--- title: "SS7 call set-up and release" subject: "Telecommunications" chapter: 5 key: "semester_8/telecommunications:ss7-call-setup-release" source: https://www.bctengineer.com/q/semester_8/telecommunications/ss7-call-setup-release site: BCT Engineering --- # SS7 call set-up and release Telecommunications · Chapter 5: Signaling Systems - Asked in: 2080 Baisakh, 2+6 marks - Video: [SS7 Protocol - ISUP - End to End Call Setup](https://www.youtube.com/watch?v=41dsiejZ3vA) — Telecom Bites - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/ss7-call-setup-release - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5 ## Question What is SS7 (Signalling System #7)? Explain the steps involved in call set up and release in a system using SS7. ## 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. ## Answer #### 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 ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. After digit analysis, how does SSP-A identify the trunk to SSP-B? A. By the point code of the STP B. By reserving an idle trunk and identifying it with its CIC in the IAM C. By the dialled number only D. By the global title Answer: B. The circuit identification code tells the far exchange which trunk the ISUP message refers to. (Key point: Trunk selection with CIC) 2. What does ACM tell SSP-A? A. B has answered and charging starts B. All digits are received and B is being alerted, so ring-back is given C. The trunk is released D. B is busy Answer: B. ACM means address complete and B is ringing. ANM is the answer message that starts charging. (Key point: ACM meaning) 3. When does charging start? A. On IAM B. On ACM C. On ANM, when the speech path is connected end to end D. On RLC Answer: C. ANM signals B answered, so charging starts. Charging stops on release. (Key point: Charging start) 4. What happens during release? A. The hanging-up exchange sends REL with a cause; the other exchange frees the trunk and replies RLC B. Both exchanges send IAM C. The STP releases the trunk D. RLC is sent first, then REL Answer: A. REL starts freeing the trunk; RLC confirms, charging stops and the CIC can be reused. (Key point: Release steps) ## Related in Telecommunications - [Common channel signaling and SS7 call set-up](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Accs-ss7-call-setup) — Telecommunications, chapter 5, asked 2× - [Sequence of call steps in SS7](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-call-steps-sequence) — Telecommunications, chapter 5 - [Signaling system and reason for SS7](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-system-ss7-reason) — Telecommunications, chapter 5