Importance of signaling; ITU-T SS7

Why signaling is important

Signaling is the exchange of control information between subscriber and exchange, and between exchanges, needed to run a call. It is important because it:

• Requests, routes and sets up calls (off-hook, dial tone, dialled digits, route selection).
• Supervises lines and trunks (idle, busy, answered, cleared) and releases resources.
• Gives alerting and progress information (ringing, busy tone, ring-back).
• Starts and stops charging.
• Enables services such as caller ID, call forwarding, mobile roaming and toll-free numbers.
• Manages the network (congestion control, rerouting, maintenance).

ITU-T Signaling System No. 7 (SS7)

SS7 is the ITU-T standard (Q.700 series) common channel signaling system for digital networks. It sends signaling as labelled messages over separate 64 kbps signaling links, forming a packet network that runs alongside the speech network.

Network elements

• SSP: exchange that originates/ends signaling.
• STP: packet router for SS7 messages (mated pairs).
• SCP: database for intelligent network services.
• Links: A, B, C, D, E, F links joined into link sets.

Signaling modes

• Associated: signaling link runs directly between the two exchanges, parallel to the trunk group.
• Quasi-associated: messages go through STPs along a fixed predetermined route.
• Non-associated: messages go through STPs on routes that may change.

Protocol architecture

 OSI layer   SS7 part
  7      | MAP | INAP | OMAP |
  4-7    |       TCAP        |  ISUP  | TUP
  3      |       SCCP        |        |
  3      |  MTP Level 3: network        |
  2      |  MTP Level 2: link           |
  1      |  MTP Level 1: physical       |

• MTP-1: 64 kbps digital bearer (e.g. TS16 of E1).
• MTP-2: reliable link: flags, CRC-16, sequence numbers, retransmission, alignment; signal units FISU, LSSU, MSU.
• MTP-3: routing by point codes (DPC, OPC, SLS), message discrimination and distribution, and network management.
• SCCP: extra addressing (subsystem numbers, global title translation) and connectionless/connection-oriented services.
• TCAP: transaction (query/response) services for database access.
• ISUP: circuit-related call control: IAM, ACM, ANM, REL, RLC.
• TUP: older telephone user part, replaced by ISUP.
• MAP / INAP / OMAP: mobile, intelligent-network and maintenance applications.

Call example: IAM → ACM → ANM → conversation → REL → RLC.

Advantages: fast set-up, signaling during the call, large message capacity, fraud resistance, efficient trunk use, and support for ISDN, mobile and IN services.