--- title: "Signaling activities and SS7 architecture" subject: "Telecommunications" chapter: 5 key: "semester_8/telecommunications:signaling-activities-ss7-architecture" source: https://www.bctengineer.com/q/semester_8/telecommunications/signaling-activities-ss7-architecture site: BCT Engineering --- # Signaling activities and SS7 architecture Telecommunications · Chapter 5: Signaling Systems - Asked in: 2078 Bhadra, 1+2+4 marks - Video: [Inside the Telecom Network: SS7 Explained](https://www.youtube.com/watch?v=Q0a9FWOzWOU) — Status 451 - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/signaling-activities-ss7-architecture - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5 ## Question Why signaling system is required? Mention the signaling activities in telephone system. Draw a common channel signaling system (SS7) protocol architecture with appropriate labels. ## Short answer Signaling enables automatic call setup, supervision, and release. Activities include call request (off-hook), address digit transmission, routing, alerting, answer supervision, disconnection, and network management. SS7 architecture comprises MTP levels 1-3 (physical, link, network), plus ISUP/TUP for call control, SCCP for enhanced routing, and TCAP for database queries. ## Answer **Signaling** is the exchange of control information between the subscriber and the exchange, and between exchanges, needed to set up, supervise and release a call. #### Why signaling is required Without signaling an exchange cannot know that a subscriber wants a call, which number is wanted, whether the called line is free, when the called party answers (for billing) or when the call ends. Signaling therefore lets the network establish, monitor, charge and clear connections automatically, and also supports network management (busy circuits, faults, re-routing). #### Signaling activities in a telephone system 1. **Call request (seizure):** calling party goes off-hook; exchange detects loop current and returns **dial tone**. 2. **Address signaling:** the dialled digits (pulse or DTMF) are sent to the exchange; between exchanges the digits are forwarded (MF or ISUP IAM). 3. **Routing and trunk seizure:** exchange analyses digits, selects a route and seizes a trunk to the next exchange. 4. **Call progress / alerting:** **ringing current** to the called party, **ring-back tone** to the caller, or **busy tone** / announcement if the call fails. 5. **Answer (supervisory) signal:** called party off-hook; ringing is stopped and charging starts. 6. **Conversation supervision:** the connection is held and monitored. 7. **Clear (disconnect):** on-hook from either side; clear-forward / clear-back signals release the trunks and charging stops. 8. **Network management/maintenance:** blocking, unblocking, circuit tests, congestion control. These are grouped as **supervisory**, **address** and **call-progress (information)** signals. #### SS7 protocol architecture ```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) | +-----------+ +-----------------------------+ ``` - **MTP Level 1 (Signaling data link):** bidirectional 64 kb/s digital path (usually a timeslot, TS16, of an E1). - **MTP Level 2 (Signaling link):** flags, error detection by CRC-16, retransmission, sequence numbers (FSN/BSN), alignment and link error monitoring. Carries MSU, LSSU and FISU. - **MTP Level 3 (Signaling network):** message routing using point codes (DPC/OPC), discrimination and distribution, and signaling network management (re-routing on link failure). - **SCCP:** adds global-title translation and connectionless / connection-oriented services; together with MTP it forms the Network Service Part (NSP), equal to OSI layer 3. - **TCAP:** transaction capabilities for non-circuit queries to databases (e.g. free-phone, roaming). - **ISUP / TUP:** set up and release of voice/ISDN circuits (IAM, ACM, ANM, REL, RLC). - **MAP, OMAP, INAP:** mobile application part, operation and maintenance part, intelligent network application part on top of TCAP. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is signaling needed in a telephone network? A. To amplify the speech signal B. So the exchange can know of a call request, the wanted number, line state, answer and clear to establish, charge and release calls C. To increase the bandwidth of the voice channel D. To encrypt conversations Answer: B. Without signaling the exchange could not detect call requests, route, supervise, charge or clear calls. (Key point: Why signaling) 2. What happens when the called party answers (off-hook)? A. Dial tone is sent to the caller B. The trunk is released C. Ringing stops and charging starts D. Busy tone is sent Answer: C. The answer (supervisory) signal stops ringing and starts charging. (Key point: Answer signal) 3. In the SS7 architecture, what does SCCP together with MTP form? A. The Network Service Part, equal to OSI layer 3 B. The application layer C. The physical layer D. The ISUP user part Answer: A. MTP plus SCCP provide OSI layer 3 service, called the Network Service Part. (Key point: SCCP and NSP) 4. What is TCAP used for in the SS7 stack? A. Delimiting frames with flags B. Setting up voice trunks using IAM C. Non-circuit queries to databases such as free-phone or roaming D. Providing the 64 kb/s link Answer: C. TCAP handles transactions with remote databases not tied to a voice circuit; ISUP does trunk set-up. (Key point: TCAP role) ## 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× - [Importance of signaling; SS7 protocol stack](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-importance-ss7-stack) — 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 - [SS7 protocol structure](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-protocol-structure) — 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 ## Related in other subjects - [FDM hierarchy in telephone system](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afdm-hierarchy-telephone) — Data Communication, chapter 6