--- title: "SS7 message transfer part levels" subject: "Telecommunications" chapter: 5 key: "semester_8/telecommunications:ss7-mtp-levels" source: https://www.bctengineer.com/q/semester_8/telecommunications/ss7-mtp-levels site: BCT Engineering --- # SS7 message transfer part levels Telecommunications · Chapter 5: Signaling Systems - Asked in: 2079 Bhadra, 4 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-mtp-levels - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5 ## Question What is the purpose of the message transfer part (MTP) in SS7 signaling? Provide the specific functions and the frame format for MTP level-1, MTP level-2 and MTP level-3 in SS7. ## Short answer MTP transports signaling messages reliably and in sequence. MTP level 1 is a 64 kbps digital channel. Level 2 provides framing (flags), error detection (CRC-16), and sequence numbering/retransmission via signal units (FISU, LSSU, MSU). Level 3 handles message routing using point codes and SLS, and network management for rerouting on failures. ## Answer The **Message Transfer Part (MTP)** is the lower three levels of SS7. Its purpose is to carry signaling messages from user parts (ISUP, SCCP, TCAP) **reliably, in sequence and without loss or duplication** between signaling points, and to keep the signaling network working when links or nodes fail. #### MTP Level 1: signaling data link - Physical, electrical and functional layer: a bidirectional **64 kbps digital channel** (usually TS16 of a 2.048 Mbps E1; 56 kbps in ANSI networks). - Transmits the bit stream; has no frame of its own. #### MTP Level 2: signaling link Functions: frame delimitation with flags, zero insertion, **error detection** (16-bit CRC), **error correction** by retransmission (basic or preventive cyclic), sequence numbering, link alignment, error-rate monitoring and flow control. **Signal unit format** (bits, sent left to right): | F | BSN | BIB | FSN | FIB | LI | Spare | SIO | SIF | CK | F | |---|---|---|---|---|---|---|---|---|---|---| | 8 | 7 | 1 | 7 | 1 | 6 | 2 | 8 | 8n | 16 | 8 | - **F:** flag 01111110. **BSN/BIB:** backward sequence number and indicator bit (acknowledgement). **FSN/FIB:** forward sequence number and indicator bit. **LI:** length indicator. **CK:** CRC check bits. - **FISU** (fill-in, LI = 0): no SIO/SIF; keeps the link alive. - **LSSU** (link status, LI = 1 or 2): carries a status field for alignment. - **MSU** (message, LI ≥ 3): carries SIO and SIF (up to 272 octets). #### MTP Level 3: signaling network Functions: - **Message handling:** discrimination (is it for this node?), distribution to the right user part (from SIO), and routing to the next link using the routing label. - **Network management:** traffic, link and route management (changeover, changeback, rerouting, congestion control). **Format used by level 3** (inside the MSU): | SIO (8) | DPC (14) | OPC (14) | SLS (4) | User data | |---|---|---|---|---| - **SIO:** service indicator (4 bits: which user part, e.g. ISUP) and subservice field (network indicator). - **Routing label (32 bits):** destination point code, originating point code and signaling link selection (load sharing). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the main purpose of the Message Transfer Part? A. To carry signaling messages reliably, in sequence and without loss or duplication between signaling points B. To carry the speech samples of the call between exchanges C. To translate dialled numbers into global titles D. To store subscriber data for free-phone services Answer: A. MTP is the lower three levels of SS7 that transports user-part messages reliably and keeps the network working on failures. (Key point: Purpose of MTP) 2. Which set of functions belongs to MTP level 2? A. Routing by DPC and changeover B. Flag delimitation, 16-bit CRC error detection and retransmission C. Global title translation D. Providing the 64 kbps physical channel Answer: B. Level 2 makes one signaling link reliable using flags, CRC, sequence numbers and retransmission. (Key point: MTP-2 error control) 3. What does the 32-bit routing label used by MTP level 3 contain? A. FSN, BSN and CRC B. SIO, LI and flag C. DPC (14), OPC (14) and SLS (4) D. CIC, SSN and GT Answer: C. The ITU routing label is 14-bit DPC + 14-bit OPC + 4-bit SLS, the SLS being used for load sharing. (Key point: MTP-3 routing label) 4. Which signal unit has LI = 0 and no SIO/SIF, and keeps the link alive? A. MSU B. LSSU C. MSU with 272 octets D. FISU Answer: D. FISU (fill-in) is sent when there is no traffic; LSSU has LI 1 or 2 and MSU has LI of 3 or more. (Key point: Signal unit by LI) ## Related in Telecommunications - [SS7 vs OSI; MTP-L2 signal units](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-vs-osi-mtp2-units) — Telecommunications, chapter 5 - [SS7 signaling points: STP, SSP, SCP](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-stp-ssp-scp) — Telecommunications, chapter 5 - [Signaling connection control part](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asccp-message-structure) — 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× - [Importance of signaling; SS7 building blocks](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-importance-ss7-blocks) — 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 - [Head, specific speed and part load in turbine choice](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aturbine-head-speed-part-load) — Power Plant Design, chapter 3 - [E1 frame, TDM hierarchy and signaling rate](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ae1-frame-hierarchy) — Communication Systems, chapter 5 - [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 - [Web-mail to POP3 message transfer protocols](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Aalice-bob-email-protocols) — Internet and Intranet, chapter 3