Ss7 Structure
SS7 is defined in terms of messages and functions; hardware architecture issues are left to equipment manufacturers. The signaling system is divided into a number of protocols (or parts), each of which handles a group of related functions. The interfaces between those parts were defined in the early stages of the specification work. The various parts could then be specified simultaneously and independently of each other. That has made the overall specification effort more manageable.
7.1.1 SS7 Hierarchy
The parts of SS7 are organized in a four-level hierarchy [1-4]. We say that a higherlevel part is a user of the services provided by a lower-level part. This arrangement is similar to the seven-layer structure of the Open Systems Interconnection (OSI) used for data communication protocols (Chapter 20). Several efforts have been made to align the SS7 levels and OSI layers. The efforts, however, have been only partially successful. OSI layers 1 and 2 correspond to SS7 levels 1 and 2, but things start to differ in the higher parts of the hierarchies. In what follows the four-level SS7 hierarchy will be used. It works well for trunk-related applications, and less well for other applications.
7.1.2 SS7 Protocols
The protocols of SS7 and their levels are shown in Fig. 7.1-1.
- Figure 7.1-1. Structure of SS7. (From Rec. Q.700. Courtesy of ITU-T.)
Message Transfer Part (MTP). This protocol provides message transfer services for its users. It is divided into three parts—denoted as MTP1, MTP2, and MTP3—that occupy levels l, 2, and 3 of the SS7 hierarchy.
A MTP-user passes its outgoing messages to—and receives its incoming messages from—the MTP3 at its signaling point. A signaling point, has one, MTP3.
A combination of a MTP2 and a MTP1 represents a signaling link at a signaling point (a signaling link between two signaling points consists of a MTP1/MTP2 combination at each point). A signaling point that terminates n signaling links has n of these combinations.
Telephone User Part (TUP). TUP (a MTP-user) is a protocol for telephony call control and for trunk maintenance. It is very similar to SS6 but includes a number of additional features.
Integrated Services User Part (ISUP). ISUP (another MTP-user) is a protocol for call-control and trunk-maintenance procedures in both the telephone network and the ISDN.
TUP and ISUP messages are trunk-related: they contain information concerning a particular trunk with TUP or ISUP signaling and are sent by the exchange at one end of the trunk to the exchange at the other end.
Signaling Connection Control Part (SCCP). This protocol (a MTP-user) provides functions for the transfer of messages that are not trunk-related. Its users are ISUP and TCAP. SCCP does not fit neatly into the four-level hierarchy, because it is at the same level as ISUP.
The combination of MTP and SCCP corresponds to OSI layers 1, 2, and 3 and is known as the network services part of SS7.
Transaction Capabilities Application Part (TCAP). Transactions are operations that are not related to individual trunks and involve two signaling points (Chapter 16). The TCAP protocol provides a standard interface to TC-users (functions at a signaling point that ate involved in transactions of various kinds). In turn, TCAP is a user of SCCP.
7.1.3 Messages and Message Transfer
In Fig. 7.1-1 we distinguish messages of several types in a signaling point. User messages are messages between a level-4 protocol and the MTP3 and are named after the level-4 protocol: we speak of TUP, ISUP, and SCCP messages. MTP3 messages are messages between the MTP3 and a MTP2. Message signal units (MSUs) are messages between the MTP2 and MTP1 of a signaling link at a signaling point, and between the MTP1s at both ends of a signaling link. A MSU contains a message originated by MTP3, or by a MTP3-user.
User messages are transferred between two "peer" level-4 protocols at two signaling points. Figure 7.1-2 illustrates the transfer of a TUP message from TUP-A
- Figure 7.1-2. Transfer of a TUP message. (1), TUP message; (2), MTP3 message; (3), message signal unit.
to TUP-C (at signaling points A and C, respectively) that is routed via signal transfer point B.
At signaling point A, TUP-A passes the TUP message downward to its MTP3, which expands it into a MTP3 message and passes it to the MTP2 of the signaling link to B. MTP2 expands the MTP3 message into a message signal unit (MSU) and passes it to its MTP1.
The MSU traverses the signaling link and arrives at the MTP1 of signaling point B, where MTP2 extracts the MTP3 message and passes it to its MTP3. MTP3 transfers the MTP3 message to the MTP2 of the signaling link to C.
The second leg of the message transfer is similar: the message is passed and expanded downward in signaling point B, traverses the signaling link between B and C (as a MSU), and is passed upward in signaling point C. It finally arrives as a TUP message at TUP-C.
Average user rating: 5 stars out of 1 votes
Post a comment