SS7 protocol structure

Common Channel Signaling System No. 7 (SS7) is an ITU-T standard in which signaling for many trunks is carried as packets on a separate 64 kb/s link. Its protocol is layered into a Message Transfer Part (MTP) and several user parts.

Protocol structure

 OSI layer        SS7 stack
+-----------+  +----------------+------+-----+
| 7 Applic. |  | MAP/INAP/OMAP  |      |     |
+-----------+  +----------------+      |     |
| 4 - 6     |  |      TCAP      | ISUP | TUP |
+-----------+  +----------------+      |     |
| 3 Network |  |      SCCP      |      |     |
|           |  +----------------+------+-----+
|           |  |  MTP Level 3 (network)      |
+-----------+  +-----------------------------+
| 2 Link    |  |  MTP Level 2 (link)         |
+-----------+  +-----------------------------+
| 1 Physical|  |  MTP Level 1 (data link)    |
+-----------+  +-----------------------------+

1. MTP Level 1 – Signaling data link

• Physical, bidirectional digital path, normally 64 kb/s (a timeslot of E1/T1); 56 kb/s in some ANSI networks.
• Equivalent to the OSI physical layer.

2. MTP Level 2 – Signaling link

• Gives reliable, in-sequence, error-free transfer on one link.
• Flag delimitation, 16-bit CRC, forward/backward sequence numbers, retransmission (basic or preventive cyclic).
• Initial alignment and signal unit error-rate monitoring.
• Signal units: MSU (messages), LSSU (link status), FISU (fill-in).

3. MTP Level 3 – Signaling network

• Message handling: discrimination (is it for this node?), distribution to the right user part (by service indicator), and routing using the routing label (DPC, OPC, SLS).
• Network management: traffic, link and route management (changeover, changeback, rerouting on failure or congestion).

4. SCCP – Signaling Connection Control Part

• Adds to MTP-3 to give full OSI layer-3 service.
• Global Title Translation (e.g. a dialled number to point code + subsystem number).
• Four protocol classes: 0, 1 (connectionless), 2, 3 (connection-oriented).

5. TCAP – Transaction Capabilities Application Part

• Supports non-circuit dialogues such as database queries (free-phone, number portability, roaming).
• Consists of a component sub-layer (invoke, result, error) and a transaction sub-layer.

6. User parts

• TUP (Telephone User Part): older, basic telephone call control.
• ISUP (ISDN User Part): circuit set-up and release for voice and ISDN: IAM, ACM, ANM, REL, RLC.
• MAP (Mobile Application Part): GSM location update, authentication, SMS.
• INAP: intelligent network services; OMAP: operation and maintenance.

Summary

MTP 1–3 plus SCCP form the Network Service Part (NSP), giving the transport. ISUP/TUP use MTP directly for circuit-related signaling, while TCAP-based applications use SCCP for non-circuit transactions.