CCS and SS7 working principle

Common Channel Signaling

Common Channel Signaling (CCS) is a method in which signaling information for a large group of speech circuits is sent as labelled data messages over a separate, shared signaling link, instead of on each speech channel. Each message carries a label (e.g. circuit identification code) showing which circuit it refers to. SS7 (ITU-T Q.700) is the standard CCS system.

Working principle of SS7

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

Network elements: SSPs (exchanges that originate/terminate messages), STPs (packet switches that route messages, used in mated pairs) and SCPs (databases).

Protocol layers used:

• MTP-1: 64 kb/s link; MTP-2: framing, CRC, sequence numbers and retransmission; MTP-3: routing by DPC/OPC/SLS and network management.
• SCCP + TCAP: database queries; ISUP: circuit set-up and release.

Steps of a call

1. Subscriber A lifts the handset and dials. SSP-A analyses the digits and picks a free trunk to SSP-B (CIC).
2. SSP-A's ISUP forms an IAM with called number, calling number and CIC. MTP-3 adds the routing label; MTP-2 frames it as an MSU with CRC; it is sent on the A-link to the STP.
3. The STP reads the DPC and forwards the MSU to SSP-B.
4. SSP-B checks B's line. If free, it rings B and returns ACM; A hears ring-back.
5. When B answers, SSP-B sends ANM; the trunk is through-connected and billing starts.
6. On hang-up, REL is sent and the other side answers RLC; the trunk becomes idle.
7. If B is busy, SSP-B sends REL (cause: user busy) at once, so no trunk is held uselessly.
8. For services such as toll-free numbers or mobile roaming, the SSP first sends a TCAP query through SCCP to an SCP or HLR and uses the reply to route the call.

Reliability: errors on a link are corrected by retransmission at MTP-2; failed links or STPs are bypassed by MTP-3 network management using the redundant STP pairs.