--- title: "Common channel signaling and SS7 call set-up" subject: "Telecommunications" chapter: 5 key: "semester_8/telecommunications:ccs-ss7-call-setup" source: https://www.bctengineer.com/q/semester_8/telecommunications/ccs-ss7-call-setup site: BCT Engineering --- # Common channel signaling and SS7 call set-up Telecommunications · Chapter 5: Signaling Systems - Asked in: 2079 Chaitra, 2+6 marks; 2075 Asoj, 2+6 marks (asked 2 times) - Video: [Common Channel Signalling](https://www.youtube.com/watch?v=7S-gG8UpDWI) — ECE Pavilion - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/ccs-ss7-call-setup - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5 ## Question What is common channel signaling (CCS)? Explain the basic call set-up process using SS7 signaling system. ## Short answer Common channel signaling sends signaling messages for many circuits over a separate shared link; SS7 is the standard. Call setup uses ISUP messages: IAM from the calling exchange, ACM when the called party rings, ANM on answer, then conversation, and REL and RLC on release. ## Answer #### 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. ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is common channel signaling? A. Signals sent on each speech circuit as tones B. Signaling of many circuits carried as labelled digital messages on a separate shared link C. Signaling sent only in TS0 D. Signaling carried by the subscriber loop current Answer: B. CCS uses a separate shared signaling link, with labels identifying which circuit each message belongs to. SS7 is the standard system. (Key point: Definition of CCS) 2. What does the IAM sent by SSP-A contain, and what is its role? A. Only an answer indication, to start charging B. Called and calling numbers, CIC of the chosen trunk and call type, to begin set-up C. A release cause for the trunk D. A database query to the SCP Answer: B. IAM is the first ISUP message and carries addressing and the trunk circuit identification code (CIC). (Key point: IAM contents) 3. What is the correct ISUP sequence for a successful call? A. IAM, ANM, ACM, REL, RLC B. IAM, ACM, ANM, conversation, REL, RLC C. ACM, IAM, ANM, RLC, REL D. IAM, REL, ACM, ANM, RLC Answer: B. ACM says B is being rung, ANM says B answered, then REL/RLC release the trunk. (Key point: Message sequence) 4. What happens if B is busy? A. The trunk is held until the caller hangs up B. SSP-B sends REL with cause user busy at once, so no trunk is held C. SSP-B sends ANM D. A TCAP query is made to the SCP Answer: B. Because signaling is separate, the busy result returns before the trunk is used, saving trunk capacity. (Key point: Busy called party) ## Related in Telecommunications - [SS7 call set-up and release](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-call-setup-release) — Telecommunications, chapter 5 - [In-channel vs CCS; SS7 link types](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ainchannel-ccs-ss7-links) — Telecommunications, chapter 5 - [In-channel vs CCS; SS7 architecture](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ainchannel-ccs-ss7-architecture) — Telecommunications, chapter 5 - [Signaling; CAS vs common channel](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-cas-vs-ccs) — 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 - [CCS and SS7 working principle](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Accs-ss7-working-principle) — Telecommunications, chapter 5, asked 2×