- 2076 Asoj · 2+6 marks
SS7 vs OSI; MTP-L2 signal units
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).
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).
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).
| 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.
✓ Answer checked by Claude Opus 5.5 (high effort)
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