--- title: "CCS and SS7 working principle" subject: "Telecommunications" chapter: 5 key: "semester_8/telecommunications:ccs-ss7-working-principle" source: https://www.bctengineer.com/q/semester_8/telecommunications/ccs-ss7-working-principle site: BCT Engineering --- # CCS and SS7 working principle Telecommunications · Chapter 5: Signaling Systems - Asked in: 2069 Chaitra, 2+6 marks - Video: [SS7 Tutorial](https://www.youtube.com/watch?v=ccy3eUG03yA) — TechLair - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/ccs-ss7-working-principle - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5 ## Question What is common channel signaling? Explain the working principle of signaling system 7 (SS7). ## Short answer Common channel signaling sends control information for many circuits over a separate shared data link. SS7 uses a packet-switched network with SSPs, STPs, and SCPs; call setup uses ISUP messages (IAM, ACM, ANM) over signaling links, while voice trunks carry speech, and TCAP queries support database services. ## Answer #### Common Channel Signaling **Common Channel Signaling (CCS)** is a method in which signaling information for a large group of speech circuits is sent as labelled data messages over a **separate, shared signaling link**, instead of on each speech channel. Each message carries a label (e.g. circuit identification code) showing which circuit it refers to. SS7 (ITU-T Q.700) is the standard CCS system. #### Working principle of SS7 ```text Tel-A Tel-B | | SSP-A ---A link--- STP pair ---A link--- SSP-B | | | +====== voice trunks (CIC) ==============+ | SCP (database) ``` **Network elements:** SSPs (exchanges that originate/terminate messages), STPs (packet switches that route messages, used in mated pairs) and SCPs (databases). **Protocol layers used:** - MTP-1: 64 kb/s link; MTP-2: framing, CRC, sequence numbers and retransmission; MTP-3: routing by DPC/OPC/SLS and network management. - SCCP + TCAP: database queries; ISUP: circuit set-up and release. **Steps of a call** 1. Subscriber A lifts the handset and dials. SSP-A analyses the digits and picks a free trunk to SSP-B (CIC). 2. SSP-A's ISUP forms an **IAM** with called number, calling number and CIC. MTP-3 adds the routing label; MTP-2 frames it as an MSU with CRC; it is sent on the A-link to the STP. 3. The STP reads the DPC and forwards the MSU to SSP-B. 4. SSP-B checks B's line. If free, it rings B and returns **ACM**; A hears ring-back. 5. When B answers, SSP-B sends **ANM**; the trunk is through-connected and billing starts. 6. On hang-up, **REL** is sent and the other side answers **RLC**; the trunk becomes idle. 7. If B is busy, SSP-B sends REL (cause: user busy) at once, so no trunk is held uselessly. 8. For services such as toll-free numbers or mobile roaming, the SSP first sends a **TCAP query** through SCCP to an SCP or HLR and uses the reply to route the call. **Reliability:** errors on a link are corrected by retransmission at MTP-2; failed links or STPs are bypassed by MTP-3 network management using the redundant STP pairs. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What defines common channel signaling? A. Signals travel on each speech channel in-band B. Signaling for many circuits is sent as labelled messages on a separate shared link C. Signals are sent as tones during conversation D. Only one circuit is signaled at a time Answer: B. CCS uses one separate link carrying labelled messages for a large group of circuits. (Key point: CCS definition) 2. Which function does MTP-2 perform? A. Routing by DPC/OPC and network management B. Framing, CRC, sequence numbers and retransmission C. Database queries D. Circuit set-up and release Answer: B. MTP-2 gives reliable link transfer; MTP-3 does routing. (Key point: MTP-2 role) 3. If the called party is busy, what does SSP-B do? A. Sends ANM then REL B. Holds the trunk until B is free C. Sends REL (user busy) at once so no trunk is held uselessly D. Sends a TCAP query to the SCP Answer: C. A REL with cause user busy releases the call immediately. (Key point: Busy called party) 4. What happens when ANM is sent? A. The trunk is through-connected and billing starts B. The called phone starts ringing C. The trunk is released D. The SCP is queried Answer: A. ANM signals answer; the trunk is connected and charging starts. (Key point: ANM) ## Close variants set in other exams - [Advantage and working of SS7](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-advantage-working) — Telecommunications, chapter 5, asked 2× ## 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× - [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 - [Importance of signaling; ITU-T SS7](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-importance-itu-ss7) — Telecommunications, chapter 5