- 2079 Bhadra · 4 marks
SS7 message transfer part levels
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.
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.
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).
✓ Answer checked by Claude Opus 5.5 (high effort)
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