Signaling activities and SS7 architecture

Signaling is the exchange of control information between the subscriber and the exchange, and between exchanges, needed to set up, supervise and release a call.

Why signaling is required

Without signaling an exchange cannot know that a subscriber wants a call, which number is wanted, whether the called line is free, when the called party answers (for billing) or when the call ends. Signaling therefore lets the network establish, monitor, charge and clear connections automatically, and also supports network management (busy circuits, faults, re-routing).

Signaling activities in a telephone system

1. Call request (seizure): calling party goes off-hook; exchange detects loop current and returns dial tone.
2. Address signaling: the dialled digits (pulse or DTMF) are sent to the exchange; between exchanges the digits are forwarded (MF or ISUP IAM).
3. Routing and trunk seizure: exchange analyses digits, selects a route and seizes a trunk to the next exchange.
4. Call progress / alerting: ringing current to the called party, ring-back tone to the caller, or busy tone / announcement if the call fails.
5. Answer (supervisory) signal: called party off-hook; ringing is stopped and charging starts.
6. Conversation supervision: the connection is held and monitored.
7. Clear (disconnect): on-hook from either side; clear-forward / clear-back signals release the trunks and charging stops.
8. Network management/maintenance: blocking, unblocking, circuit tests, congestion control.

These are grouped as supervisory, address and call-progress (information) signals.

SS7 protocol architecture

 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)    |
+-----------+  +-----------------------------+

• MTP Level 1 (Signaling data link): bidirectional 64 kb/s digital path (usually a timeslot, TS16, of an E1).
• MTP Level 2 (Signaling link): flags, error detection by CRC-16, retransmission, sequence numbers (FSN/BSN), alignment and link error monitoring. Carries MSU, LSSU and FISU.
• MTP Level 3 (Signaling network): message routing using point codes (DPC/OPC), discrimination and distribution, and signaling network management (re-routing on link failure).
• SCCP: adds global-title translation and connectionless / connection-oriented services; together with MTP it forms the Network Service Part (NSP), equal to OSI layer 3.
• TCAP: transaction capabilities for non-circuit queries to databases (e.g. free-phone, roaming).
• ISUP / TUP: set up and release of voice/ISDN circuits (IAM, ACM, ANM, REL, RLC).
• MAP, OMAP, INAP: mobile application part, operation and maintenance part, intelligent network application part on top of TCAP.