSC. The MSC stops timer T10 upon receipt of the message, and starts timer T301 to wait for the response of the MS.
5) The BSC sends a Connect message to the MSC to indicate that the call has been answered by the MS. Now the call is considered established and is in conversational state. The MSC stops timer T301 upon receipt of the Connect message from the BSC.
8.3.4 Call Clearing
When a mobile subscriber stops conversation and hooks on the phone, the BSC initiates a clearing procedure (the procedure may also result from other events).
Figure 8-7 shows the call clearing procedure initiated by BSC.
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Figure 8-7 Call clearing initiated by BSC
The procedure is as follows:
1) In case of a radio channel failure (for example, due to MS power failure or hook-on) between the MS and the BSC, the BSC sends a Clear Request to the MSC. The BSC then starts timer T300 and waits for the Clear Command from the MSC.
2) The MSC starts timer T315 and sends a Clear Command to instruct the BSC to release associated dedicated resources (such as the terrestrial circuit). The BSC stops timer T300.
3) The BSC returns a Clear Complete message. The MSC stops timer T315 upon receipt of the message and releases the resources at MSC side.
The call clearing procedure initiated by the MSC contains only Clear Command and Clear Complete messages (without Clear Request message). The flowchart is omitted here.
8.3.5 Circuit Block/Unblock
The A interface circuit between the BSC and MSC are maintained through circuit management messages. Because circuit assignment is controlled by the MSC, the BSC must inform the MSC of the circuit status at BSC side.
I. Circuit Block
The MSC must be informed if any circuit at BSC side is out of service (for example, during circuit reset initiated by the MSC). In this case, the BSC sends a Block message to the MSC to inform the later that the circuit is not available any longer. This procedure can be initiated by BSC only.
Figure 8-8 shows the circuit block procedure initiated by the BSC.
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Figure 8-8 Circuit block procedure
The procedure is as follows:
1) The BSC sends a Block message to the MSC, containing the information of the circuit to be blocked. Then it starts timer T1.
2) The MSC returns a Block Acknowledge message, indicating that the involved circuit is blocked. On receipt of the message, BSC stops timer T1.
II. Circuit Unblock
When a blocked circuit becomes available again, the BSC sends an Unblock message to the MSC to inform the status change. This procedure can be initiated by BSC only.
Figure 8-9 shows the circuit unblock procedure initiated by the BSC.
Figure 8-9 Circuit unblock procedure
The procedure is as follows:
1) The BSC sends an Unblock message to the MSC, requesting the later to unblock the circuit. Then it starts timer T1.
2) The MSC returns an Unblock Acknowledge message to the BSC, indicating that the circuit is unblocked. On receipt of the message, BSC stops timer T1.
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8.3.6 Circuit Reset
The MSC initiates circuit reset when some circuits are abnormal, such as:
Abnormal release of SCCP connections Inaccessible signaling point becoming available Intermittent trunk line failures Manual maintenance operations
Circuit reset can be initiated either by BSC or by MSC.
I. BSC-Initiated Circuit Reset
Figure 8-10 shows the circuit reset procedure initiated by BSC.
Figure 8-10 BSC-initiated circuit reset
The procedure is as follows:
1) Once the BSC detects that one or more circuits become idle, it sends a Reset Circuit message to the MSC. Then it starts timer T12.
2) Upon receipt of the Reset Circuit message, the MSC returns a Reset Circuit Acknowledge message to indicate that the circuits are reset. On receipt of the message, BSC stops timer T12.
II. MSC-Initiated Circuit Reset
Figure 8-11 shows the circuit reset procedure initiated by MSC.
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Figure 8-11 MSC-initiated circuit reset
The procedure is as follows:
1) Once the MSC detects that one or more circuits become idle, it sends a Reset Circuit message to the BSC. Then it starts timer T12.
2) Upon receipt of the Reset Circuit message, the BSC returns a Reset Circuit Acknowledge message to indicate that the circuits are reset. On receipt of the message, MSC stops timer T12.
III. MSC-Initiated Circuit Reset Failure
Figure 8-12 shows the procedure when the BSC fails to reset the circuit as requested by MSC.
Figure 8-12 MSC-initiated circuit reset failure
The procedure is as follows:
1) Once the MSC detects that one or more circuits become idle, it sends a Reset Circuit message to the BSC. Then it starts timer T12.
2) If the BSC fails to set the circuits to idle, it sends a Block message to the MSC and starts timer T1. The MSC stops Timer T12 on receipt of the Block message.
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3) The MSC returns a Block Acknowledge message, indicating the involved circuits are blocked. The BSC stops timer T1 on receipt of the message.