--- title: "In-channel vs CCS; SS7 architecture" subject: "Telecommunications" chapter: 5 key: "semester_8/telecommunications:inchannel-ccs-ss7-architecture" source: https://www.bctengineer.com/q/semester_8/telecommunications/inchannel-ccs-ss7-architecture site: BCT Engineering --- # In-channel vs CCS; SS7 architecture Telecommunications · Chapter 5: Signaling Systems - Asked in: 2075 Chaitra, 3+5 marks - Video: [T 2 2 Common Channel signaling CCS and channel associated signaling CAS](https://www.youtube.com/watch?v=y4CcbR6ypZ0) — Ramanikumar animations -Telecom and Wireless - 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-architecture - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5 ## Question Differentiate between in-channel signaling and common channel signaling. Also explain the Architecture of Common channel signaling system 7 with proper block diagram and Protocol stack. ## Short answer In-channel signaling shares the speech path, is slow, limited and fraud-prone; common channel signaling uses a separate data link, is fast, flexible and supports database access. SS7 architecture consists of SSPs, mated STP pairs, SCPs and A-F links. The protocol stack has MTP levels 1-3, SCCP, TCAP, ISUP/TUP and MAP/INAP/OMAP. ## Answer #### In-channel vs common channel signaling | Point | In-channel signaling | Common channel signaling (CCS) | |---|---|---| | Path | Same channel as the speech | Separate dedicated data link | | Examples | DC loop, 2600 Hz SF, MF R1/R2 | CCITT No. 6, SS7 | | Speed | Slow (seconds) | Fast (well under 1 s) | | Signal set | Small, fixed | Large, easily extended | | Signaling during call | Limited | Possible at any time | | Fraud risk | Tones can be imitated | Not reachable by user | | Trunk use | Trunk held during set-up | Trunk seized only when needed | | Database access | Not possible | Possible (TCAP/SCP) | #### SS7 network architecture SS7 is a separate packet network that carries signaling for the voice network. ```text mated pair mated pair SSP-A --A-- STP1 ---B--- STP3 --A-- SSP-B \ | \ / | / \ C B B C / \--A-- | / \ | --A--/ \ STP2 ---B--- STP4 / | SCP (A link to STP pair) SSP-A ==========F (direct)========= SSP-B ``` - **SSP (Service Switching Point):** telephone exchange that originates/terminates SS7 messages. - **STP (Signal Transfer Point):** packet switch that routes SS7 messages; deployed in mated pairs for reliability. - **SCP (Service Control Point):** database for 800/free-phone, number portability, HLR, prepaid. - **Links:** A (SSP–STP), B (STP pair to STP pair), C (between mated STPs), D (local to regional STP), E (SSP to alternate STP), F (direct SSP–SSP). #### SS7 protocol stack ```text OSI layer SS7 stack +-----------+ +----------------+------+-----+ | 7 Applic. | | MAP/INAP/OMAP | | | +-----------+ +----------------+ | | | 4 - 6 | | TCAP | ISUP | TUP | +-----------+ +----------------+ | | | 3 Network | | SCCP | | | | | +----------------+------+-----+ | | | MTP Level 3 (network) | +-----------+ +-----------------------------+ | 2 Link | | MTP Level 2 (link) | +-----------+ +-----------------------------+ | 1 Physical| | MTP Level 1 (data link) | +-----------+ +-----------------------------+ ``` 1. **MTP-1:** 64 kb/s physical signaling data link. 2. **MTP-2:** reliable link — flags, CRC, sequence numbers, retransmission; units MSU, LSSU, FISU. 3. **MTP-3:** message routing by point codes and signaling network management. 4. **SCCP:** global title translation, connectionless and connection-oriented classes. 5. **TCAP:** dialogues with databases (query/response). 6. **ISUP/TUP:** circuit-related call control (IAM, ACM, ANM, REL, RLC). 7. **MAP/INAP/OMAP:** mobile, intelligent network and maintenance applications. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the key difference between in-channel and common channel signaling? A. In-channel uses a separate data link, CCS uses the speech channel B. In-channel signals travel on the speech channel, CCS uses a separate dedicated data link C. Both use the speech channel D. CCS is always slower Answer: B. CCS such as SS7 carries signaling for many trunks on a separate link. (Key point: In-channel vs CCS path) 2. What is the role of an STP in the SS7 network? A. A database for free-phone numbers B. A telephone exchange that originates calls C. A packet switch that routes SS7 messages, deployed in mated pairs D. A voice trunk Answer: C. STP routes messages; SSP is the exchange and SCP is the database. (Key point: STP role) 3. Which node holds databases such as free-phone and number portability? A. SCP B. SSP C. STP D. MSC trunk Answer: A. The Service Control Point is the database node queried via TCAP. (Key point: SCP role) 4. Why does CCS use trunks more efficiently? A. It uses higher tone frequencies B. It removes the need for exchanges C. It compresses speech D. The trunk is seized only when needed, after the called line is found Answer: D. In-channel signaling holds the trunk during set-up, whereas CCS seizes it only when needed. (Key point: CCS advantage) ## Related in Telecommunications - [Importance of signaling; SS7 protocol stack](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-importance-ss7-stack) — Telecommunications, chapter 5 - [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× - [Signaling activities and SS7 architecture](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-activities-ss7-architecture) — 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 - [CCS7 stack; why CCS over in-channel](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Accs7-stack-ccs-preferred) — 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 ## Related in other subjects - [PCM block diagram; T1 signaling rate and frame](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Apcm-block-t1-rate-frame) — Communication Systems, chapter 5 - [Block diagram of digital communication system](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Ablock-diagram-digital-comm) — Data Communication, chapter 1, asked 8× - [Communication system and block diagram](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Acomm-system-block-diagram) — Basic Electronics Engineering, chapter 5, asked 7× - [Closed loop control system block diagram explained](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Awhat-is-cs-bd-closed-loop) — Control Systems, chapter 1, asked 4×