Sequence of call steps in SS7

In SS7 a call is set up and released by ISUP messages exchanged between the originating and terminating Service Switching Points (SSPs) through Signal Transfer Points (STPs), while the voice trunk itself carries only speech.

Network arrangement

 Caller--SSP-A ----- STP ----- SSP-B--Called
           |   (signaling link)  |
           +======= voice trunk ==+

Sequence of steps for a successful call

1. Off-hook and dial tone: calling subscriber lifts the handset; SSP-A detects it and gives dial tone.
2. Digit collection and analysis: SSP-A receives the dialled number, finds that the called party is on SSP-B, and selects an idle trunk (identified by its CIC, circuit identification code).
3. IAM (Initial Address Message): SSP-A sends IAM via the STP to SSP-B. It contains the called and calling numbers, the CIC and the nature of connection. The trunk is reserved.
4. Called line check: SSP-B checks that the called line is free, applies ringing to it and connects ring-back towards the trunk.
5. ACM (Address Complete Message): SSP-B returns ACM to SSP-A through the STP, telling that all digits are received and the called party is being alerted. Caller hears ring-back tone over the trunk.
6. ANM (Answer Message): when the called party lifts the handset, SSP-B sends ANM; the speech path is through-connected in both directions and charging starts.
7. Conversation on the voice trunk; no signaling traffic on it.
8. REL (Release): when either party hangs up, its switch sends REL with a cause value (e.g. normal clearing).
9. RLC (Release Complete): the other switch frees the trunk and replies RLC; the CIC becomes idle, charging stops.

Message flow

 SSP-A            STP            SSP-B
   |---IAM-------->|---IAM-------->|  ringing
   |<--ACM---------|<--ACM---------|
   |  (ring-back)                  |
   |<--ANM---------|<--ANM---------|  answer
   |====== conversation (trunk) ===|
   |---REL-------->|---REL-------->|  clear
   |<--RLC---------|<--RLC---------|

Unsuccessful call

If the called line is busy, SSP-B sends REL with cause "user busy" instead of ACM; SSP-A gives busy tone to the caller and returns RLC. If the trunk or route is congested, SSP-A itself gives a congestion tone.

Points to note

• Signaling and speech use separate paths, so the trunk is held only after IAM and is released immediately on failure.
• Because the trunk is not seized for ringing on a busy line, call set-up is fast (under about 1 s) and trunk usage is efficient.
• Database-type steps (free-phone 1660 numbers, number portability, mobile location) use TCAP queries to an SCP before the IAM is sent.