signalling

Signalling

The interchange of information

between the different functional parts

of a telecomm network

There are three distinct types of signalling:

1.Between customer and exchange

2. Within an exchange

3.Inter-exchange

There are three distinct types of signalling:

LX LX

TrunkLocal Loop Local Loop

Calling Party

Called Party

Off Hook

Digits

Ring back Tone

Call Progress

Seize

Start

Digits

Call Progress

Supervision

Ring

Off Hook

Functions of Inter-Exchange Signalling

• Supervisory functions

• Selection functions

• Operational functions

Channel Associated Signalling

Signalling is carried, associated with each channel.

For each channel Bits are reserved in channel 16 for

signalling status.

Speech Channels 1-15 Speech Channels 17-31

0 16For Frame Synchronization

For Signalling

Channel Associated Signalling

Limitations of Channel-Associated Signalling

Slow Inflexibility

Limited Capability

Closed Growth path

C7 Signalling

A worldwide standard for inter-exchange signalling.

The average call setup time for a toll call is 3 to 7 seconds, compared to 11 to 15 seconds before.

Optimum use of trunk lines.

It uses labeled messages which cab be sent on a signalling path separate from that used by the speech and data.

Each signalling path conveys signalling information about many speech/data channels. A C7 Signalling link can handle several thousand calls.

Long distance bandwidth is conserved since signalling is out of band and signalling for several trunks can be multiplexed on a single signalling channel.

C7 Signalling

Additional user services can be offered with CCS, such as closed user groups (CUGs), credit card verification, and calling party identification.

C7 is modular in structure Hence it is flexible and may serve different user sources i.e. it is a multi-user system. For example, C7 can be used to handle signalling for Telephony or ISDN users.

The modular structure of C7 allows it to meet not only present, But future needs too.

As C7 is an open system, it offers administrations the opportunity to adapt and expand their network according to their needs.

Characteristics of C7

•Fast 3 to 7 seconds, compared to 11 to 15 seconds

•High Capacity Can handle several thousand calls

•Modular Can serve different user sources

•Flexible Labeled messages for speech and data

•Economical Optimum use of trunk lines less EQP

Network Structure

Signalling PointNodes in the C7 Signalling network are referred to as Signalling Points (SPs).

Signalling Link (SL).A signalling path between two adjacent Signalling Points (SPs) is called a Signalling Link.

Link Set (LS).A number of Signalling Links between two SPs are together referred to as a Link Set.

Network Structure

Originating Point (Op)The SP from where the signalling message is generated is called Originating Point.

Destination Point (DP)The SP for which the signalling message is generated is called Destination Point.

Signalling Transfer PointA Signalling Transfer Point (STP) is an SP through which messages are routed without being processed.

Signalling Point Codes

Every Signalling Point is identified by a unique Point Code to enable the system to route a message efficiently. This is done by specifying the:

Originating Point Code (OPC)

Destination Point Code (DPC)

The OPC and DPC are contained in a part of the C7 labeled message called the Routing Label.

Structure of a C7 Node

The C7 node was originally designed:

• as a 4-level structure,

• and to support Circuit-Related applications

The modern C7 node is based on the 7-layer OSI model

Can be used for non circuit-related applications

A C7 nodes consists of three parts

UserPart

Message Transfer Part

Application Part

UserPart

Application Part

Messages

Signalling LinkMessage

Transfer Part


It acts as a common message transfer system for the different applications and user parts in the node.

Basically, the MTP is responsible for the reliable transfer of signalling messages between users.

Messages from different users are packed into a standard format by the MTP and are transported over the network in this format.

User Parts

User Parts are level 4 parts of the node and are concerned with circuit-related applications e.g.

The Telephone User Part (TUP).

The ISDN User Part (ISUP)

Application Parts

Later additions to the node which were developed in line with the OSI model.

Some of these parts are concerned with non circuit-related applications such as network operation and maintenance.

Some More Additions to C7

SCCP ( Signaling Connection Control Part )

ISP ( Intermidate services Part )

TCAP ( Transaction Capability Part )

With the addition of these C7 becomes OSI compatible. ISP & TCAP provide layers 4-7.

Structure of C7 Node

Message Transfer Part ( Level 1-3 )

SCCP

TUP

Level 4

TCAP

OMAPMAP

ISP

ISUP

Level 4


• MTP Serves as a transport system for the reliable transfer of messages.

• The MTP ensure that:– No errors are introduced into the signalling units

– The SUs are sent in correct sequence

– A SU is sent only once

– SUs are sent in the correct direction

– All SPs within the signalling network are informed of any changes in the signalling network e.g. link failure.

Types of SUs

1) Message Signal Units (MSUs)

2) Link Status Signal Units (LSSUs)

3) Fill In Signal Units (FISUs)

Message Signal Units (MSUs)

These are received from the User Parts and

the Network management functions.

They contain information relating to the control of calls

or the signalling network.

Structure of MSU

F = Flag. BSN = Backward Sequence No.BIB = Backward Indicator

BitFSN = Forward Sequence

NoFIB

= Forward Indicator Bit

L1

= Length IndicatorSIO = Service Info.

Octet.SIF

= Signalling Info.FieldCK = Check

bits

MSU

F CK SIF SIO LI FIB

FSN BIB

BSN F

8 16 8n, n>2 8 2 6 1 7 1 7 8

Structure of MSU

Length Indicator LI is used:

To distinguish between SU types and

In MSU to indicate the size of SIF

If L = 0 SU is a FISU

L = 1 or 2 SU is a LSSU

L > 2 SU is a MSU and the size of Data filed

Flag = 01111110

BIB and FIB bits are toggled to Indicate:

A faulty SU Retransmission of SU

Service Information Octet (SIO)

SIO

SISSF

SI = Service Indicator

TUP , ISUP

SSF = Sub-Service FieldThe SSF contains the Network Indicator (N1) bit. This is used to discriminate between,

national and international signalling networks,

two national signalling networks

different level 4 parts

Link Status Signal Units (LSSUs)

These are sent from level 2 of the MTP. They indicate a change in the status of the

signalling link e.g. change from the status ‘Available” to the status “Blocked’

There are 6 different status indications:

Out of Service (OS)

Processor Outage (PO)

Busy(B)

Normal Alignment (N)

Emergency alignment (E)

Out of Alignment (O)

LSSU

F CK SF LIFIB

FSNBIB

BSN

8 16 8 OR 16 2 6 1 7 1 7

F

Fill In Signal Units (FISUs)

These are from level2 of the MTP. Sent when no other message is being sent on the signalling link. They are also sent to acknowledge MSUs when no other MSUs are being sent.

F CK LIFIB

FSNBIB

BSN F

8 16 2 6 1 7 1 7 8

MTP Functional Levels

Level 1: Signalling Data Link functions (SDL)

Level 2: Signalling Link functions(SL)

Level 3: Signalling Network functions

Level 1: Signalling Data Link functions (SDL)

Level 1 defines the characteristics of :The Signalling Data Link andthe means to access it,

i.e. the transmission path. An example of a digital Signalling Data Link would be a 64 kbit/s transmission channel and semi-permanent connection through an electronic switch.

Level 1 consists of:

.Transmission Channels on the SL

.Switching Functions to Access SL


Level 2 Functions are:

•Signal Unit Delimitation

•Link Alignment

•Error Detection

•Error Correction


Signal Unit Delimitation:

The Flag is used for Signal Unit Delimitation.

The bit pattern of the flag is 01111110.

Flags are inserted at the beginning and end of a Signal Unit.

Bit Stuffing (to prevent the flag sequence being imitated )

A ‘0’ is inserted after every five consecutive ‘1S’


Link Alignment

Alignment is a level 2 function which occurs when A link :

-Is first brought into service

-Which has been out of service is brought back into service.

The link may be aligned under ‘Normal’ or ‘Emergency’ procedures.

The Signalling Link Management function of level 3 decides

whether Normal or Emergency alignment is required,

depending on whether the Link is required urgently or not.


Link Alignment

The following LSSU types are sent during Initial Alignment:

-The Status Indicator Out of Service (SIOS)This is sent on a Link after initial power up of the link

The Status Indicator Out of alignment (SIO)This is sent at the beginning of alignment to start the alignment procedure.

Status Indicators Normal (SIN)This is sent for a period of 8 seconds during normal alignment.

The Status Indicator Emergency alignment (SIE)This is sent during emergency alignments for a 0.5 second proving period

The error rate of the SINs and SIEs is checked by:- Examining the CK bits- Incrementing the alignment error rate monitor if an error is detected


Error Detection

The Error Detection function of Level 2 is used to detect errors in SUs.The CK field is used in the Error Detection function. It contains 16 checksum bits.

These CK bits are generated in Level 2.

The CK bits are assigned a value on the basis of the preceding bits of an SU. The SU is then sent on the SL. At the receiving Signalling Point (SP), the CK bits are compared with the preceding bits of the SU. If they correspond, the SU is considered to be error free.

If SU is not error free the SU will be discarded and the SUERM Unit Error Rate Monitor incremented.


Error Correction

The Signal Units are retransmitted.

One method used is the Basic Error Correction method.

All SUs that are transmitted are stored in a buffer, in the sending SP,

until they are acknowledged.

The Backward Sequence Number (BSN) is used for error correction.

For non-satellite links

Level 3 Functions

The functions of Level 3 are divided into two main categories:

1. Message Handling functions

1. Message Routing

2.Message Discrimination

3.Message Distribution

2. Network Management functions

Signalling Traffic Management (STM)

Signalling Route Management (SRM)

Signalling Link Management (SLM)

Message Handling functions

The Message Handling functions manage the messages,which come from or are sent on the Signalling Link.These messages may have been generated by the:

User Parts (UP) Level 4 SCCP and Application Parts orNetwork Management functions (also in Level 3 )

The Message Handling functions involve:

1. Message Discrimination2. Message Routing3.Message Distribution


1. Message Discrimination

Discriminates between incoming SUs which are for this Signalling Point and those destined for another Signalling Point,

The parts used in Discrimination are:The Network Indicator (NI) in SIOThe Destination Point Code (DPC) in the Routing Label.

The Network Indicator (NI) is a 2/bit code found in the SIO.The Network Indicator (NI) is used to distinguish between:

- National and International networks- Two different National Signalling networks


2. Message Routing

It involves directing the messages to the correct SL.

In User Message the DPC and SLS contain the information required for the Routing function.

In Management Messages the DPC and SLC contain the information required for the Routing.


3. Message Distribution

Directs incoming Signal Units (SUs) to the correct destination, i.e.

User Parts,SCCP orThe Network Management functions (Level 3).

The Service Indicator (SI) in the SIO field indicates to which destination the message will be sent.


UserParts Distribution Discrimination

Routing

Signalling Link

The Network Management functions

Signalling Traffic Management (STM)• Changeover• Change back• Forced rerouting• Controlled rerouting.



• Activation• Restoration• Deactivation


Signalling Traffic Management (STM)Signalling Traffic Management (STM) is responsible for Diversion and Reduction of Traffic.

There are four procedures which enable the MTP to carry out the (STM) functions:

- Changeover

- Changeback

- Foced rerouting

- Controlled rerouting.


Signalling Traffic Management (STM)1. ChangeoverTraffic diversion to an alternative link is implemented by the Changeover procedure.

Stopping the transmission on the faulty link.Retransmission of buffers in both sides.

Changeover is a handshaking procedure which ensures that:

No SUs are lostNo SUs are duplicatedThe Sus sequence is maintained

This is done by exchanging Changeover Order (COO) and Changeover Acknowledgement (COA) SUs.


Signalling Traffic Management (STM)2. Change back Procedure

When the normal link becomes available again,the Change-back procedure is initiated.The change-back Procedure is as follows:

(SP) A prevents the traffic from being transmitted to via the alternative link.

Signalling Point (SP) A send a Change-back Declaration (CBD).

Signalling Point (SP) B replies with a Change-back Acknowledge (CBA).

Traffic then resumes on the normal link.


Signalling Traffic Management (STM)Forced Rerouting

When a signalling route to a particular Signalling Point (S) is indicated as unavailable, Forced Rerouting is carried out by the Signalling Traffic Management (STM) functions.

Controlled Rerouting

Controlled Rerouting is used when:Traffic is to be directed from an alternative Signalling Route back to the normal Signalling Route, which has become available.A Transfer Restricted (TFR) SU is received, indicating congestion on a Signalling Route.


Signalling Traffic Management (STM)Rerouting Procedure.

A similar procedure is used in both cases.

In a route restriction case (where traffic is restricted on a route due to congestion), traffic is transferred from the restricted route to an alternative route using a Controlled rerouting Buffer, e.g.

- Traffic is stopped on the restricted route- MSUs are stored in the Rerouting Buffer- An alternative route is selected- Traffic resumes on the alternative route

Flow Control ( If No alternative is available )

Signalling Link Management

The Signalling Link Management (SLM) function is used to control the Signalling Link.

There are three basic Signalling Link Management procedures:

- Activation- Restoration- Deactivation


Activation-The process of bringing a SL into service.

Link Restoration

In case of SL failure, a Link Restoration procedure is defined to restore the faulty SL.

During restoration the following takes place:

(1) Initial Alignment(2) Signalling Link Test


Link Deactivation

A link in service can be taken out of service.For example a link will be taken out of service when it is required for another signalling route. This procedure is called Deactivation.

Link Deactivation will only take place if there is no traffic on the Signalling Link (SL).

The Signalling Terminal of the deactivated Signalling Link will also be taken out of service.

Signalling Route Management

The Signalling Route Management (SRM) function communicates to every SP, information regarding signalling route availability / unavailability

A signalling route between two SPs may be:

Available i.e. all SLs on that route are availableUnavailable i.e all SLs on that route ae unavailableIn restricted service i.e.some Sls on the route are available and some are not

The procedures used to communicate these states are:

- transfer-allowed for available state- transfer-prohibited for unavailable state- transfer-restricted for restricted service

A fourth procedure is defined to check if any changes have occurred in the route status. This procedure is called signalling-route-set-test procedure.

MTP Summary

Types of SUs:

1) Message Signal Units (MSUs)2) Link Status Signal Units (LSSUs)3) Fill In Signal Units (FISUs)

MTP Summary

MTP Functional LevelsThe operations of the MTP are subdivided into three functional levels.

Level 1: Signalling Data Link functions (SDL).Transmission Channels .Switching Functions


.Signal Unit Delimitation .Link Alignment

.Link Detection .Error Correction

Level 1 and 2 together ensure that:- errors are not introduced in the Signal Units (SUs)- Sus are sent in the correct sequence- Sus are sent only once

MTP Summary

Level 3: Signalling Network functions1.Signalling Message Handling functions:

DiscriminationDistributionRouting

2.Signalling Network Management functions:Signalling Traffic Management (STM)

• Changeover• Changeback• Foced rerouting• Controlled rerouting.



• Activation Restoration

Deactivation

Telephone User Part

The TUP is that part of the C7 in the telephone exchange which controls the setting up, supervision and release of calls.

The TUP is a C7 level 4 function.

It defines the functions and procedures for the control of telephone calls and circuits.

The functions in the TUP are equivalent to those in layers 4 to 7 of the OSI model.

A TUP can generate about 250 messages. 60 are defined.

Telephone User Part

TUP Messages

The part of the MSU in which the TUP Message is stored is SIF.

In the C7 TUP there are 9 groups of messages.

Call set-up phase 5 groups

Call Supervision phase 1 group

Circuit maintenance and supervision 2 groups

Node to node communication 1 group

Signalling Information Field (SIF)

SIF

Label

Destination Point Code (DPC)

Originating Point Code (OPC)

Signalling Link Selection (SLS) In case of TUP Messages

Signalling Link Code (SLC) In case of MTP message

Circuit Identification Code (CIC)

Heading CodeDataDPCOPCCIC SLC / SLSH0H1

4 4 8 4 14 14

Telephone User Part

There are 9 message groups.

Abbreviation Message Group H0 ValueFAMFSMBSMSBMUBMCSMCCMGRMNNM

Forward Address MessageForward set-up MessageBackward set-up MessageSuccessful Backward set-up MessageUnsuccessful Backward set-up MessageCall Supervision MessageCircuit Supervision MessageCircuit Group Supervision MessageNode to Node Message

123456789

H1 on the other hand indicates the message within group.

Heading CodeH0H1

H0, It indicates the message group to which the message belongs.H1, It indicates the message within that group.

Telephone User Part

Description of Message Groups

Call Set-Up FAM , FSM , BSM , SBM , UBMCall Supervision CSMCircuit Supervision and Maintenance CCM , GRMNode to Node NNM

ISDN User Part (ISUP)

ISUP provides the signalling functions required to support the range of services offered by the ISDN

The ISUP supports both circuit-related and non circuit-related signalling.

It contain both the functions of ISUP and TUP.

ISUP corresponds to OSI layers 4-7.

The ISUP supports the 3 classes of ISDN services:

1. Bearer Services2. Tele-services3. Supplementary Services


ISUP is capable of generating 256 different messages. So far, CCITT have specified 42 in blue book ( Q 762 ). In Q 767 recommendation, a subset of these messages have been defined for international applications.

There are 8 ISUP message group.

1. Forward set-up 2. General set-up

3. Backward set-up 4. Call supervision

5. Circuit supervision 6. Circuit group supervision

7. In call modification 8. End to End


--- SIF ---

ISUP MessageInformation

elementsMessage Type Label

The signalling Information Field is the part of the MSU signal unit which carries the information generated by ISUP

SIGNALLING CONNECTION CONTROL PART (SCCP)

SCCP provides for the transfer of Circuit-Related and Non Circuit-Related signalling information.While avoiding the need for major changes to the MTP itself.

Before the development of SCCP, the sending of signalling data through the network required the setting up of a physical connection.

Two network services supported by SCCP:

1) Connectionless2) Connection Oriented


Connection-Oriented Services3 Phases:

1. Establish Connection2. Data Transfer3. Release Connection

Characteristics:- Two - way exchange of information- Data sent in sequence- Received data is acknowledged- Temporary or Permanent- Reliability- Transfer of large amounts of data- Reset and Notice Services


Connectionless Services

- Flow of information is one way- Both routing information and data- No connection is established- Each piece of data is routed independently- Send and pray- Small amounts of time critical data


Layer 3 Functions

The layer 3 OSI functions provided by the SCCP are:

1. RoutingThis function routes all incoming and outgoing messages to their appropriate destinations. SCCP recognize address formats other than the 14-bit Signalling Point of the MTP.

2. TransactionThe upper layers generate extended and detailed signalling addresses called Global Titles(GT).The SCCP translates the Global Titles into network addresses which can be used by the MTP to transfer the message through the network. The network addresses used by the MTP are:

- Signalling Point Codes- Subsystem Numbers (SCCP User Identities)


Layer 3 Functions

3. Management

The function of the SCCP Management is to provide procedures to maintain network performance by rerouting traffic in the event of failure or congestion of the network. SCCP Management are of 2 types:

-Management of the Signalling Points (SPPs)-Management of the Subsystem (SSNs)

SIF (SCCP)

Optionalpart

Mandatoryvariable part

MandatoryFix part

Message Type Label

Transaction Capabilities (TC)

The CCITT Blue Book has defined that:

“The overall objective of TC is to provide the means for the transfer of information between nodes..…

And to provide generic services to applications, while being independent of any of these.”

The introduction of TC has allowed:

application dependent functioning and protocols to reside in theTC-Users (e.g. MAP, OMAP).functions and protocols common to all applications to reside in TC.


The CCITT Blue Book has defined that:

The Scope of TC

Transaction Capabilities (TC) are implemented by:

the Transaction Capabilities Part (TCAP) in layer 7

and the Intermediate Services Part (ISP),

which provides the services of OSI layers 4,5, & 6.


Connectionless Services

For applications which require a Connectionless service the ISP is not needed and the TCAP interfaces directly with the SCCP.

Connection-Oriented Services

For a Connection-Oriented service the TCAP requires the services of the ISP.


Applications which use TC can be classified into two broad categories:

1) Real-Time SensitiveFor example, checking subscriber location in the mobile network.

2) Less Real-Time SensitiveFor example, downloading customer billing information.

The following applications are recognized by TC-Users:

Mobile radio applications (E.g. location registration)Registration, activation and invocation of supplementrary servicesAccess to specialized nodes Operation and maintenance applications (e.g. bulk data transfer)

O S I and C7

A Protocol is a set of rules governing the exchange to data between a layer in one system and a similar (peer) layer in another system. In OSI (Open System Interconnection) terms Protocols govern the exchange of data between peers. The development of an Open System is facilitated by the use of common, clear and standard rules for communication (protocols)

O S I and C7

OSI has defined seven layers.These layers fall into two groups:

Layers 1-3:Define the functions for transfer of informtion through the network.

Layers 4-7:Ddefine the functions related to end-to-end communication

Seven OSI layers:

Layer 1 (Physical)Concerned with the transmission and reception of bits across telecommunication link. Changes in the media are catered for this level.

Layer 2 (Data Link)Reliable transfer of data: error detection and correction, retransmission and flow control

Layer 3 (Network)Establishment, maintenance and release of connections between systems, addressing and routing.

Layer 4 (Transport)Isolates the layers above so that they are not affected by the characteristics of the network. Provides two-way, reliable and cost effective exchange of data between end points. This may involve the blocking or de-blocking of data streams. Also flow control and sequencing.

Seven OSI layers:

Layer 5 (Session)Provides means of communications between applications. Establishes, manages, and terminates end-to end connections (called sessions) between applications.

Layer 6 (Presentation)Transforms the data into the correct code format or syntax the particular application. e.g. ASCII code is a standard format. Encryption and decryption can also be included in this layer, if security and confidentially are required.

Layer 7 (Application)

Provides service support to the end users. It effects the data exchange between two end users. It also provides System Management and takes care of such things as Passwords and Logons.

Seven OSI layers:

Layer Enveloping

As data is transferred from layer to layer,each layer add its own Protocol Control Informationand then passes the data on to the next layer.

OSI is concerned with defining the Interfaces between layers rather then the implementation of the layers themselves.

The well defined structure gives OSI a modularity which means that a functional change in one layer will not affect functions in another.

OSI Primitives

Primitives are used for communication between layers. A layer communicates with itsadjacent layers using Primitives. A primitive is a command or the respective responseassociated with the services requested of a layer by another layer. Each layer has its ownset of primitives.

The syntax of a Primitive is:

LayerIdentifier

GenericName

SpecificName

Paramter

Layer providing theservice

of the service beingrequested e.g.connects, Data ,Disconnect

direction andfunction:1.Request2.Indication3.Confirm4.Response

Elements ofinformation whichare transferredbetween layers e.g.Called and CallingParty, User Data

C7 and OSI

MTP

SCCP

MAP OMAP

TCAP

ISP

ISUP TUP

TC

OSI

7

6

5

4

3

2

1

signalling

Documents

signalling message

signalling path

c7 signalling link

signalling status

signalling information

c7 signalling network

single signalling channel

signalling link sl