---
title: "Message transfer parts and signal unit addressing"
subject: "Telecommunications"
chapter: 5
key: "semester_8/telecommunications:mtp-signal-unit-addressing"
source: https://www.bctengineer.com/q/semester_8/telecommunications/mtp-signal-unit-addressing
site: BCT Engineering
---

# Message transfer parts and signal unit addressing

Telecommunications · Chapter 5: Signaling Systems

- Asked in: 2072 Kartik, 6+4 marks
- Video: [SS7 Protocol - MTP2 - Signalling unit - LSSU, MSU, FSSU](https://www.youtube.com/watch?v=wr5rBLyYNyU) — Telecom Bites
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/mtp-signal-unit-addressing
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5

## Question

What are message transfer parts? Write addressing of the signaling units.

## 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.

## Answer

#### 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.
```text
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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does the 4-bit service indicator in the SIO specify?
   A. The destination point code
   B. The user part to which the message is delivered, e.g. ISUP or SCCP
   C. The link number
   D. The length of the message
   Answer: B. SI selects the user part (0 SNM, 3 SCCP, 4 TUP, 5 ISUP); NI says national or international. (Key point: SIO fields)

2. What is the signaling link selection (SLS) field used for?
   A. Identifying the speech trunk
   B. Selecting a user part
   C. Choosing a link for load sharing while keeping message sequence
   D. Error detection
   Answer: C. SLS is the 4-bit field in the routing label for load sharing. (Key point: SLS use)

3. Why do LSSU and FISU carry no routing label?
   A. They are link-local and travel only on that link
   B. They are encrypted
   C. They are sent by the user parts
   D. They use the CIC instead
   Answer: A. They are addressed only by the link they travel on; only MSUs carry SIO and routing label. (Key point: LSSU and FISU addressing)

4. What does the CIC following the label in an ISUP message identify?
   A. The originating node
   B. The signaling link
   C. The service
   D. The speech trunk circuit
   Answer: D. The 12-bit (ITU) circuit identification code identifies the speech trunk. (Key point: CIC)

## Related in Telecommunications

- [SS7 message transfer part levels](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-mtp-levels) — Telecommunications, chapter 5
- [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

## Related in other subjects

- [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
- [SSB wave for a low pass message signal](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Assb-lowpass-message) — Communication Systems, chapter 2
- [Unit step signal and relation to impulse](https://www.bctengineer.com/question.md?key=semester_7%2Fdigital_signal_analysis_and_processing%3Aunit-step-relation-impulse) — Digital Signal Analysis and Processing, chapter 1
- [Even and odd parts of a signal](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aeven-odd-parts) — Data Communication, chapter 2