Protocol stack for SS7 signalling over MTP

The standard protocol stack for SS7 is shown in Figure 3.25. The layers are analogous to the layers of the ISO OSI seven-layer model, with the layers in an SS7 stack referred to as parts rather than layers. The lower three layers are collectively referred to as the message transfer part (MTP).

Message transfer part level 1

This is functionally equivalent to the OSI layer 1 and defines the various physical layer interfaces. Messages are usually carried over 56 kbps or 64 kbps links for the narrow- band services. E1 (2048 kbps) consisting of 32×64 kbps channels and DS1 (1544 kbps) consisting of 24×64 kbps channels are also supported. These TDM links ensure that the bits making up the message arrive in the correct order.

Message transfer part level 2

This layer ensures that there is a reliable connection between two datalink network ele- ments. It includes error checking, flow control and sequencing. If an error is detected, MTP2 can ask for a retransmission. Layer 2 is capable of monitoring the state of the link and enables peer devices to communicate link information to one another. This

MTP level 1 MTP level 2 MTP level 3

MTP

TUP ISUP TCAP

Physical Data Link Network Transport Session Presentation Application SCCP OSI SS7

Figure 3.25 SS7 protocol stack

information could be, for example, an indication that the link is congested or has failed. This is functionally equivalent to the OSI layer 2.

Message transfer part level 3

This layer extends the functionality of level 2 so that signalling messages can be trans- ported over a complex network, i.e. there does not need to be any direct connection between signalling points (network elements). This layer deals with routing, re-routing when a link fails and congestion control. It has many similarities with the IP layer on the Internet.

Signalling connection control part (SCCP)

SCCP sits on top of the message transfer part and provides both connectionless and connection-oriented network services. Together with MTP it is referred to as thenetwork service part(NSP). Signalling points can have a number of attached applications operating simultaneously; SCCP introduces the subsystem number (SSN) to ensure that the correct application is accessed. This layer can be seen as analogous to TCP in the Internet suite of protocols. SCCP also provides address translation capabilities, known as global title translation (GTT). SCCP is not required for services such as TUP and ISUP and thus in Figure 3.25 it does not extend across the complete stack.

Telephone user part (TUP)

This deals with call setup and release but is designed for the traditional analogue circuits, which are still dominant in some parts of the world. It has been largely superseded by ISUP. TUP was one of the first applications to be defined and as such did not have provision for ISDN services.

ISDN user part (ISUP)

This layer also defines the messages that are to be used for call setup, modification, tear down, etc. It provides the services required by ISDN. ISUP supports basic telephony in a similar manner to TUP; however, it has a greater variety of messages which enable many more services to be offered. Calls that originate and terminate at the same SSP do not require the services of ISUP. Examples of ISUP messages are: Answer, Charge information, Connect, Identification request, Information request and Release.

Transaction capabilities application part (TCAP)

TCAP provides a structured method to request processing at a remote node. It defines the information flow and can report on the result. Queries and responses between SSPs and SCPs are sent via TCAP messages. Typical examples of TCAP services are registration of roaming users in a mobile network, and intelligent network services such as free-phone or calling card. TCAP is employed for the non-circuit-related information exchange, for example, if a subscriber’s PIN is required for using a calling card, mobile application part (MAP; see below) messages which are sent between mobile switches and databases to support subscriber authentication, equipment identification and roaming are carried by TCAP. As a mobile subscriber roams into a new MSC area, the VLR requests the sub- scriber’s profile information from the HLR using MAP information carried within TCAP messages. Although SCCP can locate the database, the actual query for data is performed by a TCAP message. TCAP can also be used for transporting billing information.

GSM user part

A cellular network requires additional features to a fixed network; for example, a mobile subscriber needs to be tracked when roaming from location to location. This extra func- tionality is performed by MAP and BSSAP. Figure 2.26 below shows how these GSM components fit into the SS7 protocol stack.

BSSAP MTP level 1 MTP level 2 MTP level 3 MTP MTP level 1 MTP level 2 MTP level 3 MTP DTAP BSSMAP SCCP TCAP MAP SCCP mobile station BSS MSC

Message application part (MAP)

MAP is a key protocol in many cellular networks. Both GSM and ANSI-41 networks use MAP for accessing roaming information, paging devices, controlling handover and sending SMS messages. Although GSM and ANSI-41 use MAP, these messages are not directly compatible and interworking functions are required when communication between the two types of MAP is required. MAP messages are carried by TCAP.

Base station system application part (BSSAP)

BSSAP messages are used for signalling both between the mobile device and the MSC and also between the BSC and the MSC. For this reason the BSSAP is divided into two sub-layers:

• The direct transfer application part (DTAP) is used to transfer the messages between the MSC and mobile device. These messages, which include MM and CM, are not interpreted by the BSS and are carried transparently.

• BSS management application part (BSSMAP) messages are transferred between the BSC and the MSC to support procedures such as resource management, handover control and paging of the mobile device.