--- title: "SS7 protocol structure" subject: "Telecommunications" chapter: 5 key: "semester_8/telecommunications:ss7-protocol-structure" source: https://www.bctengineer.com/q/semester_8/telecommunications/ss7-protocol-structure site: BCT Engineering --- # SS7 protocol structure Telecommunications · Chapter 5: Signaling Systems - Asked in: 2074 Chaitra, 8 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/ss7-protocol-structure - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5 ## Question Describe common channel signaling system (SS7) protocol structure. ## Short answer SS7 is layered into MTP and user parts. MTP1 is the 64 kb/s data link, MTP2 provides reliable link transfer with flags, CRC and retransmission, and MTP3 handles routing and network management. SCCP adds subsystem addressing and global title translation; TCAP supports database dialogues, while ISUP/TUP handle circuit control and MAP/INAP/OMAP are applications. ## Answer **Common Channel Signaling System No. 7 (SS7)** is an ITU-T standard in which signaling for many trunks is carried as packets on a separate 64 kb/s link. Its protocol is layered into a **Message Transfer Part (MTP)** and several **user parts**. #### Protocol structure ```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 Level 1 – Signaling data link - Physical, bidirectional digital path, normally 64 kb/s (a timeslot of E1/T1); 56 kb/s in some ANSI networks. - Equivalent to the OSI physical layer. #### 2. MTP Level 2 – Signaling link - Gives reliable, in-sequence, error-free transfer on one link. - Flag delimitation, 16-bit CRC, forward/backward sequence numbers, retransmission (basic or preventive cyclic). - Initial alignment and signal unit error-rate monitoring. - Signal units: **MSU** (messages), **LSSU** (link status), **FISU** (fill-in). #### 3. MTP Level 3 – Signaling network - **Message handling:** discrimination (is it for this node?), distribution to the right user part (by service indicator), and routing using the **routing label** (DPC, OPC, SLS). - **Network management:** traffic, link and route management (changeover, changeback, rerouting on failure or congestion). #### 4. SCCP – Signaling Connection Control Part - Adds to MTP-3 to give full OSI layer-3 service. - **Global Title Translation** (e.g. a dialled number to point code + subsystem number). - Four protocol classes: 0, 1 (connectionless), 2, 3 (connection-oriented). #### 5. TCAP – Transaction Capabilities Application Part - Supports non-circuit dialogues such as database queries (free-phone, number portability, roaming). - Consists of a component sub-layer (invoke, result, error) and a transaction sub-layer. #### 6. User parts - **TUP (Telephone User Part):** older, basic telephone call control. - **ISUP (ISDN User Part):** circuit set-up and release for voice and ISDN: IAM, ACM, ANM, REL, RLC. - **MAP (Mobile Application Part):** GSM location update, authentication, SMS. - **INAP:** intelligent network services; **OMAP:** operation and maintenance. #### Summary MTP 1–3 plus SCCP form the **Network Service Part (NSP)**, giving the transport. ISUP/TUP use MTP directly for circuit-related signaling, while TCAP-based applications use SCCP for non-circuit transactions. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is SS7 protocol structured? A. A single flat layer B. Only the three MTP levels C. A Message Transfer Part plus several user parts and application parts D. Only SCCP and TCAP Answer: C. SS7 has MTP (levels 1-3) with SCCP and user parts such as ISUP, TUP, TCAP and applications above. (Key point: Structure overview) 2. Which are the two main groups of MTP level 3 functions? A. Flag delimitation and CRC B. Message handling (discrimination, distribution, routing) and network management C. Global title translation and subsystem numbers D. Alignment and error-rate monitoring Answer: B. Level 3 handles messages and manages traffic, links and routes. (Key point: MTP-3 functions) 3. What does Global Title Translation do? A. Converts a dialled number into point code plus subsystem number B. Adds CRC bits C. Allocates timeslots on E1 D. Starts charging Answer: A. GTT lets SCCP route by a number such as MSISDN to a destination node and subsystem. (Key point: SCCP GTT) 4. How do ISUP/TUP use the stack for circuit-related signaling? A. Through TCAP B. Through SCCP only C. Through MAP D. Directly on MTP, without SCCP Answer: D. ISUP/TUP sit directly on MTP, while TCAP applications use SCCP for non-circuit transactions. (Key point: ISUP route) ## 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 - [Signaling activities and SS7 architecture](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-activities-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 ## Related in other subjects - [UNIX goals; NFS protocol and structure](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Aunix-goals-nfs) — Operating System, chapter 5