Importance of signaling; SS7 protocol stack

Importance of signaling

Signaling is the control information that lets a telephone network work automatically. It is important because it:

• detects a call request and returns dial tone;
• carries the called number so the exchange can route the call;
• tells whether the called party is free, alerts it (ringing) and informs the caller (ring-back or busy tone);
• detects answer and clear, so the connection is made, charged and released correctly;
• supports network management (blocking circuits, re-routing on faults) and advanced services such as caller ID, free-phone and mobile roaming.

SS7 protocol stack

 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: 64 kb/s digital bidirectional path; corresponds to the OSI physical layer.
2. MTP Level 2 – Signaling link: reliable transfer over one link: flags, 16-bit CRC, FSN/BSN sequence numbers, retransmission, alignment and error-rate monitoring. Uses MSU (messages), LSSU (link status) and FISU (fill-in) units.
3. MTP Level 3 – Signaling network: message discrimination, distribution and routing using the routing label (DPC, OPC, SLS); signaling network management for link failures and congestion.
4. SCCP – Signaling Connection Control Part: extends MTP-3 with global title translation and subsystem numbers; offers connectionless and connection-oriented classes. MTP + SCCP give OSI layer-3 service (Network Service Part).
5. TCAP – Transaction Capabilities Application Part: carries query/response dialogues not tied to a voice circuit (e.g. 800-number lookup, HLR query).
6. ISUP – ISDN User Part: controls trunk circuits: IAM, ACM, ANM, REL, RLC. TUP is the earlier telephone user part.
7. Applications: MAP (GSM mobility, SMS), INAP (intelligent network), OMAP (operation and maintenance).