- 2072 Kartik · 6+4 marks
Message transfer parts and signal unit addressing
Short answer
MTP is the lower part of SS7 that reliably transfers messages using three levels: data link, link functions, and network functions. Addressing is in the MSU via SIO (service indicator, network indicator) and routing label containing DPC (14 bits), OPC (14 bits), and SLS (4 bits); LSSU and FISU have no routing label.
What are message transfer parts? Write addressing of the signaling units.
Message Transfer Part (MTP)
The MTP is the lower part of the SS7 stack that transfers signaling messages reliably from one signaling point to another for all user parts (ISUP, TUP, SCCP). It has three levels.
MTP Level 1 – Signaling data link
- A bidirectional transmission path at 64 kb/s (56 kb/s in ANSI), usually a timeslot of E1/T1.
- Defines electrical and physical characteristics (OSI layer 1).
MTP Level 2 – Signaling link functions
- Delimitation: each signal unit starts and ends with flag 01111110 (bit stuffing used).
- Error detection: 16-bit CRC check bits.
- Error correction: basic method (go-back-N using FSN/BSN and FIB/BIB) or preventive cyclic retransmission on long-delay links.
- Initial alignment of a new link and error-rate monitoring (SUERM).
- Flow control using LSSU "busy".
- Signal units: MSU, LSSU, FISU.
MTP Level 3 – Signaling network functions
- Message handling: discrimination (is the DPC this node?), distribution (by service indicator to SCCP, ISUP, TUP...) and routing (choice of outgoing link set; load sharing with SLS).
- Network management: traffic management (changeover, changeback, forced rerouting), link management (activate, restore links), route management (transfer-prohibited/allowed messages between STPs).
Addressing of signal units
Addressing lives in the MSU, in the SIO and the routing label at the start of the SIF.
MSU: |F|BSN|BIB|FSN|FIB|LI|SIO| SIF |CK|F|
|
+-----------------+------------+
| Routing label | User data |
| DPC | OPC | SLS | |
Service Information Octet (SIO), 8 bits
| Sub-field | Bits | Meaning |
|---|---|---|
| Service indicator (SI) | 4 | User part: 0 SNM, 1 test/maint., 3 SCCP, 4 TUP, 5 ISUP |
| Network indicator (NI) | 2 | International / national network |
| Spare/priority | 2 | Spare (priority in ANSI) |
Routing label (ITU-T), 32 bits
| Field | Bits | Meaning |
|---|---|---|
| DPC – destination point code | 14 | Node the message is for |
| OPC – originating point code | 14 | Node that sent it |
| SLS – signaling link selection | 4 | Picks link for load sharing, keeps sequence |
In ANSI networks point codes are 24 bits (network–cluster–member, 8 bits each) and SLS is 5 or 8 bits, giving a 56-bit label. For ISUP messages a CIC (circuit identification code, 12 bits in ITU) follows the label and identifies the speech trunk.
LSSU and FISU are link-local, so they carry no routing label: they are "addressed" only by the link on which they travel.
✓ Answer checked by Claude Opus 5.5 (high effort)
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