--- title: "SS7 vs OSI; MTP-L2 signal units" subject: "Telecommunications" chapter: 5 key: "semester_8/telecommunications:ss7-vs-osi-mtp2-units" source: https://www.bctengineer.com/q/semester_8/telecommunications/ss7-vs-osi-mtp2-units site: BCT Engineering --- # SS7 vs OSI; MTP-L2 signal units Telecommunications · Chapter 5: Signaling Systems - Asked in: 2076 Asoj, 2+6 marks - Video: [Signaling System 7 (SS7): MTP, TCAP, ISUP, SCCP, and Network Elements](https://www.youtube.com/watch?v=BP2gMEkE0wc) — Typhon’s Workshop - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/ss7-vs-osi-mtp2-units - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5 ## Question Compare the SS7 protocol stack with the OSI layers. Describe the purpose and the format of the signal units transferred by the second message transfer part (MTP-L2). ## Short answer SS7 is a four-level stack: MTP1 physical, MTP2 data link, MTP3 plus SCCP network, and TCAP/ISUP/TUP application; ISUP and TUP use MTP3 directly. MTP2 gives reliable links using flags, 16-bit CRC, sequence numbers, retransmission, alignment and error monitoring. Its signal units are MSU (SIO/SIF messages), LSSU (link status field) and FISU (fill-in, LI=0). ## Answer #### SS7 vs OSI layers | OSI layer | SS7 equivalent | Function | |---|---|---| | 1 Physical | MTP Level 1 | 64 kb/s signaling data link | | 2 Data link | MTP Level 2 | Error control, sequencing, flags | | 3 Network | MTP Level 3 + SCCP | Routing by point code; global title | | 4–6 | (none; TCAP covers part) | Not separately defined | | 7 Application | TCAP, ISUP, TUP, MAP, OMAP | Call control and database queries | ISUP and TUP sit directly on MTP-3 (they skip SCCP), so SS7 is a 4-level stack rather than a strict 7-layer one. #### Purpose of MTP Level 2 MTP-2 makes a single signaling link reliable: it delimits units with flags, detects errors with a 16-bit CRC, keeps the order with sequence numbers, retransmits lost or errored units, aligns a new link, and monitors the error rate so a bad link is taken out of service. Three kinds of **signal units (SU)** are used. #### 1. Message Signal Unit (MSU) Carries real signaling messages from MTP-3 users (ISUP, SCCP, network management). ```text | F | BSN|BIB| FSN|FIB| LI |Sp| SIO | SIF | CK | F | | 8 | 7 | 1 | 7 | 1 | 6 |2 | 8 | 8n, n>=2 | 16 | 8 | ``` #### 2. Link Status Signal Unit (LSSU) Carries link status (alignment, out of service, processor outage, busy) in a **Status Field (SF)** of 8 or 16 bits in place of SIO+SIF. #### 3. Fill-In Signal Unit (FISU) Sent when there is no traffic, to keep the link synchronised and to carry acknowledgements; it has no SIO/SIF (LI = 0). #### Fields - **F (Flag):** 01111110, opens and closes each unit. - **BSN / BIB:** backward sequence number and indicator bit, used to acknowledge or ask for retransmission (BIB inverted = negative ack). - **FSN / FIB:** forward sequence number of this unit and its indicator bit. - **LI (Length Indicator):** number of octets after LI: 0 = FISU, 1 or 2 = LSSU, 3 or more = MSU (63 if above 62). - **SIO (Service Information Octet):** service indicator (which user: SNM, SCCP, TUP, ISUP) and network indicator (national/international). - **SIF (Signaling Information Field):** routing label (DPC, OPC, SLS) and user data, up to 272 octets. - **CK:** 16-bit CRC check bits. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which SS7 element corresponds to OSI layer 3? A. MTP level 1 B. TCAP only C. MTP level 2 D. MTP level 3 plus SCCP Answer: D. Routing by point code (MTP-3) and global title (SCCP) together give network-layer function. (Key point: SS7 vs OSI mapping) 2. How does the length indicator (LI) identify the signal unit type? A. LI = 0 FISU, 1 or 2 LSSU, 3 or more MSU B. LI = 0 MSU, 1 LSSU, 2 FISU C. LI = 63 FISU D. LI is not related to unit type Answer: A. LI counts octets after the LI field; 0 means FISU, 1-2 LSSU, 3 or more MSU. (Key point: LI meaning) 3. What does an inverted BIB signal? A. A new message has begun B. A negative acknowledgement, requesting retransmission C. The link is aligned D. The flag is corrupted Answer: B. BSN/BIB acknowledge received units; inverting BIB asks for retransmission. (Key point: BIB use) 4. What does the Link Status Signal Unit carry? A. Routing label and user data B. Speech samples C. Link status such as alignment or processor outage in the status field D. The CRC of the previous unit only Answer: C. LSSU has an 8 or 16 bit status field in place of SIO+SIF. (Key point: LSSU purpose) ## Related in Telecommunications - [SS7 message transfer part levels](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-mtp-levels) — Telecommunications, chapter 5 - [Importance of signaling; SS7 protocol stack](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-importance-ss7-stack) — Telecommunications, chapter 5 - [Message transfer parts and signal unit addressing](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amtp-signal-unit-addressing) — Telecommunications, chapter 5 - [Communication protocol and OSI architecture](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aprotocol-osi-architecture) — Telecommunications, chapter 8 - [SS7 protocol structure](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-protocol-structure) — Telecommunications, chapter 5 ## Related in other subjects - [Explain OSI model and its layers](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aosi-model-layers) — Computer Networks and Security, chapter 1, asked 2× - [Significance of OSI layers](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aosi-significance-layers) — Computer Networks and Security, chapter 1 - [Tasks done at OSI layers (set 2)](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aosi-layer-task-mapping-2) — Computer Networks and Security, chapter 1 - [Tasks done at OSI layers (set 1)](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aosi-layer-task-mapping-1) — Computer Networks and Security, chapter 1