Common Channel Signalling

DX200 MSC/HLR

Introduction to Circuit Switch Core Network

Common Channel SignallingTraining Document

M12

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51Objectives

2Introduction63Signalling concepts73.1Signalling definitions and uses73.2Signalling types73.2.1Circuit-related signalling83.2.2Non-circuit-related signalling93.3Chapter review104Common Channel Signalling System Number 7114.1OSI reference model vs. CCS7 protocols114.1.1User part (common part)134.1.2Application part (GSM specific part)134.2CCS7 protocols in various network elements134.3Multimedia Gateway144.4Chapter review155Functional parts of CCS7165.1User parts165.1.1Telephone User Part and National User Part165.1.1.1Example of a TUP connection175.1.2ISDN User Part195.1.2.1Example of the ISUP connection205.2Application parts225.2.1Base Station Subsystem Application Part235.2.2Mobile Application Part245.2.2.1MAP interfaces245.2.2.2SCCP subsystems of MAP in the DX 200 architecture255.2.3Intelligent Network Application Part265.2.4Radio Access Network Application Part275.2.5Signalling conversion in the MGW325.2.6Signalling protocol stack in MGW325.2.7GSM to UMTS handover335.2.8Summary355.3Chapter review386Signalling routing methods396.1Message Transfer Part406.1.1Factors that effect the routing of a signalling message406.1.2Routing of a signalling message by the MTP426.1.2.1MTP level 2426.1.2.2MTP Level 3446.1.2.3Signalling message handling446.1.3Example of MTP routing476.1.3.1Signalling network management496.1.4Summary of MTP506.2Signalling Connection Control Part506.2.1SCCP services516.2.2SCCP routing function526.2.2.1Global Title546.2.2.2IMSI and VMSC/SGSN address sent to the SMSC in MT-SMS556.2.2.3GT Analysis556.2.3Example of SCCP routing location update566.2.3.1IMSI analysis566.2.3.2IMSI analysis with routing on GT566.2.3.3IMSI analysis with routing on label576.3Chapter review617CCS7 management in the DX 200 MSC/VLR627.1Creating an MTP627.2Creating an ISUP or TUP637.3Creating an SCCP637.4Creating an SCCP subsystem637.5Creating a GT analysis647.6MML interfaces658Architecture661AppendixTransaction Capabilities Application Part672AppendixIP trunking692.1Protocol stack692.2Call setup procedures over IP trunking702.2.1TGSU702.2.2IPET713Check your understanding724Glossary774.1Definitions774.2Abbreviations77

2 Objectives

On completion of this module, you should be able to:

List two examples of circuit-related signalling and four examples of non circuit-related signalling

List necessary CCS7 protocols needed in the MSC, HLR, BSC, MGW, and the PSTN exchange

State the reason why some protocols have the name User Parts and some of them have the name Application Parts

Give an example case in which SCCP routing is needed and MTP routing cannot be used to route the signalling message to the destination

List at least one function of MTP-level 3, SCCP, MAP, BSSAP, RANAP, ISUP, and INAP

List the main purpose of IMSI analysis

Give two different IMSI analysis types and explain why and when these two types are used

Write a list of necessary definitions that have to be done in our MSC when a new MSC is added to the existing GSM/UMTS network and the new MSC has a direct signalling connection to the MSC

Write a list of necessary CCS7 definitions that have to be created in core networks when an SMSC is integrated to the network

Describe the protocol stack used in case of IP trunking (optional)

List the procedures of a call setup over IP trunking (optional)

3 Introduction

This training document explains how Common Channel Signalling Number 7 (CCS7) is used in the General forward Set-up information Message (GSM) network, especially in the NSS area. It also includes the necessary CCS7 protocols used in the Mobile Services Switching Centre (MSC), Home Location Register (HLR), Base Station Controller (BSC), Multimedia Gateway (MGW), and Public Switched Telephone Network (PSTN) exchange, along with the function of each protocol.

In addition, it explains ways that the signalling message is routed from the origination to the destination over the GSM networksignalling routing by Message Transfer Part (MTP)-level 3 and Signalling Connection Control Part (SCCP) level.

4 Signalling concepts

What is signalling? What are the functions of signalling? How many categories can signalling be divided into? And what is the recommended method for signalling transfer from, to, or within a GSM network?

4.1 Signalling definitions and uses

For PSTN, signalling is any transfer of data that enables speech and data connections between users.

For GSM, signalling is any transfer of data that enables speech and data connection between users and supports mobility management and GSM services handling.PSTN needs signalling for call establishing, call release and call maintaining. In GSM, signalling can also be independent from speech. The different functions of signalling are call control, control of services, and charging control. Mobile networks have some special functions, such as location update, handover, subscriber administration, and short message service.

GSM signalling can be divided into three main categories:

Speech and data connection (set up, supervise and release a call)

Mobility management (location update, handover)

Subscriber administration (including all basic and supplementary GSM services)

4.2 Signalling types

Signalling messages can be categorised into two main types, as shown in Figure 1:

Circuit-related signalling

Non-circuit-related signalling

Figure 1.Signalling message structure

4.2.1 Circuit-related signalling

Circuit-related signalling occurs when the signalling message contains information about the circuit to be signalled and the origin and destination of the message.

An example of circuit-related signalling is call-setup signalling between two exchanges, as shown in Figure 2. When the originating exchange sends a call-setup signalling message to the terminating exchange, this message contains the Mobile Station Roaming Number (MSRN) and the Circuit Identification Code (CIC) to identify the circuit that is already reserved for use by this call.

The CIC identifies the Pulse Code Modulation (PCM) and the time slot of the speech or data connection for which the call-setup message is sent.

Figure 2.Example of circuit-related and non-circuit-related signalling

4.2.2 Non-circuit-related signalling

Non-circuit-related signalling occurs when the signalling message does not contain information about the circuit, but instead contains other information depending on the use of the signalling.

Other information can be:

Location Area Identity (LAI) data and Temporary Mobile Station Identity (TMSI)/International Mobile Subscriber Identity (IMSI) in case of location update

Short messages in case of sending or receiving a short message

Forwarding numbers in case of activation of the call forward supplementary service

B-Mobile Subscriber International ISDN Number (MSISDN) in the Send Routing Information (HLR Inquiry)

4.3 Chapter review

1. Which of the following is the correct definition of signalling?

a.The facilitation of speech connections only.

b.All transfers of data that enable mobility management.

c.Any transfer of data that enables speech and data connections between users.

d.The facilitation of subscriber administration.

2. What are the two categories of signalling?

a.Circuit-related signalling and non-circuit-related signalling

b.mobile unit signalling and circuit related signalling

c.circuit related signalling and location update signalling

d.mobile unit signalling and non-circuit related signalling

3. True or FalseThe following graphic represents non-circuit related signalling:

True

False

5 Common Channel Signalling System Number 7

Common Channel Signalling (CCS) is a signalling method that uses addressed messages to convey signalling information over a single channel. CCS supports both circuit-related and non-circuit-related signalling.

Common Channel Signalling System Number 7 can be abbreviated CCS7, CCS#7, SS7 and SS No. 7. CCS7 is the abbreviation used in this training document.

CCS7 is an internationally standardised CCS system that is optimised for operation in digital telecommunication networks such as GSM. One of the main advantages of CCS7 is that the signalling does not have to go along the same path as the speech. Therefore, CCS7 can support both circuit-related and non-circuit-related signalling.

5.1 OSI reference model vs. CCS7 protocols

Originally, CCS7 consisted of two parts. The first part, the MTP, was responsible for transferring messages within a signalling network. The second part, the User Part (UP), was the user of the MTP. The UP was only used for circuit-related signalling.

However, CCS7 was created for GSM-type networks. In GSM, CCS7 is also divided into two parts, but they are called the Functional Part (FP) and the MTP. The user is the functional part that sends, receives, and acts depending on received signalling information. A comparison of the Open System Interconnection (OSI) reference model and the CCS7 protocols is shown in Figure 3.

The functional part of CCS7 is divided into two parts:

User part (common part)

Application part (GSM specific part)

Figure 3.The OSI reference model vs. CCS7 protocols

Example 1:

When a subscriber makes a call to the PSTN, the mobile sends the dialled number to the MSC via the BTS and BSC. Since only the BSC and MSC use CCS7, a functional part in the BSC will send the dialled number to the MSC, which will receive this dialled number by using another functional part in the MSC. In this example, the dialled number acts as signalling information and there are two functional parts in use (one is in the BSC and another is in the MSC).

Example 2:

When a subscriber turns on a mobile, there is a location update request message sent from the mobile to the VLR (Visitor Location Register) that informs the VLR that the mobile now wants to become active in the GSM network. In this case, there are at least two functional parts in use (one is in the BSC and another in the VLR). It is also possible that the VLR will ask the HLR for subscriber data. In this case one more functional part has to be used in the HLR to be able to receive the signalling information from the VLR.

5.1.1 User part (common part)

The user part is commonly used in any type of digital telecommunication networks such as PSTN, ISDN and GSM. All signalling messages in this common part have the CIC defined. The common part is used for circuit-related signalling. User parts need the services from the MTP level only. Three different user parts are used in GSM:

Telephone User Part (TUP)

National User Part (NUP)

ISDN User Part (ISUP)

5.1.2 Application part (GSM specific part)

The application part is used in the GSM network only. In GSM networks, signalling is not as simple as in the PSTN (only circuit-related signalling). GSM requires circuit-related signalling as well as a large amount of non-call-related signalling such as location update, SMS, and handover.

Three application parts are used in GSM:

Base Station System Application Part (BSSAP)

Mobile Application Part (MAP)

Intelligent Network Application Part (INAP)

All application parts (BSSAP, MAP, INAP) need the services provided by the Signalling Connection and Control Part, SCCP. The SCCP will be explained later in this chapter.

5.2 CCS7 protocols in various network elements

Referring to Example 1, when a subscriber makes a call, a functional part in the BSC sends the dialled number to a functional part in the MSC. The functional part in the BSC is the BSSAP and the functional part in the MSC is also the BSSAP. After the MSC analyses the dialled number, the MSC knows that the call has to be routed to the PSTN because it is a PSTN number. Thus, another functional part in the MSC will start sending call setup message (the dialled number together with the CIC are included) to the PSTN. The ISUP is the functional part in the MSC and in the PSTN in this case.

Referring to Example 2, the location update message is sent from the mobile to the VLR via the BSC. The BSSAP is a functional part in both the BSC and VLR. If the VLR needs to ask subscriber data from the HLR, another functional part will be used in the VLR and HLR. This functional part is used only in the NSS areait is a GSM specific functional part. As a result, a MAP is used in both the VLR and the HLR. The CCS7 protocols are shown in Figure 4.

Figure 4.CCS7 protocols in various network elements

5.3 Multimedia Gateway

Multimedia Gateway (MGW) is a digital switching product for 3G networks. The main function of the MGW is to enable interworking between the MSC and the UMTS Radio Access Network (UTRAN).

MGW contains extensive support for signal processing. Signal processing is employed in the speech coding (transcoding) and circuit switched data adaptation functions.

MGW allows transmission of circuit-switched speech and data between the MSC and the UTRAN. CS speech requires different connection management and signal processing than CS data. At the user plane, MGW converts the traffic channel information from the RNC to PCM traffic channel format and vice versa as follows:

Transcoding for speech channels

Conversion of data format for data channels

5.4 Chapter review

1. Which of the following are the functional parts of CCS7?

a.User part and application part

b.user part and GSM common part

c.SCCP part and application part

d.GSM common part and network layer part

4. Which of the following is the correct definition of Multimedia Gateway (MGW)?

a.A common channel-signalling tool that uses the OSI reference model.

b.A signalling method that uses addressed messages to convey signalling information over a single channel

c.A 3G-network feature that enables the facilitation of subscriber administration

d.A digital switching product for 3G networks that enables interworking between the MSC and the UTRAN.

5. What are the three user parts used in GSM?

__________________________________________________

__________________________________________________

__________________________________________________

6 Functional parts of CCS7

This chapter reviews the functional parts of CCS7, including a detailed review of both the user parts and the application parts.

6.1 User parts

The user parts in CCS7, including TUP, NUP, ISUP, and MTP, are shown in Figure 5.

Figure 5.User parts

6.1.1 Telephone User Part and National User Part

The Telephone User Part (TUP) is the CCS7 protocol that provides signalling functions that are required in international telephone call control signalling. The TUP handles the normal call setup and release of voice and data calls between the originating and destination exchanges, and handles abnormal situations by using its release procedures.

The ITU-T allowed that the TUP could be slightly modified to allow for national use within one country. These national TUP versions are known as National User Parts (NUPs). The variations are minor and very similar to the TUP. Therefore, we will only take a look at the TUP message structure, shown in Figure 6.

Figure 6.The TUP message structure

TUP information is contained in the SIFs (Signalling Information Fields) of the MSUs (Message Signal Units) of the MTP (Message Transfer Part) shown in Figure 6. The TUP message includes the identification of the destination and originating exchanges (Destination Point Code, DPC, and Originating Point Code, OPC), and the CIC.

The number of bits in the DPC and OPC depends on the used country. If the country is China or any country with reference to ANSI (American National Standards Institute), there are 24 bits for these two fields. Otherwise, there are 14 bits.

The CIC indicates the PCM system and the used time slot within the range of circuits connecting the originating and destination exchanges.

The heading codes H0/H1 uniquely identify the message as shown in Table 1.

Table 1.H1 and H0 coding

H1/H0Message name

06hANU, Answer

21hIAI, Initial address

14h ACM, Address complete

17hRLG, Release guard

The user data field contains two main types: signals and indicators. For instance, the Initial Address with additional Information (IAI) has the dialled digits as signals and the indicator is used to identify the type of call (international or national number, direct or redirect call).

6.1.1.1 Example of a TUP connection

Assume that the MSC/VLR, PSTN-T and PSTN-B have the TUP as CCS7 interfaces. The call setup of the TUP is shown in Figure 7.

Figure 7.Example of a TUP connection

Example:

Call setup (mobile subscriber calls to a PSTN):

2. Subscriber A dials subscriber B number and sends it to the MSC/VLR.

6. Exchange A sends the initial address message (IAI) to the transferring PSTN exchange (PSTN exchange-A), which relays the message to PSTN exchange-B.

7. When PSTN exchange-B realises that it has received all the necessary digits for routing the call to Subscriber B, it sends an Address Complete message (ACM) to the PSTN exchange-A, which relays the message to the MSC/VLR. PSTN exchange-B sends the ringing tone to the mobile subscriber and sends the ringing signal to the PSTN subscriber. The phone is ringing.

8. When the PSTN subscriber answers, the PSTN exchange B sends an Answer Signal message (ANU) to the PSTN exchange-A, which relays the message to the MSC/VLR. The ringing tone is switched off from the line and the speech connection is set up. The call is now set up.

6.1.2 ISDN User Part

ISDN User Part (ISUP) is the user part specifically defined for the ISDN. It provides the facilities for handling the ISDN services and supplementary services for voice and data applications.

ISUP is well suited for applications in dedicated telephone and circuit-switched data networks, both national and international. Within the GSM network, the ISUP is used between the MSC and another MSC, as well as between the MSC and the PSTN.

ISUP is on level 4 in the CCS7 signalling system architecture. The MTP is used for carrying the ISUP information situated in the SIFs (Signalling Information Fields) of the MSUs (Message Signal Units) in the MTP. The ISUP message structure is shown in Figure 8.

Figure 8.The ISUP message structure

The ISUP message includes the identification of the destination and originating exchanges (DPC, and OPC), a load sharing parameter (Signalling Link Selection, SLS) and the CIC.

The Signalling Link Selection (SLS), which is a field of the routing label, is typically used by the message routing function (MTP-level 3) to perform load sharing among different signalling links/link sets. It can be shown by ZNEO command.

The user data field contains information for identifying a type of ISUP messages such as Initial Address Message (IAM), ACM, Answer Message (ANM) and other parameters, such as type of number. For example, if the call is a mobile originating call, the PSTN parameter is the dialled number. If, on the other hand, the call is a mobile terminated call to a different MSC, the parameter is the roaming number.

Figure 9 shows an SLS table and Figure 10 shows the signalling link set definition.

NEO:NA0,30:;

DX 200 MSC 2000-12-12 06:34:00

SIGNALLING POINT LOAD SHARING

SP EXT OUT

NET SP CODE H/D NAME SLS NET SP CODE H/D LINK PCM-TSL SLS

=== ================== ===== ==== === ================== ===== ======= ===

NA0 0030/00048 BSC1 0 NA0 0030/00048 2 72-16 0

1 NA0 0030/00048 9 75-16 1

2 NA0 0030/00048 3 73-16 2

3 NA0 0030/00048 8 74-16 3

4 NA0 0030/00048 9 75-16 4

5 NA0 0030/00048 2 72-16 5

6 NA0 0030/00048 8 74-16 6

7 NA0 0030/00048 3 73-16 7

8 NA0 0030/00048 2 72-16 8

9 NA0 0030/00048 8 74-16 9

10 NA0 0030/00048 2 72-16 10

11 NA0 0030/00048 3 73-16 11

12 NA0 0030/00048 8 74-16 12

13 NA0 0030/00048 9 75-16 13

14 NA0 0030/00048 9 75-16 14

15 NA0 0030/00048 3 73-16 15

Figure 9.SLS tableNSI:NA0,30;

DX 200 MSC 2000-12-12 06:34:20

INTERROGATING SIGNALLING LINK SET DATA

NET SP CODE H/D LINK SET LS STATE LINK SLC

--- ------------------ ---------- -------- --------

NA0 0030/00048 17 BSC1 AV 2 0

3 1

8 2

9 3

Figure 10.Signalling Link set definition

6.1.2.1 Example of the ISUP connection

An example of the ISUP connection is shown in Figure 11.

Figure 11.Example of an ISUP connection

Example:

Call setup (A calls to a PSTN subscriber B):

9. Subscriber A dials the complete number of subscriber B and starts the call establishment by pressing the send button of the mobile. With the SETUP message, the B-number is sent to the MSC/VLR.

10. The MSC/VLR analyses the dialled digits and seizes a traffic timeslot toward the PSTN. The initial address message (IAM) is sent. The IAM contains the originating point code (A) and the destination point code (T) of the seized traffic timeslot and the CIC.

11. PSTN-A receives the IAM. After an analysis of the contained digits a new traffic timeslot toward the PSTN is seized, and a new IAM is sent to PSTN-B.

12. When PSTN-B receives the message, it will recognise that the call establishment request is for a subscriber connected to it.A SETUP is sent to subscriber B and an ACM message is sent in backward direction toward the MSC/VLR.

13. In the MSC/VLR, an ALERT message is sent to subscriber A, who hears the ringing tone.

14. Subscriber B hangs up, and a CONN message is sent to exchange B. There it is mapped onto an ANM message, which is sent through the network.

15. The receipt of ANM in exchange A starts the charging, and a CONN message is sent to subscriber A.

16. The call has been established.

6.2 Application parts

The information in BSSAP, MAP, and INAP messages is handled by the SCCP, as shown in Figure 12. The application parts can be called SCCP subsystems. All application parts are specific to GSM networks. TCAP is explained in the Appendix of this training document.

Figure 12.Application parts

6.2.1 Base Station Subsystem Application Part

The Base Station Subsystem Application Part (BSSAP) is responsible for transferring the GSM-specific messages between the MSC and BSC, and between the MSC and a mobile station. The BSSAP can be divided into two subpartsDTAP and BSSMAP.

DTAP (Direct Transfer Application Part) handles the transfer of the signalling information between the MSC and the MS such as call setup, alerting, and location update request messages. They are all sent transparently via the BSC to the MS.

BSSMAP (Base Station System Management Application Part) handles the signalling functions between the MSC and the BSS and performs other procedures such as handover control and paging. The messages used in these procedures are sent directly from the MSC to the BSC.

From the examples of DTAP and BSSMAP, highlighted in Figure 13, it becomes clear that the BSSAP supports sending both circuit and non-circuit-related signalling by using SCCP-level services.

Figure 13.Base Station Subsystem Application Part

6.2.2 Mobile Application Part

The Mobile Application Part (MAP) is a GSM-specific protocol for only non-circuit-related applications inside the NSS area. AP procedures can be location registration, handling of supplementary services, HLR inquiry, Inter-MSC handover, authentication, International Mobile Equipment Identity (IMEI) checking, and support of short message services.

The operations defined within the MAP protocol are sent through the signalling network by using TCAP, SCCP, and MTP services. For SCCP, MAP uses only a connectionless service.

6.2.2.1 MAP interfaces

The MAP interface enables the MSCs, HLRs, VLRs, and Equipment Identity Registers (EIRs), to communicate smoothly and efficiently with each other. The GSM Technical Specification defines six different interfaces between network elements (Figure 14).

Figure 14.MAP

Interface B is situated between the MSC and the VLR. Messages concerning, for example, location updates or call control (check subscriber A info), go through this interface. In the DX 200 architecture, the MSC and the VLR are integrated, and thus, the interface B is an internal interface.

Interface C is situated between the MSC and the HLR. Messages related to, for example, HLR-inquiries, are sent via this interface.

Interface D is situated between the VLR and the HLR. Messages concerning, for example, location updates, are sent via this interface.

Interface E is situated between two MSCs. It is used, for example, for inter-MSC handover-related signalling.

Interface F is situated between the MSC and the EIR. Messages for IMEI check are sent via this interface.

Interface G is situated between two VLRs. It is used, for example, for location updates.

6.2.2.2 SCCP subsystems of MAP in the DX 200 architecture

In the DX 200 GSM implementation there are the following MAP SCCP subsystems, as illustrated in Figure 15:

MAP-M:MSC MAP handles messages going through the interfaces B, C, E,and F.

MAP-V:VLR MAP handles messages going through the interfaces B, D, and G.

MAP-H:HLR MAP handles messages going through the interfaces C and D.

MAP-E:EIR MAP handles messages going through interface F.

Figure 15.SCCP subsystems of MAP

6.2.3 Intelligent Network Application Part

The Intelligent Network Application Part (INAP) protocol is part of the Intelligent Network (IN) implementation used for communication between a Service Switching Point (SSP) and a Service Control Point (SCP). Both the SSP and SCP are part of the basic IN network architecture. Regarding the Nokia solution, these are the DX 200 switch and a UNIX-server.

The operations defined within the INAP protocol are sent through the signalling network using TCAP, SCCP connectionless service, and MTP.

Examples of INAP operations are Initial Detection Point (IDP), SSP -> SCP, used to start the dialogue, and Release Call, which is used by SCP to instruct SSP to release the call as shown in Figure 16.

Figure 16.INAP

6.2.4 Radio Access Network Application Part

In the Iu interface the control plane is maintained by the signalling protocol Radio Access Network Application Part (RANAP). In order to use it over the ATM, some convergence protocols are required.

In 3GPP Specification Set 99 these convergence protocols are expected to be primarily CCS7-based. This is MTP (either normal or broadband) and SCCP offering both connections-oriented and connectionless services the RANAP uses over the Iu interface, as shown in Figure 17.

Figure 17.Iu interface control plane

RANAP is a very important protocol containing plenty of procedures; it maintains the Iu interface control plane thus handling activities between the RAN and the core network domains. Due to its location it is able to handle both circuit-switched and packet-switched traffic-related activities. This section shows some examples about the RANAP activities.

The RANAP performs two kinds of activities. Some activities are related only to the connection management between the RNC and the core network domains. The Iu interface must however carry information that is not directly related into it: the UE and the CN domain(s) exchange signalling information on the control plane. For instance, terminal or subscriber authentication could be this kind of procedure where the RAN (RNC) has no role but where it carries the related signalling information through itself.

The 3G network is able to handle all kinds of traffic created by different services the subscribers use. Some of the services used are so called RT (real-time) services. These services require dedicated connection through the 3G network. The connection should provide a constant, fixed bit rate. NRT (Non-real-time) traffic does not require a constant bit rate, as shown in Figure 18.

Figure 18.RAB and CN domains

The dedicated connection established over RAN (that is, between the UE and the core network) is called Radio Access Bearer (RAB). The core network domains are the entities setting up, modifying, maintaining and deleting bearers. In the CN circuit domain, the bearer is established by the Serving MSC/VLR, which negotiates the RAB and its features over the Iu interface with the Serving RNC (SRNC). In the CN packet domain the same task is performed by the SGSN. Before the RAB can be allocated, there must be at least one active radio link established between the UE and the SRNC. The RAB can be understood as a collection of resource point definitions attending to the connection between the UE and the core network. These kinds of resource points are, for instance, AAL2 ID and bearer ID (defining uniquely the RAB in SRNC and Serving MSC/VLR or SGSN), as shown in Figure 19.

Figure 19.Bearer between the UE and core network circuit domain

Bearer allocation always starts from the core network side. The signalling resources required for that purpose are supplied with the signalling protocol RANAP with the Iu interface. Inside the RAN another protocol, NBAP (Node B Application Part) is also attending to the procedure, as shown in Figure 20.

Figure 20.Bearer between the UE and core network packet domain

In a 3G network, the term bearer and its management has the same content in both of the CN domains delivering traffic, and the procedures related to the RAB assignment are also the same. Because the CN packet domain is an IP based entity, the parameters of the bearer assignment are different between the CN domains. The parameters used for RAB definition in the CN packet domain are NSAPI, CN IP Address and GTP Flow Label.

As examples, this section presents three procedures, in which the RANAP is involved. The first one is the Bearer Assignment. As it was explained in transaction examples, the core network domain is responsible for Bearer Assignment. In 3G, the procedure itself is somewhat simpler than in 2G: there are two messages allocated: RAB Assignment Request and RAB Assignment Complete, as shown in Figure 21.

Figure 21.RANAP Bearer Assignment

The RAB Assignment Request may or may not contain information about several bearers and what to do with them: Create, Modify or Delete. Hence, there is no separate procedure for bearer modification or deletion. When the RAB Assignment Request message arrives to the RNC, the RNC actually binds together the bearer and related radio links thus enabling the user traffic between the UE and the core network, as shown in Figure 22.

Figure 22.RANAP Bearer Deletion

The deletion of the bearer may be initiated either by the RAN or by the core network. If initiated by the RAN, the RAB Release Request message is used like in the figure above. If initiated by the core network domain, the RAB Assignment Request message is used. From the RNC point of view, the RAB deletion means that the binding between the RAB and the radio links is released and the NBAP/RNSAP/RRC procedures for radio links release can be started, provided that the UE does not have any traffic ongoing through another bearer(s), as shown in Figure 23.

Figure 23.RANAP Serving RNC Relocation

When the UE moves in the RAN, there will be a situation where the UE context originally created is not controlled with a reasonable SRNC. Thus, the SRNC functionality must be carried from one RNC to another. This procedure is called Serving RNC Relocation and it requires activities to be performed in two interfaces, Iu and Iur.

When the original Serving RNC realises that there is a need for SRNC Relocation (the very first radio link created for the UE context is about to be released), it informs the desired new SRNC through the core network domain(s) about the need for the relocation. The new possible RNC acknowledges this request through the core network, and the core network starts preparations for bearer switching by assigning bearers towards the new Serving RNC.

When the Relocation Command reaches the original Serving RNC (this informs that the core network is aware of the issue and the bearers towards the new Serving RNC have been allocated), the original Serving RNC starts the relocation in the Iur interface by sending the Relocation Commit. After this has been realised by the new Serving RNC, the information about the readiness comes to the original Serving RNC through the core network in the message Relocation Detect. This is also a sign to start the bearer switching: new bearers towards the new Serving RNC are taken into use and the old bearers towards the original Serving RNC are released.

When the new Serving RNC realises that the new bearers are working properly, it sends the message Relocation Complete thus informing that the rest of the connections through the Iu interface towards the original Serving RNC can be released.

6.2.5 Signalling conversion in the MGW

The MGW for 3G-MSC (ATM Module) contains extensive support for signal processing. These functions are implemented with the TCU (Transcoding Units), which are CDSP (Configurable Dynamic Signal Processing) plug-in units. Each of these cards has several state-of-the-art DSPs (Digital Signal Processors) as well as a control processor. The signal processing tasks can be configured and altered dynamically for each DSP. This CDSP platform with application independent, scalable and dynamic DSP resources provides a flexible and future-proof solution for the MGW for 3G-MSC (ATM Module).

Signal processing is employed in the speech coding (transcoding) and circuit switched data adaptation functions. Because the tasks performed in each DSP can be changed dynamically, the same MGW for 3G-MSC (ATM Module) hardware configuration can handle different kinds of service distributions.

Speech coding is employed in the radio interface to increase the traffic capacity of the system. Furthermore, terrestrial transmission resources are saved when carrying the traffic channel in transcoded form in the UTRAN network up to the MGW for 3G-MSC (ATM Module). The PSTN (toll) speech quality can be obtained with the bit rate of 8-16 kbit/s. Discontinuous transmission (DTX) is also supported by the speech-coding scheme.

6.2.6 Signalling protocol stack in MGW

The following are the SS7 Protocols in MGW for 3G-MSC(ATM Module), as shown in Figure 24:

MGW for 3G-MSC (ATM Module) network element connects narrowband and broadband SS7 signalling networks 3G mobile network - interface uses TDM based and Iu interface uses ATM based signalling links Generic MTP-3 uses both types of signalling links Generic SCCP adopts to different segmentations used in ATM and TDM links Own Signalling Point Code (SPC) is assigned to MGW for 3G-MSC (ATM Module)

Figure 24.Signalling Protocol Stack in MGW6.2.7 GSM to UMTS handover

The GSM to UMTS handover feature provides support for handovers from GSM to UMTS, and vice versa, and for intra-UMTS inter-MSC relocation. Also the necessary changes in the MSC for the support of UMTS are implemented in this feature. This includes the support for BSSMAP R99 signalling, MAP V3 and RANAP on the E-interface, and for the BSSMAP RANAP conversion mechanism in the MSC-B. The possible environment for this feature is shown in Figure 25.

Figure 25.Environment of inter-system handover6.2.8 Summary

PartCCS7 userType of signallingRequired services fromExampleUsed between

User PartsTUPCircuit-related signallingMTPMessage Transfer PartSetup (IAM)Supervise(ACM, ANM)Release (REL) a callPSTN-MSCMSC-MSC

NUP

ISUP

Application PartsBSSAPBoth circuit-related and non-circuit-relatedSCCPSignalling Connection Control PartPaging, control handover and tracing procedureMSC-BSC(BSSMAP)

Set up a call, authentication checking, activate/ deactivate supplementary services and send/receive short message servicesMSC-MS(DTAP)

MAPNon-circuit-relatedHLR inquiry (only MTC)IMEI checkingUpdate locationInter-MSC handoverInter-VLR inquiryMSC-HLRMSC-EIRVLR-HLRMSC-MSCVLR-VLR

INAPNon-circuit-relatedInitial Detection PointRelease callSSP-SCPSCP-SSP

RANAPBoth circuit-related and non-circuit-relatedMTP and SCCPPaging, Set up a call, Set up the bearers, RANAP Serving RNC RelocationAuthentication checking, activate/ deactivate supplementary services, and send/receive short message servicesHandover control, power control, bearers allocation, codes allocationRNC-Core Network Domain

Core Network Domain-UE

RNC-UE

RNSAPServing RNC Relocation in Iur interface by RNSAP SRNC Relocation CommitRNC-RNC

All user parts are commonly used in digital telecommunications networks (not only in the GSM network). The TUP and NUP are mainly used in the PSTN, but can also be used in GSM. The ISUP is introduced for handling services and supplementary services in the ISDN.

The only signalling information transferred by these three user parts is circuit-related signalling. To be able to transfer circuit-related signalling, MTP services are required. This means that the MTP should know how to route signalling messages from one user to another and vice versa.

All application parts are GSM-specific parts. BSSAP is used only between the MSC and the BSC, and the MSC and the MS. MAP transfers only non-circuit-related signalling in the NSS area (MSC, VLR, HLR, EIR). INAP is needed for communication between the SSP and the SCP when there is IN implementation in GSM.

All GSM-specific application parts need the SCCP services. This means that:

the SCCP should be able to perform routing functions for signalling messages that are sent from its users (SCCP subsystems, as shown in Figure 26)

only MTP services cannot fulfil the application part requirement

Figure 26.SCCP subsystems

6.3 Chapter review

3. The information in BSSAP, MAP. and INAP messages is handled by the

a.TCAP

b.SCCP

c.MAP

d.CIC

17. True or False:ISUP provides the facilities for handling the ISDN services and supplementary services for voice and data applications.

True

False

18. List two of the six interfaces between the network elements in the MAP interface.

__________________________________

__________________________________

7 Signalling routing methods

To be able to transfer messages from both user parts and application parts, there are two levels that perform the routing function: the MTP level for user part messages and the SCCP level for application part messages.

Since the MTP was defined for general digital telecommunications networks such as the PSTN, the MTP is well suited for the TUP and NUP. But because the BSSAP, MAP, and INAP are defined especially for GSM, a special protocol has to be established between the application parts and the MTP level. This special protocol is called the Signalling Connection Control Part, SCCP.

Signalling routing methods are shown in Figure 27.

Figure 27.Signalling routing methods

In this chapter, the Message Transfer Part along with its different levels and the routing of the signalling message will be explained.

7.1 Message Transfer Part

Message Transfer Part (MTP) is a functional part of the CCS7 system. It transfers signalling messages, as required by, all users and performs necessary subsidiary functions (for example, error control and signalling security).

7.1.1 Factors that effect the routing of a signalling message

There are three main factors that have to be taken into account every time a message is transferred.

Factor 1:A message should be transferred to the correct destination, for example, a location registration message is sent between MAP-V in the VLR and MAP-H in the HLR.Factor 2:A destination should receive an error-free message or, if there are some errors, the destination should be able to detect and correct these errors. For example, MAP-H in the HLR should receive the correct IMSI, which is included in the location registration message.Factor 3:Since there can be more than one signalling link between two adjacent signalling points and more than one route toward the same destination, the MTP should know which signalling route and signalling link to use. Sometimes, a signalling link may fail because of transmission or traffic overload problems, or because of how the MTP informs the adjacent nodes that this signalling link is unavailable. Therefore, the MTP must have some kind of management for signalling traffic, signalling links, and signalling routes.These three factors are handled by the three levels of the MTP as shown inFigure 28.

Figure 28.Routing by MTP

The MTP contains the functions of the three lowest levels of the CCS7 architecture: level 1 - signalling data link functions; level 2 - signalling link functions; and level 3 - signalling network functions.

Level 1:Signalling data link functions define physical and electrical characteristics of the signalling link The standard signalling link is a 64 kbit/s time slot on a 2.048 Mbit/s digital connection. Bit is a unit of transfer data in this level. Factor 1 and factor 3 need a function of MTP level 1.Level 2:The signalling link functions provide a reliable method of transferring data between two adjacent signalling points (error detection, correction and synchronisation). The information transfer unit on this level is a Signal Unit (SU). Factor 2 and factor 3 need a function of MTP level 2.Level 3:The link contains two function blocks: Message Handling and Network Management. The data transfer unit on this level is Message.

The Message Handling function block is responsible for the routing and distribution of the received signal units, as well as of the signal units to be sent. Factor 1 requires the message handling function.

The Network Management function block contains all procedures and functions for maintaining the signalling network in case of loss of a signalling link, link set, or route set, as well as being a mechanism for traffic control. Factor 3 requires the network management function.

7.1.2 Routing of a signalling message by the MTP

This section begins with a discussion of MTP level 2 and then continues with MTP level 3. Level 1 is a physical level and therefore will not be discussed.

7.1.2.1 MTP level 2

To be able to understand how MTP level 2 performs error and synchronisation handling between two adjacent points, the basic structure of a signal unit (SU) must be explained.

The SU types can be seen in Figure 29. Signal units contain additional information to guarantee a secure transmission. This additional information enables the following level 2 functions:

delimitation of signal units by means of flags

flag imitation prevention by bit stuffing

error detection by checking bits included in each signal unit

error control by re-transmission and signal unit sequence control by means of explicit sequence numbers in each signal unit and explicit continuous acknowledgements

signalling link failure detection by means of signal unit error rate monitoring and signalling link recovery by means of special procedures.

Figure 29.Signal unit types of MTP level 2

There are three different kinds of signal units:

Message Signal Units (MSU)

Link State Signal Units (LSSU)

Fill In Signal Units (FISU)

Fill-In Signal Unit

The FISU is used in times when there is no traffic at all to be sent on the link, and in times when the own side has no messages to send, but the remote end expects acknowledgements for the MSUs that it has sent. The FISU contains no additional information. The FISU can be sent only between two adjacent nodes.

Link State Signal Unit

The LSSU is used to control the signalling link. It contains information about the state of the signalling link. The possible states are link out of service or busy. The LSSU can be sent only between two adjacent nodes.

Message Signal Unit

The MSU is used to transport the user part and SCCP messages to the destination. It contains information received from level 4: user part and application part (via SCCP level) information. The MSU is the only signal unit which is sent through the whole network (not only between two adjacent nodes).

7.1.2.2 MTP Level 3

After level 2 receives a signal unit (for example an MSU or an LSSU), it will interpret that SU. If no error is detected and a SU is the MSU, the MSU will be routed to the level 3 function block Signalling Message Handling. If there is no error, the LSSU will be sent to the level 3 function block Signalling Network Management as shown in Figure 30.

Figure 30.Functions of MTP level 2 and level 3

7.1.2.3 Signalling message handling

Questions:

19. Why does only the MSU (and not the FISU or the LSSU) need a message handling function?

As we already know, the FISU and LSSU are sent between two adjacent nodes only. The LSSU contains the signalling link status so it should be routed directly to the network management. The FISU is used only when there is no MSU or LSSU sent in the link.

20. What is the purpose of message handling?

To determine two things:

The right destination (that is, whether the destination of the message is this network element or another).

If the destination of the message is this network element, the next thing to determine is the right user. The recipient of the message depends on the message type. If the message is used for network management, the message handling will send it to the network management function block. If the message is of ISUP type, it will be sent to the ISUP level. If the message is an SCCP message, message handling will send it to the SCCP level.

21. What are the necessary processes in message handling?

There are three processes needed in message handling function, as shown in Figure 31:

Message discrimination decides whether the incoming message is for its own signalling point or not by checking the DPC contained in the message.

Message distribution determines upon receipt of a signalling message to which user part the signalling is to be delivered by using the Service Information Octet (SIO).

Message routing decides which signalling link to use for each outgoing signalling message by using the Signalling Link Selection (SLS) or the Signalling Link Code (SLC).

Figure 31.Message handling in MTP level 3

The discrimination function evaluates the DPC of the MSU:

DPC = own SPC -> distribution function

DPC own SPC

-> routing function.

The distribution function checks the Service Information Octet, SIO, to determine the suitable user part.

The routing function finds the suitable signalling link for sending the signal unit to another network element.

Message handling can only route a message that contains three bits of necessary information: DPC, SIO and SLS, or SLC. DPC is not the only bit needed for routing purposes; the OPC is also needed when the destination wants to reply to the original source of the message so it can use the OPC that is contained in the received message.

Finally, there are four bits of information that should be stored in every message: DPC, OPC, SIO and SLS, or SLC. Figure 32 shows the structure of the message from MTP users (ISUP, TUP, SCCP, and network management).

Figure 32.Structure of messages from MTP users

The DPC, OPC and SLS, or SLC, as shown in Table 2, are used to route the message between the source and the destination, so they are called Labels or Routing Labels. Figure 33 shows the structure of the standard routing label.

Table 2.Summary of necessary information for message handling

Used to identifyInformationFormat

1. Destination or OriginationDPC (Destination Point Code)14 or 24 bits

OPC (Origination Point Code)14 or 24 bits

SLS (Signalling Link Selection)

or SLC (Signalling Link Code)4 bits

2. MTP userSIO (Service Information Octet)8 bits

(Subservice field and Service indicator)

Figure 33.Structure of SIF, SIO and routing label

The Service Indicator indicates the user part that produces the message included in the MSU.

The Subservice Field indicates the network to which the message has to be delivered.

7.1.3 Example of MTP routing

Figure 34 shows an example of a call to the PSTN originating from a mobile subscriber. The MSC/VLR sends the IAM to a PSTN exchange by the ISUP. The ISUP uses the message handling function of the MTP to route the IAM to a PSTN exchange and within the PSTN exchange. The message handling function is needed in the PSTN exchange to receive and deliver a message to the correct user part (ISUP) by using the Service Information Octet (SIO) contained in the message.

Figure 34.

MTP routing example

7.1.3.1 Signalling network management

The signalling network management message contains a label and management functions of the signalling link as shown in Figure 35.

Figure 35.Structure of the network management messages

There are three subsidiary functions to carry out signalling network management:

Signalling Traffic Management

Signalling Link Management

Signalling Route Management.

Signalling Traffic Management is responsible for the availability of a signalling link or a signalling route by using the following procedures:

Changeover

Changeback

Forced rerouting

Controlled rerouting

Signalling traffic flow control

Signalling traffic usage limitations (inhibiting, uninhibiting, inhibit test procedure)

Restarting a signalling point.

Signalling Link Management controls the signalling links and is responsible for state changes by using the following procedures:

Signalling link activation

Signalling link restoration

Signalling link deactivation

Signalling link set activation.

Signalling Route Management is responsible for the availability of a destination by using the following procedures:

Transfer prohibited:

indicating the unavailability of a destination

Transfer allowed:

indicating the availability of a destination

Transfer controlled:

indicating the overload situation of a

destination

Route set test:

testing the state of a signalling route set

Transfer restricted:

indicating the restricted availability of a

route.

The management functions use the LSSUs to perform operations such as changeover between signalling links or routes and restoration of failed signalling links.

The heading codes H0 and H1 identify the message type such as changeover and out of service.

The user information subfield contains additional information depending on the message type.

7.1.4 Summary of MTP

The MTP level can route the signalling message from the source and destination only if the label is known. A label consists of DPC, OPC and SLS, or SLC. It means that if the source knows the DPC, it can use the MTP for routing purposes. If, however, the source does not know the DPC of the destination, but knows a number that can be used to identify the destination, the source has to use routing by the SCCP level instead of routing by the MTP level.

7.2 Signalling Connection Control Part

Because the application parts are GSM-specific, the MTP does not fit them. Non-circuit-related signalling messages, for example, location registration, HLR inquiry, and short message service, require services provided by the Signalling Connection Control Part (SCCP).

7.2.1 SCCP services

SCCP provides two different services to the application parts, as shown in Figure 36.

Figure 36.Routing by SCCP

The overall set of services is grouped into:

connection-oriented services

connectionless services

Connection-oriented services provide the means to form a virtual connection between two network elements. The messages that are sent from one SPC to another can be connected as a single communication link, and the routing operations are made only during the connection establishment.

Connectionless services provide the applications a way to transfer signalling messages through the signalling network without knowing the network structure. These services are used for very short transfers of data between network elements.

When using the connectionless service, all messages must be routed separately.

In GSM networks, both service groups are used. MAP uses only connectionless service (mostly service class 0; only inter-MSC handover uses service class 1). The BSSAP uses the connection-oriented service class 2 and the connectionless service uses service class 0. INAP uses only connectionless service, as shown in Figure 37.

Figure 37.Use of SCCP services

7.2.2 SCCP routing function

The messages coming from the SCCP subsystems (BSSAP, MAP, and INAP) must be processed by the SCCP. The information contained in the message determines which way the message is routed. This is illustrated in Figure 38 and Figure 39.

If there is a label (DPC, OPC, SLS) inside, the SCCP will not perform any special routing function. The SCCP just sends a message through the MTP and uses the normal message handling function of MTP level 3. We call this type of routing SCCP routing on label.

If, on the other hand, a label is not contained in the message but there is an address of the destination contained in a message instead, the SCCP will perform a routing function by using this address instead of the label. The address is called a Global Title (GT). Thus, this routing type is called SCCP routing on GT.

Figure 38.Routing on label and on GT

Figure 39.Signalling routing (MTP and SCCP)

7.2.2.1 Global Title

A GT is an address used to identify the destination of the signalling message, as shown in Figure 40. The GT is sent by application parts (MAP-V, MAP-M, MAP-H, and MAP-E) and analysed by the SCCP level.

Figure 40.Global title

Most GTs are in the format of MSISDN (E.164), such as a Short Message Service Centre (SMSC) address, HLR, VLR, MSC, and EIR numbers. Only location update for a roaming subscriber, E.214 or a hybrid number, will be used instead of E.164.

As an example of using E.164, in case of location update for a new visitor (HPLMN subscriber), the VLR will send an Update Location message to the HLR so that the destination of this message is identified by the HLR number. When the HLR replies, the message contains the VLR number as the destination.

If the new visitor is a subscriber roaming from another operator, the VLR will not know the HLR number of this subscriber. The VLR will use a hybrid number, which is a result of an IMSI analysis, to route the Update Location message to the HLR of the other operator network.

Furthermore, a GT is used to route a signalling message to the right destination. Thus, a GT itself should have enough information to route a message toward a destination: Country Code (CC) and National Destination Code (NDC). The CC and NDC are in both E.164 and E.214.

E.214 (Hybrid number): operator number + MSIN

This consists of operator number and the Mobile Subscriber Identification Number (MSIN). Operator number is a unique number that identifies the GSM operator. Normally we can use the CC + the NDC to identify the GSM operator. However, some countries cannot use only the CC + NDC to identify their network. They need to add a few more digits, a shown in Table 3.

Table 3.Hybrid numbers for different operators

OperatorCC+NDCOperator numberMSIN(part of E.212)Hybrid number (E.214): operator number + MSIN

Finnish3585035850123456789035850+1234567890

China A8613986139000000000286139+0000000002

China B8613086130000000123486130+0000001234

All GSM operators in Finland can use only CC + NDC to tell that a number can be used to route a signalling message to their network. In their case, the E.214 is CC + NDC + MSIN.

7.2.2.2 IMSI and VMSC/SGSN address sent to the SMSC in MT-SMS

The subscriber B IMSI number and the destination VMSC/SGSN address are sent to the SMSC as supporting statistics in the SMSC. This is because the SMSC requires this data for statistical purposes. In addition, if the SMSC is charging for SMS, the VMSC address is a useful piece of information for locating subscriber B.

7.2.2.3 GT Analysis

GT analysis is used to analyse a GT and identify which way a signalling message should be routed out of the exchange. This is illustrated in Figure 41.

Because the SCCP uses the MTP service, one of the outputs of GT analysis is the DPC, which is a necessary parameter for message handling function on MTP level 3. The other output of GT analysis is the Routing Indicator (RI), which is used to inform the next exchange whether the GT analysis has to be performed.

Figure 41.GT analysis

Table 4 lists the necessary GT analyses for when a SMSC is integrated to a Nokia network.

Table 4.Necessary GT analyses

Stepin SMS-GMSCin HLRin VMSC

1aNP=E164, DIG= MSISDN-B(HLR, RI=GTNP=E164, DIG= MSISDN-B(HLR (own), RI=SSN

1bNP=E164, DIG= SMSC-ISDN(SMSC, RI=SSNNP=E164, DIG=SMSC-ISDN(SMSC, RI=GT

2NP=E164, DIG= VMSC-ISDN(SMSC, RI=GTNP=E164, DIG= VMSC-ISDN(VMSC (own), RI=SSN

5NP=E164, DIG= SMSC-ISDN(SMSC (own), RI=SSNNP=E164, DIG= SMSC-ISDN(SMSC , RI=GT

6aNP=E164, DIG= HLR-ISDN(HLR, RI=GTNP=E164, DIG= HLR-ISDN(HLR (own), RI=SSN

6b(=1b)NP=E164, DIG= SMSC-ISDN(SMSC (own), RI=SSNNP=E164, DIG= SMSC-ISDN(SMSC, RI=GT

7.2.3 Example of SCCP routing location update

The following examples demonstrate what can happen when a subscriber makes a location registration.

7.2.3.1 IMSI analysis

If the VLR does not have the subscriber data before, the subscriber's IMSI will be analysed by the means of an IMSI analysis. The purpose of the IMSI analysis is to determine where the subscriber's HLR is. When the VLR knows the subscriber's HLR, the VLR will send a request message to the subscriber's HLR for subscriber data including authentication and GSM services.

The application part responsible for sending an update location message to the HLR is MAP-V (VLR). MAP uses the connectionless service of SCCP. On the SCCP level, there are two types of signalling routing. Thus, the IMSI analysis can use both SCCP routing on label and SCCP routing on GT, depending on who the subscriber is.

7.2.3.2 IMSI analysis with routing on GT

If the subscriber is a roaming subscriber, the VLR does not know the DPC of the HLR. Therefore, the VLR needs IMSI analysis to translate the IMSI (E.212) to a hybrid number (E.214). The hybrid number will be analysed later in the GT-analysis. This is illustrated in Figure 42.

Figure 42.IMSI analysis

IMSI analysis with routing on GT is always used with roaming subscribers, as shown Figure 43.

Figure 43.International roaming, the first location update

7.2.3.3 IMSI analysis with routing on label

If the subscriber is a home subscriber, the use of IMSI analysis with routing on label depends on whether the VLR knows the DPC of the HLR or not. If the VLR knows the DPC of the HLR, it can use routing on label. Otherwise it has to use routing on GT. In this case, the result of the IMSI analysis is the DPC of the HLR, as shown in Figure 44.

Figure 44.IMSI analysis

IMSI analysis with routing on label can be used with home subscribers if the VLR knows the DPC of the HLR, as shown in Figure 45.

Figure 45.Home subscriber, the first update

SCCP routing by PSTN is shown in Figure 46, and IMSI and GT analysis is shown in Figure 47.

Figure 46.SCCP routing by PSTN

Figure 47.IMSI and GT analysis

7.3 Chapter review

4. Which level performs the routing function for user part messages?

a.TCAP

b.SCCP

c.INAP

d.MTP

22. Which level performs the routing function for application part messages?

a.TCAP

b.SCCP

c.INAP

d.MTP

23. List the three main factors that must be taken into account every time a message is transferred.

___________________________________________________________

___________________________________________________________

___________________________________________________________

8 CCS7 management in the DX 200 MSC/VLR

In this chapter, we take a look at how to create an MTP, user parts, an SCCP, and SCCP subsystems in the DX 200 MSC/VLR. We will learn the necessary procedures and the Man-Machine Language (MML) commands needed for implementing them.

Figure 48 shows an overview of the procedure to use to add a new MSC to a network that has direct connection to an MSC.

Figure 48.Adding a new MSC to the network

8.1 Creating an MTP

Use the following steps to create an MTP.

Creating an MTP

1. Create signalling link(s) (SL) ZNC.

2. Create signalling link set (SLS) ZNS.

3. Create signalling route set (SRS) ZNR.

4. Activate states of signalling link(s), signalling route(s) ZNL, and NV.

8.2 Creating an ISUP or TUP

Use the following steps to create an ISUP or TUP.

Creating an ISUP or TUP

24. Create Service Information Octet (SIO) for ISUP or TUP if it does not exist ZNP.

2. When the ET is connected, define the User Part for that ET so ET knows that it is an ISUP-ET or a TUP-ET ZWUC.

8.3 Creating an SCCP

Use the following steps to create an SCCP.

Creating an SCCP

25. Check Service Information Octet (SIO) for SCCP. If it does not exist, create it ZNP.

2. Define SCCP ZNFD.

3. Activate SCCP state ZNG.

8.4 Creating an SCCP subsystem

Use the following steps to create an SCCP subsystem.

Creating an SCCP subsystem

26. Add SCCP subsystem(s) (BSSAP, MAP-M, MAP-V, MAP-H, MAP-E or INAP) ZNFD or ZNFB.

2. Activate SCCP subsystem state(s) ZNH.

8.5 Creating a GT analysis

Use the following steps to create a GT analysis.

Creating a GT analysis

5. Create GT results ZNA.

6. Create GT analysis ZNB.

8.6 MML interfaces

Figure 49 highlights the CCS7 network administration command class.

DX 200 MSC03 2000-12-15 09:36:44

CCS7 NETWORK ADMINISTRATION

? ..... DISPLAY MENU

A ..... GLOBAL TITLE RESULT HANDLING

B ..... GLOBAL TITLE ANALYSIS HANDLING

C ..... SIGNALLING LINK DATA HANDLING

E ..... SIGNALLING NETWORK STATE INTERROGATION

F ..... SCCP DATA HANDLING

G ..... SCCP STATE HANDLING

H ..... SCCP SUBSYSTEM STATE HANDLING

L ..... SIGNALLING LINK STATE HANDLING

M ..... CCS7 LEVEL 3 PARAMETERS

N ..... SIGNALLING ROUTE SET PARAMETER HANDLING

O ..... SIGNALLING LINK PARAMETER HANDLING

P ..... SERVICE INFORMATION DATA HANDLING

R ..... SIGNALLING ROUTE SET DATA HANDLING

S ..... SIGNALLING LINK SET DATA HANDLING

V ..... SIGNALLING ROUTE STATE HANDLING

Z; .... RETURN TO MAIN LEVEL

Figure 49.CCS7 network administration command class

9 Architecture

The CCS7 architecture is illustrated in Figure 50.

Figure 50.Hardware used in CCS7

10 AppendixTransaction Capabilities Application Part

In GSM networks the Transaction Capabilities Application Part (TCAP) is the interface between the Mobile Application Part (MAP) and the SCCP.

Because the connectionless services of the SCCP only send datagrams, a function enabling a virtual dialogue between two network elements is needed.

The TCAP provides all functions that enable this virtual dialogue.

The TCAP contains the following functions:

Component Handling

Transaction Handling

Component Handling provides the assignment between an operation sent by MAP and the received answer.

Example:HLR-enquiry

MSC sends toward the HLR: INVOKE (Send Routing Information)

HLR answers:

RESULT (MSRN)

The following components are used:

INVOKE

operation

RESULT

related to invoke

ERROR

Transactions Handling binds these components into a logical dialogue.

The real TCAP message looks like:

MSC -> HLR:

BEGIN [ INVOKE (Send Routing Information) ]

HLR -> MSC:

END [RESULT (MSRN) ]

The following TCAP messages are defined:

BEGIN

dialogue

CONTINUE

dialogue

END

dialogue

ABORT

dialogue

The TCAP dialogue is shown in Figure 51.

Figure 51.TCAP dialogue

11 AppendixIP trunking

The protocol stack and IP routing feature implements the layered set of protocols that make up the TCP/IP protocol suite.

11.1 Protocol stack

The implementation supports unicast IP forwarding and can be configured to operate as a host or as a router. The protocol stack includes a host implementation of IP multicasting. A host can send and receive multicasts, but does not forward multicast packets or participate in multicast routing.

IP routing is the exchange of information that takes place in order to have the information necessary to make correct IP forwarding decisions. Routing information is communicated between routers using routing protocols. This feature implements the Open Shortest Path First (OSPF) protocol.

Session Initiation Protocol (SIP) is a protocol intended to be used to manage multimedia sessions between client and servers in an IP telephony environment. In the MSC IP trunk application, speech calls are considered a multimedia session, where only speech information is exchanged, as shown in Figure 52. Other protocols include:

Real-time Transport Protocol (RTP), used for transmitting user plane traffic

Real Time Control Protocol (RTCP), used to control the quality of service in the user plane

Transmission Control Protocol (TCP), used as a reliable transport protocol

User Datagram Protocol (UDP), used as an unreliable transport protocol

Figure 52.IP trunking protocol stack

11.2 Call setup procedures over IP trunking

The basic principle of the Integrated IP Trunk Solution is to connect MSCs together through an IP network and the needed hardware, which can be implemented as an option to existing M98 exchange mechanics.

This feature is implemented with a Trunk Gateway Signalling Unit (TGSU), which handles the signalling over IP network and IP Exchange Terminal (IPET) units. These convert speech frames between TDM and IP packets.

Figure 53.IP trunk architecture

The speech is driven through DSPs located in the IPET, and signalling is handled by the TSGU, which is also attached to the IP network. Data/fax, handover calls, and emergency calls are still routed via TDM.

11.2.1 TGSU

TGSU uses SIP with ISUP tunnelling as call control protocol over IP. TGSU dynamically negotiates the media source and destination IP addresses with SIP parameters in the beginning of each call setup.

The routes and circuits are hunted and selected in the same manner as with normal trunk calls, and the hunted circuit appears as a normal trunk. The exception is that the traffic in this trunk is forwarded to the IP network.

11.2.2 IPET

IPET units can be associated with traditional ETs. IPET is a functional unit that is implemented using a half of a Packet Control Unit (PCU)-A plug-in unit. IPET forwards the speech frames arriving from the switch matrix to the IP network. Each IPET has an IP address that is used to identify it. In a certain direction, any of exchange Bs IPET can be accessed by any of exchange As IPET. Addresses of these two IPETs are defined in the call setup as an outcome of the media address negotiation.

TGSU and IPET are attached to the IP network by LAN switches. Switches are connected to routers that act as an access to the operators IP network, which is used for trunk bypass. This access router is located in exchange premises.

12 Check your understanding

7. In Figure 54, fill in the protocols used between different network elements.

Figure 54.Protocols used within PLMN

27. List two examples of circuit-related signalling.

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28. List four examples of non-circuit-related signalling.

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29. List the necessary CCS7 protocols needed in the MSC, HLR, BSC, MGW, and PSTN exchange.

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30. True or FalseThere is no significant difference between protocols that are user parts and protocols that are application parts.

True

False

31. Give an example of a case in which SCCP routing is necessary because MTP routing cannot be used.

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32. List a function of each of the following:

MTP-level 3___________________________________________

SCCP

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MAP

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BSSAP

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RANAP

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ISUP

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INAP

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33. True or FalseThe purpose of an IMSI analysis is to determine where the subscriber's HLR is located.

True

False

34. Which of the following is a valid reason to use IMSI analysis with routing on label?

a.Use with roaming subscribers.

b.Use with home subscribers if the VLR knows the DPC of the HLR.

c.Use whenever and wherever a subscriber makes a location registration.

d.Use when the MAP sends an update location message to the HLR.

35. Which of the following is a valid reason to use IMSI analysis with routing on GT?

a.Use with roaming subscribers.

b.Use with home subscribers if the VLR knows the DPC of the HLR.

c.Use whenever and wherever a subscriber makes a location registration.

d.Use when the MAP sends an update location message to the HLR.

36. List the definitions that must be done in a Nokia MSC when a new MSC is added to the existing GSM/UMTS network. This new MSC has a direct signalling connection to the Nokia MSC.

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37. List the SS7 definitions that must be created in core networks when an SMSC is integrated with a Nokia network.

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38. List the protocol stack used with IP trunking.

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39. List the procedures of a call setup with IP trunking.

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13 Glossary

The following sections provide definitions for key words, phrases, and acronyms used in this training document.

13.1 Definitions

TermDefinition

Base Station System Application PartA subsystem that contains the process for radio resource control and management known as the Base Station System Management Application Part (BSSMAP).

Circuit-related signallingSignalling messages that contain information about the circuit to be signalled and the origin and destination of the message.

Common Channel SignallingA signalling method that uses addressed messages to convey signalling information over a single channel.

Common Channel Signalling Number 7An internationally standardised CCS system that is optimised for operation in digital telecommunication networks such as GSM.

Mobile Subscriber Identification NumberA code that uniquely identifies the mobile subscriber within a PLMN area.

Non-circuit-related signallingSignalling messages that do not contain information about the circuit, but instead contain other information depending on the use of the signalling.

Pulse Code ModulationA process in which a signal is sampled, and each sample is quantised independently of other samples and converted by encoding to a digital signal.

SignallingFor PSTN, any transfer of data that enables speech and data connections between users.

or

For GSM, any transfer of data that enables speech and data connection between users and supports mobility management and GSM services handling

13.2 Abbreviations

AbbreviationTerm

BSSAPBase Station System Application Part

BSSMAPBase Station System Management Application Part

CFUCall Forwarding Unconditional

CICCircuit Identification Code

CCSCommon Channel Signalling

CCS7Common Channel Signalling Number 7

GSMGeneral forward Set-up information Message

GSWGroup Switch

GTGlobal Title

H0Heading Code 0

H1Heading Code 1

IMSIInternational Mobile Subscriber Identity

IPETIP Exchange Terminal

MGWMultimedia Gateway

MSCMobile Services Switching Centre

MSINMobile Subscriber Identification Number

MSRNMobile Station Roaming Number

MSUMessage Signal Unit

MTPMessage Transfer Part

NUPNational User Part

OSIOpen System Interconnection

PCMPulse Code Modulation

PCUPacket Control Unit

PSTNPublic Switched Telephone Network

RANAPRadio Access Network Application Part

SCCPSignalling Connection Control Part

SCPService Control Point

SMSShort Message Service

TCAPTransaction Capabilities Application Part

TGSUTrunk Gateway Signalling Unit

UTRANUMTS Radio Access Network

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