05 signaling in sccp

Signaling in SCCP

Chapter 5

OBJECTIVES: Upon completion of this chapter the student will be able to:

• Describe the functional structure of the SCCP

• Describe the general format of SCCP messages

• List some important SCCP procedures

Signaling No.7 in GSM

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5 Signaling in SCCP

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5 Signaling in SCCP

Table of Contents

Topic Page

INTRODUCTION TO SCCP.......................................................................101

PURPOSE OF SCCP..................................................................................104

SCCP SERVICES .......................................................................................107

PROTOCOL CLASSES..............................................................................109

SCCP PRIMITIVES .....................................................................................112

SCCP FUNCTIONAL STRUCTURE .......................................................115

SCCP MESSAGES .....................................................................................118

CONNECTIONLESS SERVICE ............................................................................. 118

CONNECTION-ORIENTED SERVICE .................................................................... 118

SCCP MESSAGE PARAMETERS...........................................................122

SCCP MESSAGE STRUCTURE..............................................................123

ROUTING LABEL................................................................................................ 123

MESSAGE TYPE CODE ..................................................................................... 123

PARAMETERS ................................................................................................... 123

FORMATS AND CODES............................................................................127

SCCP MESSAGE EXAMPLE ............................................................................... 128

ADDRESSING AND ROUTING IN SCCP...............................................129

SIGNALING PROCEDURES.....................................................................131

CONNECTION-ORIENTED PROCEDURE.............................................................. 131

CONNECTIONLESS PROCEDURE....................................................................... 132

SET-UP OF AN SCCP NETWORK..........................................................134

CALLING AND CALLED ADDRESS ...................................................................... 134


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SCCP ADDRESSING .......................................................................................... 135

SCCP ROUTING ................................................................................................. 138

SCCP NETWORK ............................................................................................... 138

SCCP NETWORK DEFINITION............................................................................. 141

SCCP ROUTING DEFINITION............................................................................... 142

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INTRODUCTION TO SCCP

In order to meet the needs for extended services the Signaling Connection Control Part (SCCP) was introduced by CCITT in 1984. Extended services are, e.g. communication processes with databases without any speech circuit in use.

SCCP provides additional functions to the ones offered by MTP. It is able to transfer circuit-related and non-circuit-related signaling information between exchanges and specialized centers in telecommunication networks.

SCCP supports both ConnectionLess (CL) and Connection-Oriented (CO) modes to transfer the signaling information in the Signaling No.7 network.

Connection-Oriented (CO) services allow the sending of signaling messages on an established signaling connection between SCCP users.

ConnectionLess (CL) services transfer signaling information without establishing a signaling connection. The routing of messages is based only on the address information included in each data packet.

For GSM, new protocols have been developed to handle the signaling procedures. The protocol between the MSC/VLR and the BSC is called Base Station System Application Part (BSSAP). It uses both Connection-Oriented (CO) and ConnectionLess (CL) modes for signaling. The protocol for communication between the MSC/VLR, the HLR and the GMSC is called Mobile Application Part (MAP). Its messages are always sent in ConnectionLess (CL) mode. See the figure 5-1.


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CL COSCCP

MTP

MAP

BSSAPTCAP

ISUP

TUPTUP

ISUP

TCAP

Figure 5-1 GSM protocols

Abbreviations:

BSSAP Base Station System Application Part CL ConnectionLess services CO Connection-Oriented services ISUP ISDN User Part MAP Mobile Application Part MTP Message Transfer Part SCCP Signaling Connection Control Part TCAP Transaction Capabilities Application Part TUP Telephony User Part

SCCP offers full addressing capacity. Full addressing capacity implies end-to-end signaling: an SCCP user can send a message to any other network node, without analysis of the called address, by layers above SCCP in an intermediate node.

The SCCP was introduced by CCITT in 1984 (Red Book). SCCP makes use of the functions provided by the Message Transfer Part (MTP).

The SCCP and the MTP together form the Network Service Part (NSP). The Network Service Part (NSP) meets the requirements for the Network Layer (layer 3) as defined in the OSI reference model and in the CCITT recommendation X.200. SCCP makes it possible to use a Signaling No.7 network based on MTP, as a carrier between applications using OSI protocols on upper layers.

The SCCP is described in the CCITT recommendations Q.711 -

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Q.716.


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PURPOSE OF SCCP

Non-Circuit-Related Signaling

Traditionally, all signaling messages in PSTN were related to a speech circuit, and MTP could provide all necessary signaling procedures.

Later one of the objectives was to implement additional functions in the signaling network, so that the users are able to send and receive messages without establishing a speech or data connection.

This objective was achieved by adding the SCCP layer above MTP. MTP was designed for circuit related link-by-link signaling and does not meet the requirements for non-circuit-related signaling.

With SCCP it is possible to transfer signaling messages from one point to another in the network when there is no need for setting up a speech or data connection. In these cases the messages do not relate to a circuit (speech or data).

An example of non-circuit-related signaling is the dialogue between the MSC/VLR and the HLR for location updating in the HLR.

The SCCP can transfer both circuit-related and non-circuit-related information. It may be signaling or user information.

Signaling Connections

Another purpose of the SCCP is to cater for both ConnectionLess (CL) and Connection-Oriented (CO) network services. MTP only offers CL transfer of signaling messages.

SCCP can handle the establishment, the data transfer, and the release of logical signaling connections. This is necessary for the Connection-Oriented (CO) services. It can also transfer ConnectionLess (CL) data messages, where all messages are routed independently, and do not belong to a logical connection.

The combination of MTP and SCCP is called Network Service Part (NSP). The NSP is said to provide a transparent logical connection service to the user (a layer above SCCP). Whether the service is CL or CO is determined by the SCCP user.

The ISDN-User Part (ISUP) is an example of a network service

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user that may use the CO functions for end-to-end signaling of short messages.

The SCCP interfaces to the upper layer (SCCP user) and to the lower layer (MTP) are described by means of primitives and parameters. An SCCP user is any functional entity that uses the network provided by SCCP.

Peer-to-peer communication between two SCCP users is performed by means of a protocol. The protocol caters for:

• The set-up of logical signaling connections;

• The release of logical signaling connections;

• The transfer of data with or without logical signaling connections.

The signaling communication between two nodes is described as an abstract model. See figure 5-2.

User of theNetwork Service

NSP

User of theNetwork Service

NSP

Node A Node B

A to B

B to A

Figure 5-2 Model for inter-node communication using the NSP

Abbreviations:

NSP Network Service Part

If a logical signaling connection is used between the origin node and the destination node, it may be divided into a number of connection sections separated by intermediate nodes.


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User

SCCP

User signaling connection

End Node End NodeIntermediate Node

MTP

SCCP

MTP

connection

section

connection

section

physical connection

SCCP

MTP

Figure 5-3 Signaling connection and connection sections

The SCCP Connection-Oriented (CO) services establish a temporary signaling connection, and they transfer data on this connection. The ConnectionLess (CL) services transfer data without establishing a signaling connection.

A connection is identified by a reference number, which uniquely identifies a signaling connection at the interface between the SCCP and an SCCP user.

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SCCP SERVICES

There are two kinds of services provided by the SCCP protocol:

• ConnectionLess services, (CL-services)

• Connection-Oriented services, (CO-services)

Connectionless Service

In the ConnectionLess (CL) service all signaling messages are routed independently.

All address information required to route the messages to their destination, must be included in each data packet. No logical connection is established between the end nodes. The CL transfer mode has only one phase: the data transfer phase.

The CL mode is typically used to transfer small amounts of real-time critical information between two users. Sending a request for a Roaming Number from the GMSC to the HLR or sending an alarm from an exchange to a remote O&M center are examples where the CL mode is used.

Another example of an application, which may use the CL service, is the cellular mobile application where a Mobile Switching Center (MSC) requests information from a database about the location of a mobile subscriber.

Connection-oriented Service

Connection-Oriented (CO) network service is a way to rationalize the exchange of signaling information between two network service users by establishing a logical connection between them. This logical signaling connection is achieved by giving a local reference number to all signaling messages belonging together.

CO service means the ability to transfer signaling messages over an established signaling connection. The signaling connection may either be temporary or permanent.

A temporary signaling connection is initiated and controlled by the service user. It is comparable with a dialed telephone connection.

A permanent signaling connection is controlled by O&M functions and is provided by the service operator on a semipermanent basis. It


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is comparable with a leased telephone line.

The CO transfer mode can be divided into three phases:

• Connection establishing

• Data transfer

• Connection release

An example of an application, which might use the CO service, is the Operation and Maintenance Application Part (OMAP), which requests the downloading of test results from an exchange in the network.

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PROTOCOL CLASSES

Four protocol classes (0-3) are provided by SCCP, two for ConnectionLess (CL) services and two for Connection-Oriented (CO) services. Each protocol class represents a set of services offered to the users of SCCP. See figure 5-4.

Protocol Classes

CL

CO

UDT, UDTS,XUDT, XUDTS

DT1, DT2

2 Basic Connection-orientated Class

3 Flow Control Connection-oriented Class (not provided)

0 Basic Connectionless Class

1 Sequenced (MTP) Connection-less Class (same SLS for all packets)

Figure 5-4 SCCP protocol classes

Abbreviations:

UDT Unitdata DT1 Data Form 1 DT2 Data Form 2 XUDT Extended Unitdata

The SCCP user determines the protocol class to be used for the transfer of the user data, called Network Service Data Units (NSDUs). SCCP adds control information to the NSDUs and sends them to the destination node.

SCCP classes 0, 1, and 2 are supported. The ConnectionLess (CL) protocol classes (0 and 1) are similar to the X.25 datagram service.

The CL protocol classes transfer one NSDU in the data field of a UnitDaTa (UDT) message or eXtended UnitDaTa (XUDT) message. When one message is not sufficient, the data is segmented and sent in several eXtended UnitDaTa (XUDT) messages. At the destination node, the NSDU is reassembled.


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Protocol Class 0 - Basic Connectionless Class

The Network Service Data Units (NSDUs) are passed to SCCP by the higher layers in the node of origin. These data units are delivered by the SCCP to the higher levels in the destination node.

The NSDUs are transmitted independently of each other and may therefore be delivered out-of-sequence. Thus, this protocol class corresponds to a pure ConnectionLess (CL) network service.

Protocol Class 1 - Sequenced Connectionless Class.

In protocol class 1, the features of class 0 are complemented by an additional feature which allows the higher layer to indicate to the SCCP that a given stream of NSDUs must be delivered in sequence.

The originating SCCP chooses the Signaling Link Selection (SLS) field for outgoing messages. This is based on a sequence control parameter received from the SCCP user. The SLS code chosen for a stream of NSDUs with the same sequence control parameter will be identical.

Thus, protocol class 1 corresponds to an enhanced CL service, where an additional feature offers the delivery of NSDUs in sequence.

Protocol Class 2 - Basic Connection-orientated Class

In protocol class 2, transfer of NSDUs between the SCCP users is performed by setting up a temporary or permanent signaling connection.

Each signaling connection is identified by a pair of reference numbers, referred to as Local Reference Numbers (LRN).

Messages belonging to a given signaling connection will contain the same value in the SLS field to ensure sequencing in the same way as for protocol class 1.

The quality of service, in terms of message loss, undetected errors, mis-sequencing, etc., is the same as that offered by MTP to the User Parts (UPs) . Thus the protocol class 2 is a simple Connection-Oriented (CO) network service.

The data is transferred by means of Data Form 1 (DT1) packets.

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Protocol Class 3 - Flow Control Connection-oriented Class

In protocol class 3, the features of protocol class 2 are complemented by additional flow control functions. With flow control the data rate is controlled. The flow control function permits a receiving node to limit the data flow from the sending node.

Moreover an additional capability of detecting message loss or sequencing errors is included. When such faults occur, the connection is reset and a corresponding notification is given to SCCP. Class 3 also allows bypassing the flow control system when sending expedited data units.

The normal data transfer is done using Data Form 2 (DT2) packets.


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SCCP PRIMITIVES

The communication between different layers is done by using service primitives.

SCCP

Upper Layers

MTP

Primitive

Primitive

Service user

Service provider

Service user

Service provider

Figure 5-5 SCCP Service Primitives

Service primitives are functional units containing specific data. They are sent between layers to invoke procedures. Primitives are used to describe the interfaces between different layers.

A lower layer provides a service to a higher layer, this is called the service user. All lower layers are transparent to the service user. The primitives contain parameters with information that is used in the interaction between the service user and the service provider.

Peer to Peer Communication

For a full peer-to-peer communication (such as the set-up of a signaling connection) there are four types of service primitives used:

• Request primitive

• Indication primitive

• Response primitive

• Confirmation primitive.

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SCCP SCCP

User ofnetworkservice

User ofnetworkservice

ConfirmationRequest Response

Protocol Elements

Indication1. 2.3.4.

Figure 5-6 Peer-to-peer communication - CO

The request and response primitives are sent from the service user to the provider. The indication and confirmation primitives are sent from the service provider to the user.

The Connection-Oriented (CO) services use all four types of primitives. The ConnectionLess (CL) services use only request and indication primitives.

Service Primitive Format

The general syntax of a primitive is shown in figure 5-7.

Layer Identif ier

GenericN a m e

Specif ic N a m e P a r a m e t e r

C O N N E C T C O N N E C T C O N N E C TC O N N E C T

E x a m p l e : N -N -N -N -

R E Q U E S TR E S P O N S E I N D I C A T I O NC O N F I R M A T I O N

Figure 5-7 Service primitive - general syntax and examples

Layer Identifier

The Layer Identifier, e.g. N for Network Service Part, specifies which layer is providing the service.

Generic Name

The Generic Name defines the action to be performed e.g.


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CONNECT for establishing a signaling connection, UNITDATA for data transfer - CL, DATA for data transfer - CO, DISCONNECT for releasing a signaling connection.

Specific Name

The specific name indicates the purpose of the primitive, e.g. a request for a service. It also indicates the direction of the primitive flow. See figure above.

Parameter

The parameters contain the elements of supporting information transferred by the primitive, e.g. the called address or user data.

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SCCP FUNCTIONAL STRUCTURE

The SCCP consists of the following parts:

• SCCP Connection-Oriented Part (SCOP)

• SCCP ConnectionLess Part (SCLP)

• SCCP Management and Operation Part (SCMOP).

Figure 5-8 shows the functional structure of the SCCP, some of the primitives and SCCP messages. For more details see Appendix A.

SCCP Users

MTP

N-CONNECT req.N-CONNECT resp.N-DATA req.N-DISCONNECT req.

N-UNITDATA req.

N-UNITDATA ind.N-CONNECT ind.N-CONNECT conf.N-DATA ind.N-DISCONNECT ind.

SCOP SCMOP SCLP

SCOC SCLC

SCOR SCLR

CR, CC, CREF,RLSD, RLC,DT1, ERR, IT

UDT, UDTS,XUDT, XUDTS

MTP - TRANSFERreq.

MTP - TRANSFERind.

MTP - TRANSFERreq.

MTP - TRANSFERind.

SCCP

Figure 5-8 SCCP functional structure


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Abbreviations:

SCLC SCCP ConnectionLess Control

SCLP SCCP ConnectionLess Part

SCLR SCCP ConnectionLess Routing

SMOP SCCP Management and Operation Part

SCOC SCCP Connection-Oriented Control

SCOP SCCP Connection-Oriented Part

SCOR SCCP Connection-Oriented Routing

SSA SubSystem Allowed

SSP SubSystem Prohibited

SST SubSystem Test

SCCP ConnectionLess Part (SCLP)

The SCCP ConnectionLess Part (SCLP) functions consist of two sub-functions:

• SCCP ConnectionLess Control (SCLC)

• SCCP ConnectionLess Routing (SCLR).

The ConnectionLess (CL) control-procedures allow a user of the SCCP to request transfer of data without first requesting establishment of a signaling connection.

Transfer of data is accomplished by including the user data in UnitDaTa (UDT) or eXtended UnitDaTa (XUDT) messages. These messages are delivered to SCCP Routing to determine the destination.

The SCCP ConnectionLess Routing (SCLR) function receives internal messages from SCLC and performs the necessary routing functions before passing the messages to MTP. SCLR also receives messages from MTP for routing and discrimination.

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SCCP Connection-Oriented Part (SCOP)

The SCCP Connection-Oriented Part (SCOP) functions consist of two sub-functions:

• SCCP Connection-Oriented Control (SCOC)

• SCCP Connection-Oriented Routing (SCOR).

The SCCP Connection-Oriented Control (SCOC) function provides procedures for the establishment, the supervision, and the release of a temporary signaling connection, as well as for the transfer of data on the connection.

The SCCP Connection-Oriented Routing (SCOR) function handles the routing of Connection-Oriented (CO) messages.

SCCP Management and Operation Part (SMOP)

The SCCP Management and Operation Part (SMOP) function provides procedures to maintain the network performance in the event of failure or congestion. SMOP updates the SCCP routing tables.


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SCCP MESSAGES

Messages are used by the peer-to-peer protocol of SCCP. All messages are uniquely identified by a message type code.

CONNECTIONLESS SERVICE

For the ConnectionLess (CL) service there are four different types of messages. See table 5-1.

Table 5-1: Coding of ConnectionLess (CL) SCCP Messages

Message Type Code Protocol Classes

UDT (Unitdata) 0000 1001 0,1 XUDT (Extended Unitdata) 0001 0001 0,1 UDTS (Unitdata Service) 0000 1010 0,1 XUDTS (Extended Unitdata Service) 0001 0010 0,1

An SCCP user may request transfer of data using ConnectionLess (CL) mode. The data will then be included in a UDT or XUDT message.

The length of the user data is checked by SCLC to determine if it can be carried in a single message, or if it must be segmented and carried in several messages.

If segmentation is necessary, the user data is transferred in several XUDT messages. If a single message may be used, the user data is transferred in a UDT or in an XUDT. The choice of message type depends on an application parameter.

The UDT or XUDT messages are returned to the originating SCCP when a message can not be delivered to its destination. UDTs and XUDTs also contain information about the cause of the problem.

CONNECTION-ORIENTED SERVICE

When using the Connection-Oriented (CO) service, it is not only necessary to send data transfer messages, signaling messages for the establishment and release of virtual connections must also be transmitted.

In table 5-2 the messages for Connection-Oriented (CO) services

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are shown.

Table 5-2: Coding of Connection-Oriented (CO) SCCP Messages

Message Type Code Protocol Class

CR (Connection Request) 0000 0001 2 CC (Connection Confirm) 0000 0010 2 CREF (Connection Refused) 0000 0011 2 RLSD (Released) 0000 0100 2 RLC (Release Complete) 0000 0101 2 DT1 (Data Form 1) 0000 0110 2 ERR (Protocol Data Unit Error) 0000 1111 2 IT (Inactivity Test) 0001 0000 2

Connection Establishment Phase

A Connection Request (CR) message is sent by the calling SCCP to request the set-up of a virtual signaling connection between two SCCP users. On reception of the CR message the called SCCP initiates the set-up of the signaling connection, if possible.

A Connection Confirm (CC) message is sent by the called SCCP to inform the calling SCCP that the signaling connection has been set-up. On reception of the CC message the calling SCCP completes the set-up of the signaling connection, if possible.

A Connection REFused (CREF) message is sent by the called or any intermediate SCCP to inform the calling SCCP that the set-up of the signaling connection has been refused.

Data Transfer Phase

A Data Form 1 (DT1) message is used to transfer data transparently between two SCCP nodes through a signaling connection.

An Inactivity Test (IT) message may be sent by either end of a connection section to check if the section is active at both ends and to audit the consistency of connection data at both ends.

A Protocol Data Unit Error (ERR) message is sent on detection of any protocol error.


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Connection Release Phase

A ReLeaSeD (RLSD) message is sent in the forward or backward direction to indicate that the sending SCCP wants to release the signaling connection. The message indicates that the connection is ready to be put into the idle state on receipt of the ReLease Complete (RLC) message.

A ReLease Complete (RLC) message is sent in response to the RLSD message when the connection has been brought into the idle condition.

Implementation of Signaling Connection

The implementation of the signaling connection in a node is done by means of a Local-Reference-Number table (LRN table).

The Local Reference Number (LRN) uniquely identifies a signaling connection in a node. It is an internal working number chosen by each node independently from the destination node.

A signaling connection is set-up by the CR and CC messages. These messages leave behind a trace in every node passed, so that the user data packets can follow the same path. During the connection establishment both a source and a destination LRN are assigned independently to a Connection Section. Once the destination LRN is known, it must be included in all messages transferred on a Connection Section.

In figure 5-9 the nodes A, B, and C are involved in the virtual connection. Node A includes an originating LRN (A1) into the CR message. It sends the message to B on Signaling Link 1. Node B also chooses an LRN (B2) and sends it with the CR message to node C. Signaling Link 2 is used. The numbers A1 and B2 are linked and stored in the LRN table.

Node C replies using the CC message and includes the originating LRN C1 and the destination LRN B2. Node B recognizes B2 and generates an LRN B1, stores it in the table and sends it with the CC message to node A.

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A B CSL1 SL2

CR (A1,Dest) CR (B2,Dest)

CC (C1,B2)CC (B1,A1)

LRN TABLE

B1-C1-SL2SL1-A1-B2

DT1(B1)

DT1(A1) DT1(B2)

DT1(C1)

RLSD(B1) RLSD(C1)

RLC(A1) RLC(B2)

Figure 5-9 Implementation of Signaling Connection

After connection establishment the LRN table is used for sending the user data packets (DT1 messages) containing the destination LRN which is then recognized at the destination.


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SCCP MESSAGE PARAMETERS

Table 5-3 shows some of the parameters used in SCCP messages.

Table 5-3 : Parameters in SCCP Messages

Parameter Name Parameter Name Code Destination Local Reference 0000 0001 Source Local Reference 0000 0010 Called Party Address 0000 0011 Calling Party Address 0000 0100 Protocol Class 0000 0101 Release Cause 0000 1010 Error Cause 0000 1101 Data 0000 1111 Segmentation 0001 0000

The Local Reference Number (LRN) parameter uniquely identifies a signaling connection within a node. At least one LRN is to be found in any message sent on a Signaling Connection (Connection-Oriented (CO) service).

The Destination Local Reference identifies a Connection Section for incoming messages. The Source Local Reference identifies a Connection Section for outgoing messages.

The Called Party Address identifies the destination Signaling Point (SP) and / or the SCCP user. The Calling Party Address identifies the originating Signaling Point (SP) and / or the SCCP user. The addresses can be a combination of a Global Title (GT), a Signaling Point Code (SPC), and a SubSystem Number (SSN). The SubSystem Number (SSN) identifies the SCCP user.

The Release Cause parameter contains the reason for the release of a connection. The Error Cause parameter indicates the exact protocol error, e.g. LRN mismatch.

The Data parameter is a variable length field containing SCCP user data coming from upper layers or from SCCP management.

The Segmentation parameter indicates that a message has been segmented.

Detailed information about all of the parameters can be found in the ITU-T recommendations Q.712 and Q.713.

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SCCP MESSAGE STRUCTURE

The Signaling Information Field (SIF) of each SCCP message consists of the fields shown in the figure 5-10.

Mandatoryvariable Part

Mandatoryfixed Part

MessageType Code

RoutingLabel

Parameters

OptionalPart

Figure 5-10 General SCCP Message Structure

ROUTING LABEL

The Routing Label is used by MTP to route a message towards its Destination Point (DP). It contains the Destination Point Code (DPC), the Originating Point Code (OPC), and the Signaling Link Selection (SLS) field.

MESSAGE TYPE CODE

The Message Type Code is used to distinguish between different types of messages. It uniquely defines the function and format of each SCCP message.

PARAMETERS

The remaining fields of the message represent the parameters and the user data (which is a parameter as well). Some of the parameters are mandatory and others are optional. They can be of fixed or variable length.

Each SCCP message has its specific number and type of parameters. The SCCP user decides which message is sent.

An SCCP message usually contains the calling party and / or called party address.


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Mandatoryfixed Part

MessageType Code

RoutingLabel

UserData

Calling PartyAddress

Called PartyAddress

Pointers

8 bit

Protocol Class

Parameters

Figure 5-11 Divisions of Parameter Groups

Protocol Class

The Protocol Class parameter defines the classes for either Connection-Oriented (CO) or ConnectionLess (CL) services and its subdivisions.

Pointers

The length of the parameters may vary, so pointers are used to indicate the beginning of a parameter.

Called Party Address

The Called Party Address parameter contains three fields. These are the Length of Address (LA), the Address Indicator (AI), and the Address. See Figure 5-12.


Mandatoryfixed Part

MessageType Code

RoutingLabel

UserData


Called PartyAddress

Pointers

8 bit

Protocol Class

Address AddressIndicator (AI)

Length ofAddress (LA)

Parameters

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Figure 5-12 Called Party Address parameter

The address consists of one or any combination of the following elements:

• Signaling Point Code (SPC),

• Global Title (GT),

• SubSystem Number (SSN).

In figure 5-13 the elements of the Address Field are shown.


Mandatoryfixed Part

MessageType Code

RoutingLabel

UserData


Called PartyAddress

Pointers

8 bit

Protocol Class

GlobalTitle

SubsystemNumber

twoZeroes

Signaling Point Code

Address AddressIndicator (AI)

Length ofAddress (LA)

Parameters

Figure 5-13 Subdivisions of the Address Field

Some of the address elements may or may not be present in the Address field.

Thus, the Address Indicator (AI) is needed to indicate the type of address information contained in the Address field, e.g. if the Global Title (GT) is included or not.

The SubSystem Number (SSN) identifies an SCCP user, e.g. ISDN User Part (ISUP), Mobile Switching Center (MSC), or Home Location Register (HLR). Note that subsystems mentioned in this context, are the SCCP users, not to be confused with the AXE subsystem concept.

The Global Title (GT) is an address, such as dialed digits, which does not explicitly contain information that would allow routing in the


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signaling network, i.e. a translation function is required in SCCP. A Global Title (GT) translation results in a Global Title Routing Case.

User Data

The Data parameter field is a variable length field containing SCCP user data to be transferred transparently between the SCCP users.

Message Signal Unit

All information added in SCCP is assembled as either a fixed or a variable parameter. After adding the Message Type Code and the Routing Label SCCP hands over the complete message to its lower layer, the MTP. The whole SCCP message is put into the Service Information Field (SIF) of the MSU. See figure 5-14.

SCCP

MTP

CallingAddress

CalledAddress

Pointer toUser Data

Pointer toCalling Add.

Pointer toCalled Add.

ProtocolClass

Message

SIF SIOCK Error Correction FF LI

physical

Userdata

MSU

RoutingLabelCode

Figure 5-14 Embedded SCCP Message

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FORMATS AND CODES

The SCCP messages are carried on Signaling Data Links by means of MSUs. The Service Indicator (SI) in the Service Information Octet (SIO) is coded 0011 for SCCP messages.

The Signaling Information Field (SIF) of each MSU containing an SCCP message consists of an integral number of octets (max. 272 octets).

Like ISUP messages each SCCP message contains a number of parameters with signaling information. These parameters may have fixed or variable length. They can be mandatory or optional. An SCCP message consists of the parts shown in figure 5-15.

Order of octet

Routing Label Message Type Code

Mandatory parameter A

Mandatory parameter FPointer to parameter M

Pointer to parameter PPointer to start of optional part

Length indicator of parameter M

Parameter M

Length indicator of parameter P

Parameter PPointer name = X

Length indicator of parameter X

Parameter X

Parameter name = ZLength indicator of parameter Z

Parameter Z

End of optional parameters

Optional Part.O

Mandatory variable Part. V

Mandatoryfixed Part . F

Order of bit transmission

transmission

Figure 5-15 SCCP messages Structure

The structure of an SCCP message is very similar to that of an ISUP


– 128 – EN/LZT 123 4734 R3A

message. One difference is that the Circuit Identification Code (CIC) is missing for obvious reasons.

SCCP MESSAGE EXAMPLE

An example of an SCCP message is the Connection Request message. Its structure is shown in table 5-4.

Table 5-4 : Structure of the Connection Request message

Parameter Name Type Length in Octets

Message Type M,F 1 Source Local Reference M,F 3 Protocol Class M,F 1 Called Party Address M,V 3 minimum Calling Party Address O 4 minimum Credit O 3 Data O 3 - 130 Hop Counter O 3 End of Optional Parameters O 1

Abbreviations :

F: Fixed length M: Mandatory O: Optional V: Variable length

The structure of the SCCP messages is described in the ITU-T recommendation Q.713.

5 Signaling in SCCP

EN/LZT 123 4734 R3A – 129 –

ADDRESSING AND ROUTING IN SCCP

Before the Telephony User Part (TUP) uses the MTP to transfer messages, the called party address (the B-Number) is analyzed to determine the DPC. Then the DPC is used by MTP to route the signaling message to the next node.

When the SCCP messages are sent, two parameters, the Called Party Address and the Calling Party Address are used to route the messages to the next SCCP node.

Both the Called and the Calling Party Addresses are included in the ConnectionLess (CL) messages UnitDaTa (UDT) and eXtended UnitDaTa (XUDT), while only the Called Party Address is included in the Connection-Oriented (CO) message Connection Request (CR). Other CO messages do not contain the Called and the Calling Party Address. They are transferred on Connection Sections, i.e. they are sent to a predetermined destination. The Destination Local Reference and the Source Local Reference numbers are used.

The SCCP addresses are a combination of a Global Title (GT), a Signaling Point Code (SPC), and a SubSystem Number (SSN).

There are two basic categories of addresses for SCCP routing: the Global Title (GT) and the Destination Point Code (DPC) in combination with the SubSystem Number (SSN).

Global Title

A Global Title (GT) is a destination address used by SCCP messages. Only the SCCP layer can use the GT. SCCP translates the GT into a DPC as a part of the routing label. MTP uses the DPC for routing in the signaling network.


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DPC and SSN

A Destination Point Code (DPC) and a SubSystem Number (SSN) allow direct routing by the SCCP and MTP, i.e. the translation function of the SCCP is not required.

A SubSystem Number (SSN) is an address identifying a part of an SCCP node which uses the SCCP, either directly like ISUP or indirectly via the transaction capabilities like MAP. Examples of subsystems are:

- SCCP management - ISDN User Part (ISUP) - Operation, Maintenance and Administration Part (OMAP) - Mobile Application Part (MAP)

Called Party Address

The figure 5-16 shows the format of the Called Party Address parameter.

GT SSN SPC AddressIndicator

RoutingIndicator

GTIndicator

SSNIndicator Indicator

SP Code

Figure 5-16 Called Party Address

The Address Indicator (AI) indicates the type of address information contained in the address field. One or more of the following elements can be included: SPC, SSN, GT.

The Routing Indicator determines whether routing should be based on SSN or GT.

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EN/LZT 123 4734 R3A – 131 –

SIGNALING PROCEDURES

CONNECTION-ORIENTED PROCEDURE

Connection Establishment

The connection establishment procedure consists of functions required to establish a temporary signaling connection between two SCCP users. An SCCP user initiates connection establishment by invoking the N-CONNECT request primitive. See figure 5-17.

Originating IntermediateNodeNode

Destination Node

SCCPuser

SCCP SCCP SCCP SCCPuser

N-CONNECTrequest CR

CRN-CONNECTindication

N-CONNECTresponseCC

CCN-CONNECTconfirm

source LRN

dest LRN

source LRN

dest LRN

DT1DT1

DT1dest LRN dest LRNN-DISCON.

RLSDRLSD

N-DISCON

RLCRLC

request

indication

dest LRNdest LRN

dest LRNdest LRN

N-DATA indication

N-DATA request

Figure 5-17 Sequence of Connection-Oriented (CO) data transfer

The procedure starts with the originating SCCP sending a Connection Request (CR) message. The message includes the addresses of the source and destination SCCPs.

On reception of the CR message the destination SCCP answers by


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sending a Connection Confirm (CC) message.

These two messages are sent to set-up a virtual connection between the end users. The signaling connection will be used for transferring the user data in DT1 messages. However it is possible to include some user data in CR and CC messages as well. This technique is known as Piggybacking.

The intention of establishing a virtual connection is to leave a trace in all nodes. That way the user data can follow a pre-determined path. During the connection establishment both a source and a destination local reference number are assigned independently to a Connection Section. Once the destination reference number is known it will be used to transfer the messages.

Data Transfer

An SCCP user at a node requests transfer of user data by invoking the N-DATA request primitive. Data Form 1 (DT1) messages are used to transfer the user information on the signaling connection.

Connection Release

A connection release is initiated when the N-DISCONNECT request primitive is invoked by an SCCP user. Two messages are sent to release the connection: the ReLeaSeD (RLSD) and the ReLease Complete (RLC).

CONNECTIONLESS PROCEDURE

The ConnectionLess (CL) procedure allows an SCCP user to request transfer of data without first requesting establishment of a signaling connection.

The N-UNITDATA request primitive is invoked by the SCCP user to request ConnectionLess (CL) data transfer. The N-UNITDATA indication primitive is used by the destination SCCP to indicate delivery of data to the destination user.

Transfer of the user data is accomplished by including the user data in UnitDaTa (UDT) messages. Called and Calling Party Addresses are parameters included in the messages. These parameters contain the information necessary for the SCCP to determine a destination and an originating node.

5 Signaling in SCCP

EN/LZT 123 4734 R3A – 133 –

Originating IntermediateNodeNode

Destination Node

SCCPuser

SCCP SCCP SCCP SCCPuser

N-UNIT DATArequest

UDTUDT N-UNIT DATA

indication

N-UNIT DATA request

UDTUDTN-UNIT DATA indication

calling/calledparty address

calling/calledparty address

Figure 5-18 ConnectionLess (CL) data flow


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SET-UP OF AN SCCP NETWORK

An SCCP network must be set-up, to be able to send messages. The following is an example of SCCP message routing. An MSC/VLR sends a MAP message to an HLR. See figure 5-19.

HLRMSC/VLR

calling addresscalled address47401123005 46705456008

Figure 5-19 An MCS/VLR sends MAP messages to an HLR

CALLING AND CALLED ADDRESS

Addresses must be defined for the involved network nodes. SCCP includes these addresses in its messages.

Different elements (SPC, SSN, GT) may be present in the Calling Party Address and in the Called Party Address.

The Address Information (AI) is a part of the Global Title (GT). It must be defined for the MSC/VLR and the HLR.

The structure of the Address Information (AI) is defined by the Numbering Plan (NP), which is also a part of the Global Title (GT). The AI often uses the Numbering Plan E.164 and has the same structure as a Mobile Station ISDN Number (MSISDN). The elements of AI are:

- Country Code (CC) - National Destination Code (NDC) - Subscriber Number (SN)

In the example 47 401 123005 and 401 123005 are defined as the international and national Address Information (AI) for the MSC/VLR.

5 Signaling in SCCP

EN/LZT 123 4734 R3A – 135 –

46 705 456008 and 705 456008 are defined as the international and national Address Information (AI) for the MSC/VLR.

SCCP ADDRESSING

A MAP message contains MTP, SCCP, TCAP, and MAP parameters. SCCP enables a signaling network to route MAP messages. Routing is always based on addresses. SCCP uses the following addresses:

• Called Party Address

• Calling Party Address

SCCP addressing is very flexible and makes use of three separate elements:

• Signaling Point Code (SPC)

• Global Title (GT)

• SubSystem Number (SSN)

See figure 5-20.

MTP

SCCP

TCAP

MAP

Routing Label DPC

OPC

Calling Address

Called AddressGlobal Title

SSN

AI

NA

NP

TT

SPC

Figure 5-20 SCCP addressing

Abbreviations:

AI Address Information DPC Destination Point Code MAP Mobile Application Part MTP Message Transfer Part


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NA Nature of Address NP Numbering Plan OPC Originating Point Code SCCP Signaling Connection Control Part SSN SubSystem Number TCAP Transaction Capabilities Application Part TT Translation Type

Global Title

The Global Title (GT) is of variable length, and includes the following information fields:

- Address Information (AI) - Nature of Address (NA) - Numbering Plan (NP) - Translation Type (TT)

The GT does not contain information that allows routing in the MTP signaling network. The translation function is required.

The following sections contain details and typical values for the information fields previously listed.

Address Information (AI)

The Address Information (AI) is an address according to the Numbering Plan (NP) indicated.

In the previous example the number 47 401 123005 is the international Address Information (AI) for the MSC /VLR. 47 is the country code for Norway. 401 is the National Destination Code, that identifies a specific, Norwegian GSM network operator. 123005 identifies one MSC/VLR in the network.

Nature of Address (NA)

The Nature of Address (NA) indicates whether the Address Information (AI) is national or international. The coding is done in the following way:

3 - national 4 - international

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EN/LZT 123 4734 R3A – 137 –

Numbering Plan (NP)

The Numbering Plan (NP) indicates the numbering scheme from which the address originates:

1 - ISDN / Telephony Numbering Plan (E.163 / E.164), e.g. MSISDN, GT address.

7 - ISDN / Mobile Numbering Plan (E.214), e.g. IMSI, MGT (Location Updating).

Translation Type (TT)

A Global Title (GT) requires a translation function. The Translation Type (TT) directs the message to the appropriate Global Title (GT) translation function. Thus it is possible to have different translation results for GTs, that only differ by their Translation Type (TT) value.

SubSystem Number (SSN)

The terminating node examines the SubSystem Number (SSN) to identify the concerned SCCP user.

The SubSystem Number (SSN) may have the following values.

Table 5-5: Coding of Subsystems

Subsystem Name SSN SSN not known / not used 0 SCCP management 1 ISUP 3 OMAP 4 MAP 5 HLR 6 VLR 7 MSC, GMSC 8 EIR 9 AUC 10 SC 12 BSC (BSSAP), ANSI signaling 222 BSC (BSSAP), CCITT signaling 254


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SCCP ROUTING

Two basic categories of addresses are distinguished by SCCP routing:

• Global Title (GT):

A Global Title (GT) does not explicitly allow routing in the signaling network. It requires the SCCP translation function.

• Destination Point Code (DPC) and SubSystem Number (SSN):

A DPC and an SSN allow direct routing by the SCCP and MTP. The translation function is not required.

Global Title translation

The results of the Global Title (GT) translation are parameters, which are required to forward the message in the network or to distribute the message.

An SCCP message entering or originating from an exchange must either be a message to be routed to another exchange or a terminating message. The SCCP therefore requires data to determine the handling of the message. This data is called Global Title Series (GTS). It is similar to the B-Number analysis table.

By analyzing the Global Title (GT) of the called address the SCCP will either route the message to another node with the help of Global Title Routing Case (GTRC) or terminate the message in the node.

In the terminating node the message will be distributed to the correct user with help of the SubSystem Number (SSN).

SCCP NETWORK

Imagine a network with five nodes A, B, C, D and E.

Node A is an international Exchange.

Nodes B, C, and D are national exchanges.

Node E is an MSC/VLR/HLR with own calling address 46 705 230.

5 Signaling in SCCP

EN/LZT 123 4734 R3A – 139 –

SCCP

MTP

SCCP

MTP

SCCP

MTP MTP

SCCP

MTP

Node A Node B

Node C, SP=2-50

Node D Node ESP=0-20SP=2-30

SP=2-40 SP=2-60 SP=2-70

MAP

TCAP

MSC/VLR/HLR

Figure 5-21 SCCP example network

The nodes have the following defined Signaling Point Codes (SPC):

Node A SPC = 0-20 and SPC = 2-30

Node B SPC = 2-40

Node C SPC = 2-50

Node D SPC = 2-60

Node E SPC = 2 -70

Node A has got two Signaling Point Codes (SPC). SPC 0-20 is used for signaling in the international network, SPC 2-30 is used for the national signaling.

A MAP message is received in node A from the international network. Figure 5-22 shows the SCCP and MTP data included in the message.


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MTP

SCCP

TCAP

MAP

DPC

OPC

Calling Address


SSN=6

Address

NA

NP

TT

Global Title

SSN=7

Address

NA

NP

TT

Routing Label 0-20

0-30

0

1

4

4740180

0

1

4

46705230

Figure 5-22 A received MAP message

According to the address information the message will be routed to the HLR functionality in the MSC/VLR/HLR node. The called Global Title (GT) belongs to the MSC/VLR/HLR node and SSN=6 indicates the HLR. The message is sent from a Norwegian VLR.

The next sections in this chapter will show how the message is routed through the network to its destination.

In the example network the SCCP functionality is included in node A, B, C and E. Node D is a stand-alone Signaling Transfer Point (STP) with no SCCP software.

5 Signaling in SCCP

EN/LZT 123 4734 R3A – 141 –

SCCP NETWORK DEFINITION

The SCCP network is a logical network of Signaling Points (SPs) , each containing an SCCP. These SPs may or may not have subsystems attached to them.

This SCCP network requires that an MTP network to exist below it. Before the SCCP network can be defined, the complete MTP network including Signaling Points (SPs) , Signaling Links (SLs) , signaling Link Sets (LSs) , and routes must be defined.

Definition of Cooperating SCCP Nodes

When the MTP network is defined, the SCCP network can be set-up. In each SCCP node the cooperating SCCP nodes must be defined at SCCP level. It is not necessary to define the own node as an SCCP node.

Node A has got B and C as cooperating SCCP nodes.

Node B has got A and E as cooperating SCCP nodes.

Node C has got A and E as cooperating SCCP nodes.

Node E has got B and C as cooperating SCCP nodes.

As an example, in node A, the nodes B and C must be defined as cooperating nodes.

Note that node D includes MTP functions only. It is therefore not defined as a cooperating node.

Subsystem Definition

The next step is to define the subsystems used in the cooperating nodes. Node E has three subsystems: MSC (SSN=8), VLR (SSN=7), and HLR (SSN=6).

The SCCP nodes must define the subsystems in their cooperating nodes. In the previous example nodes B and C must define SSN 6, 7, and 8 in their exchange data.


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SCCP ROUTING DEFINITION

After the SCCP network has been defined, it is possible to define the SCCP routing.

The MAP message is received from the international network in node A. The MTP reads the DPC of the message and compares it with its own SPC. Since DPC and SPC are equal, MTP will pass the messages to the SCCP layer.

To be able to route SCCP messages, Global Title Routing Cases (GTRCs) must be defined. GTRCs are routing specifications used to route messages, containing a Global Title (GT). The messages are routed to one Signaling Point (SP) or to one subsystem attached to an SP.

The received Global Title (GT) is translated to a GTRC. The GTRC is a routing specification that contains a primary and an optional secondary routing alternative. The GTRC leads to Signaling Points (SPs) . The Signaling Point (SP) can be defined to be either a terminating (PTERM/STERM) or an intermediate (PINTER/SINTER) Signaling Point (SP).

If the SP is defined to be terminating, it is also possible to define the destination subsystem by giving the corresponding SubSystem Number (SSN). The secondary routing alternative gives a possibility to have duplicated subsystems in the network.

In the example GTRC has the value 1. The message will be routed to node B as a first alternative. The second alternative will be to route the message to node C. The final destination of the message is node E.

From GTRC 1 it is possible to get the Primary Signaling Point (PSP) 2-40 and the Secondary Signaling Point (SSP) 2-50.

The second alternative is chosen, if there are communication problems with node B.

5 Signaling in SCCP

EN/LZT 123 4734 R3A – 143 –

SCCP

MTP

SCCP

MTP MTP

SCCP

MTP

SCCP

MTP

Node A Node B

Node C, SP=2-50

Node D Node ESP=0-20SP=2-30 national

SP=2-40 SP=2-60 SP=2-70

MAP

TCAP

MSC/VLR/HLR

GT

analysis

GTRC

analysis

GT46705

GTRC

1

PSP2-40, PINTER

SSP2-50 , SINTER

Figure 5-23 SCCP routing at node A

Routing Definition in Node A

The principle of analyzing the Global Title (GT) is very similar to the analysis of the B-number in the Traffic Control Subsystem (TCS). The GT analysis returns a Global Title Routing Case (GTRC), which is analyzed to find the Primary Signaling Point (PSP) and the Secondary Signaling Point (SSP).

In node A it is specified, that the number series 46 705 gives GTRC 1 as a translation result. The SCCP message is routed according to this routing case.

Analysis data must be defined for the different GTRCs. Node B is defined as the first routing alternative (PSP) and node C is defined as the secondary routing alternative.

The routing specification is made to Signaling Points (SPs). The Signaling Points (PSP or SSP) can be defined to be either an intermediate (PINTER / SINTER) or a terminating (PTERM /


– 144 – EN/LZT 123 4734 R3A

STERM) Signaling Point (SP).

PINTER stands for primary intermediate and means that the next node is not the terminating node but an intermediate node.

Node B is the primary intermediate node. If the primary alternative is chosen, the next GT translation will take place in node B. SINTER stands for secondary intermediate node. If the second alternative is used, a GT translation will take place in node C.

By means of the previously defined data, the MAP message will be routed to node B.

The contents of the MAP message sent from A to B is shown in figure 5-24.

MTP

SCCP

TCAP

MAP

DPC

OPC

Calling Address


SSN=6

Address

NA

NP

TT

Global Title

SSN=7

Address

NA

NP

TT

Routing Label 2-40

2-30

0

1

4

4740180

0

1

4

46705230

Figure 5-24 Contents of a MAP message, sent to B

The MAP message from node A is received in node B. The MTP reads the DPC of the message and compares it with its own SPC.

5 Signaling in SCCP

EN/LZT 123 4734 R3A – 145 –

Since the DPC and its own SPC are equal, MTP will pass the messages to SCCP where the GT is analyzed.

SCCP

MTP

SCCP

MTP MTP

SCCP

MTP

Node A Node B Node D Node ESP=0-20SP=2-30 national

SP=2-40 SP=2-60 SP=2-70

MAP

TCAP

MSC/VLR/HLR

GT

analysis

GTRC

analysis

GT46705

GTRC

1

PSP2-70 , PTERM

SSP

Figure 5-25 SCCP routing in node B

Routing Definition in Node B

In node B it is specified that the number series 46 705 456 gives GTRC 5 as a translation result. Node E is defined as the Primary Signaling Point (PSP) and as the terminating node (PTERM).

PTERM stands for primary terminating and means that the message shall be terminated in the next SCCP node. Node D is only an MTP network node.

The information, that the next node is a terminating node, e.g. PTERM, travels with the message. On reception of the message, the SCCP in the next node is informed that it is the terminating node. Thus it can skip the GT analysis and save processor capacity. In the example node E is the primary terminating node.

The MAP message will be routed from node B to node E. The contents of the MAP message is shown in figure 5-26.


– 146 – EN/LZT 123 4734 R3A

MTP

SCCP

TCAP

MAP

DPC

OPC

Calling Address


SSN=6

Address

NA

NP

TT

Global Title

SSN=7

Address

NA

NP

TT

Routing Label 2-70

2-40

0

1

4

4740180

0

1

4

46705230

Figure 5-26 Contents of a MAP Message sent to Node E

In node D the message will be transferred to node E. The discrimination function of MTP reads the DPC in the label. As the message is destined for a different Signaling Point (SP), (DPC 2-70), the message is passed to the routing function of MTP and is sent on the Signaling Link (SL) to node E.

Message Handling in Node E

When the message is received in node E, MTP will pass it to SCCP.

SCCP will then pass the message to TCAP. Global Title (GT) translation is not necessary, since the message was marked as terminating in node E.

TCAP will contact one of the incoming coordinator blocks MAPTC or HMAPTC. MAPTC is the incoming coordinator for the

5 Signaling in SCCP

EN/LZT 123 4734 R3A – 147 –

MSC/VLR and HMAPTC is the equivalent block for the HLR.

If the SSN is 6 (HLR), HMAPTC will be called. If the SSN is 7 or 8 (VLR or MSC), MAPTC will be called. The example message is sent to the HLR.

SP=2-40 SP=2-60 SP=2-70

SCCP

MTP

SCCP

MTP MTP

SCCP

MTP

Node B

Node C, SP=2-50

Node D Node E

MAP

TCAP

HMAPTC MAPTC

SSN= 7 or 8SSN = 6

Figure 5-27 SCCP Network

If the message had not been marked as terminating (PTERM) in node B, then node E would have to analyze the GT (as described earlier). The result would be a terminating case.

SCCP

MTP

MTP

SCCP

MTP

Node C, SP=2-50

Node D Node ESP=2-60 SP=2-70

MAP

TCAP

MSC/VLR/HLR

analysis

GT46705230

TERMGT

Figure 5-28 SCCP Network

In node E it is specified, that the number series 46 705 456008 gives the terminating translation result (TERM).

Technology

05 signaling in sccp