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SS7 call set-up and release

Short answer

SS7 is an ITU-T common channel signaling system where call-control messages travel as packets over separate 64-kbps links. For call setup, SSP-A sends IAM to SSP-B via STP; B rings and returns ACM (ringback) and ANM (answer) to connect speech. Release uses REL from the disconnecting side, then RLC to free the trunk.

What is SS7 (Signalling System #7)? Explain the steps involved in call set up and release in a system using SS7.

SS7

Signaling System No. 7 (SS7) is the ITU-T common channel signaling system. Call-control messages travel as data packets over separate 64 kbps signaling links between exchanges (SSPs), via STPs, while speech uses trunks. Call set-up and release use the ISDN User Part (ISUP) over the Message Transfer Part (MTP).

Call set-up and release

 Caller  SSP-A          STP          SSP-B   Called
  |off-hook|              |              |        |
  |<-dial--|              |              |        |
  |-digits>|              |              |        |
  |        |--- IAM ----->|---- IAM ---->|        |
  |        |              |              |--ring->|
  |        |<-- ACM ------|<--- ACM -----|        |
  |<-ringbk|              |              |        |
  |        |<-- ANM ------|<--- ANM -----|<-answer|
  |<=========== conversation on trunk ===========>|
  |on-hook>|              |              |        |
  |        |--- REL ----->|---- REL ---->|        |
  |        |<-- RLC ------|<--- RLC -----|        |

Set-up

  1. Call origination: caller A goes off-hook; SSP-A gives dial tone and collects the digits.
  2. Digit analysis and trunk selection: SSP-A finds that B is on SSP-B and reserves an idle trunk, identified by its CIC.
  3. IAM (Initial Address Message): SSP-A sends IAM containing the called number, calling number, CIC and call type. MTP-3 routes it by the destination point code; the STP forwards it.
  4. Continuity check (optional): the trunk may be tested (COT message).
  5. Called side: SSP-B checks B is free, reserves the trunk and rings B.
  6. ACM (Address Complete Message): SSP-B tells SSP-A that all digits are received and B is being alerted. Caller hears ring-back tone.
  7. ANM (Answer Message): B answers; SSP-B sends ANM. The speech path is connected end to end and charging starts.
  8. Conversation takes place on the trunk.

Release 9. REL (Release): when A (or B) hangs up, that exchange sends REL with a cause value (e.g. normal clearing) and starts freeing the trunk. 10. RLC (Release Complete): the other exchange disconnects, makes the trunk idle and replies with RLC. Charging stops and the CIC can be reused.

Busy case: if B is busy, SSP-B replies to the IAM with REL (cause "user busy"); SSP-A gives busy tone, and the trunk is freed at once.

Since all messages travel at 64 kbps over the signaling network, set-up takes about a second, and trunks are held only for real conversations.

✓ Answer checked by Claude Opus 5.5 (high effort)

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