Advantage and working of SS7

Advantages of SS7

• Fast call set-up (whole number sent in one message at 64 kb/s).
• Trunks are seized only after the called party is free, so trunk efficiency is higher.
• One signaling link serves thousands of trunks; less per-circuit equipment.
• Large, extendable message set: ISDN, caller ID, call forwarding, mobile (MAP), IN services.
• Signaling possible during conversation; no fraud by tone imitation.
• Database access (toll-free, number portability, HLR) through SCCP/TCAP; high reliability with CRC and redundant STPs.

Working principle

SS7 sends all control information as digital messages on a separate signaling network, while the voice trunks carry only speech.

  Tel-A                                   Tel-B
   |                                        |
  SSP-A ---A link--- STP pair ---A link--- SSP-B
   |                    |                   |
   +====== voice trunks (CIC) ==============+
                        |
                       SCP (database)

1. Message creation: the originating SSP's user part (ISUP) builds a message (e.g. IAM) containing called/calling number and the CIC of the chosen trunk.
2. Routing label: MTP-3 adds DPC (destination), OPC (origin) and SLS. MTP-2 puts it in an MSU with sequence numbers and CRC; MTP-1 sends it on the 64 kb/s link.
3. Transfer: the STP reads the DPC and forwards the message on a link toward SSP-B; MTP-2 at each hop checks the CRC and acknowledges the message or asks for retransmission.
4. Delivery: SSP-B's MTP-3 sees its own point code and passes the message (by the service indicator) to ISUP.
5. Call set-up sequence: IAM → ACM (called party ringing) → ANM (answer; charging starts) → conversation on the trunk → REL → RLC (trunk freed).
6. Non-circuit services: for an 800 number, SSP-A first sends a TCAP query through SCCP/STP to the SCP, which returns the real routing number; then the normal ISUP sequence follows.
7. Network management: if a link fails, MTP-3 changes traffic over to another link or STP automatically.