Common channel signaling and SS7 call set-up

Common channel signaling (CCS)

Common channel signaling carries the signaling of many speech circuits as digital messages over a separate shared signaling link, instead of sending signals on each speech circuit. Each message has a label that identifies the circuit it belongs to. SS7 (Signaling System No. 7) of ITU-T is the standard CCS system; it uses 64 kbps links (e.g. TS16 of an E1) and its own packet network of signaling points.

Basic call set-up using SS7 (ISUP)

Network: subscriber A on exchange SSP-A, subscriber B on exchange SSP-B; signaling messages pass through one or more STPs; speech goes over a trunk between the exchanges.

 SSP-A          STP          SSP-B
   |---- IAM ---->|---- IAM ---->|   seize trunk, dial info
   |<--- ACM -----|<--- ACM -----|   B is ringing
   |<--- ANM -----|<--- ANM -----|   B answers
   |====== conversation on trunk =====|
   |---- REL ---->|---- REL ---->|   A hangs up
   |<--- RLC -----|<--- RLC -----|   trunk released

1. Off-hook and digits: A lifts the handset; SSP-A gives dial tone, collects the digits and analyses them. It finds that B is on SSP-B and selects a free trunk (circuit) to SSP-B.
2. IAM (Initial Address Message): SSP-A sends IAM with called and calling numbers, the circuit identification code (CIC) of the chosen trunk and call type. The STP routes it to SSP-B using the destination point code.
3. Called side check: SSP-B checks that B is free, reserves the trunk and rings B.
4. ACM (Address Complete Message): SSP-B returns ACM; SSP-A connects ring-back tone to A (or B's exchange sends it over the trunk).
5. ANM (Answer Message): when B answers, SSP-B sends ANM; both exchanges complete the speech path and charging starts.
6. Conversation takes place on the trunk; no signaling uses the speech channel.
7. REL (Release): when either party hangs up, its exchange sends REL.
8. RLC (Release Complete): the other exchange frees the trunk and replies with RLC; the circuit is free for other calls.

If B is busy, SSP-B sends REL with cause "user busy" at once, so the reserved trunk is freed immediately and is never used for ringing, which saves trunk capacity. For services like toll-free numbers, an SSP may first send a TCAP query to an SCP database to get the real number before sending the IAM.