--- title: "In-channel vs CCS; SS7 link types" subject: "Telecommunications" chapter: 5 key: "semester_8/telecommunications:inchannel-ccs-ss7-links" source: https://www.bctengineer.com/q/semester_8/telecommunications/inchannel-ccs-ss7-links site: BCT Engineering --- # In-channel vs CCS; SS7 link types Telecommunications · Chapter 5: Signaling Systems - Asked in: 2080 Chaitra, 4+4 marks - Video: [what is SS7 ? - Signalling System Number-7 Architecture](https://www.youtube.com/watch?v=anfSFQvE8RE) — HackIsOn - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/inchannel-ccs-ss7-links - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5 ## Question Compare and contrast between in-channel and common channel signaling. Explain different types of signalling links available in SS7 network. ## Short answer In-channel signaling uses the same channel as speech (tones, pulses, CAS bits), giving slow setup and limited capacity. Common channel signaling (SS7) uses a separate shared data link with labelled messages, giving fast setup, high capacity and database services. SS7 link types: A access, B bridge, C cross, D diagonal, E extended, F fully associated. ## Answer #### In-channel signaling vs common channel signaling **In-channel signaling** sends the signals of a call over the **same channel** (or a channel tied to it) as the speech, e.g. DC loop pulses, in-band tones (SF 2600 Hz, MF), or out-of-band/CAS bits in TS16 of E1. **Common channel signaling (CCS)** sends signals for many circuits as **messages over a separate signaling link** shared by all of them (SS7). | Point | In-channel signaling | Common channel signaling | |---|---|---| | Path of signals | Same channel as speech (or tied to it) | Separate shared data link | | Signal form | Tones, pulses, bits | Labelled digital messages | | Speed of call set-up | Slow (seconds) | Fast (≈ 1 s or less) | | Signals during speech | Limited (may disturb speech) | Any time, no interference | | Information capacity | Few signals | Large (caller ID, services) | | Trunk holding time | Trunk held during set-up | Trunk seized only when needed | | Fraud risk | High (tones can be imitated) | Low | | Database / IN services | Not possible | Possible (TCAP, SCP) | | Equipment | Signaling unit per trunk | Shared signaling terminals | | Reliability | Failure affects one circuit | Link failure affects many; links duplicated | #### Types of signaling links in SS7 SS7 links connect signaling points (SSP, STP, SCP). They are named by their role: ```text SSP1 SSP2 | \ A A / | | STPa----STPb | C = mated pair link F | | \ D / | | | | STPc | | B/D = between pairs SSP3 -- E --> STPd ``` 1. **A-link (Access):** connects an SSP or SCP to its home STP pair. Each SSP has A-links to both STPs of the pair for redundancy. 2. **B-link (Bridge):** connects two mated STP pairs at the same level (peer pairs); usually a quad of links. 3. **C-link (Cross):** connects the two STPs of a mated pair; used only when other links fail (for recovery). 4. **D-link (Diagonal):** connects STP pairs at different hierarchy levels (e.g. local STP to regional/gateway STP). 5. **E-link (Extended):** connects an SSP to a remote STP pair other than its home pair; an alternative route for reliability. 6. **F-link (Fully associated):** connects two SSPs directly, without STPs, when there is heavy traffic between them (associated mode). Links between the same two points form a **link set** (up to 16 links); MTP level 3 shares load among links and reroutes traffic on failure. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How do in-channel and common channel signaling differ in trunk holding? A. In-channel seizes the trunk during set-up, whereas CCS seizes it only when needed B. CCS holds the trunk during set-up, in-channel does not C. Both hold trunks equally D. Neither holds the trunk Answer: A. In-channel signals use the speech circuit, so it is held during set-up. CCS uses a separate link, so the trunk is seized only for conversation. (Key point: Trunk holding) 2. What does an A-link (Access) connect? A. Two SSPs directly B. An SSP or SCP to its home STP pair C. The two STPs of a mated pair D. STP pairs at different levels Answer: B. A-links go from SSP/SCP to both STPs of the home pair, giving redundancy. (Key point: A-link) 3. Which pairing is correct? A. C-link connects two SSPs directly; F-link connects mated STPs B. C-link connects the two STPs of a mated pair; F-link connects two SSPs directly (fully associated) C. C-link links an SSP to a remote STP; F-link links different levels D. Both connect SCPs to SSPs Answer: B. C-links cross-connect mated STPs (used mainly on failure), and F-links connect SSPs directly for heavy traffic. (Key point: C-link vs F-link) ## 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 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 - [SS7 building blocks and links](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-building-blocks-links) — 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× - [Importance of signaling; ITU-T SS7](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-importance-itu-ss7) — Telecommunications, chapter 5 ## Related in other subjects - [Compare and contrast pollution in a city](https://www.bctengineer.com/question.md?key=semester_5%2Fcommunication_english%3Acompare-contrast-pollution-city-nepal) — Communication English, chapter 1 - [Link analysis in web vs cloud](https://www.bctengineer.com/question.md?key=semester_8%2Finformation_system%3Alink-analysis-web-vs-cloud) — Information System, chapter 7