--- title: "Signaling connection control part" subject: "Telecommunications" chapter: 5 key: "semester_8/telecommunications:sccp-message-structure" source: https://www.bctengineer.com/q/semester_8/telecommunications/sccp-message-structure site: BCT Engineering --- # Signaling connection control part Telecommunications · Chapter 5: Signaling Systems - Asked in: 2073 Shrawan, 2+6 marks - Video: [4 SCCP](https://www.youtube.com/watch?v=NyG-GedyIf0) — eLearning By Experts - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/sccp-message-structure - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5 ## Question What is signaling connection control part? Explain its message structure. ## Short answer SCCP is the SS7 layer above MTP3 that adds subsystem numbers and global title translation, forming the Network Service Part with MTP. Its message has routing label, message type code, mandatory fixed part, mandatory variable part with pointers, and optional part. It supports connectionless classes 0/1 and connection-oriented classes 2/3. ## Answer #### Signaling Connection Control Part (SCCP) **SCCP** is the SS7 layer above MTP Level 3. MTP-3 can only route to a node by **point code** and deliver to a user part. SCCP adds addressing of **subsystems** inside a node (by Subsystem Number, SSN) and **Global Title Translation (GTT)**, so a message addressed by a dialled number or IMSI can be routed. MTP + SCCP form the **Network Service Part**, equal to OSI layer 3. SCCP is used by TCAP, MAP and INAP. It offers four protocol classes: | Class | Service | |---|---| | 0 | Basic connectionless | | 1 | Sequenced connectionless | | 2 | Basic connection-oriented | | 3 | Connection-oriented with flow control | #### SCCP message structure An SCCP message is carried in the SIF of an MTP MSU (Service Indicator = 3). ```text +---------------------------------------+ | Routing label (DPC, OPC, SLS) | +---------------------------------------+ | Message type code (1 octet) | +---------------------------------------+ | Mandatory fixed part | +---------------------------------------+ | Mandatory variable part | | (pointers + length + value) | +---------------------------------------+ | Optional part (name, length, value) | +---------------------------------------+ ``` 1. **Routing label:** DPC (14 bits), OPC (14 bits), SLS (4 bits) in ITU format; used by MTP-3. 2. **Message type code:** one octet giving the message, e.g. CR (connection request), CC (connection confirm), CREF (refused), DT1/DT2 (data form 1/2), RLSD (released), RLC (release complete), UDT (unitdata), UDTS (unitdata service), XUDT (extended unitdata). 3. **Mandatory fixed part:** parameters of fixed length and order for that message type, so no name or length is needed, e.g. protocol class, source/destination local reference. 4. **Mandatory variable part:** variable-length parameters such as **called party address**, **calling party address** and **data**. Pointers at the start give the offset of each parameter; each parameter starts with its length. 5. **Optional part:** parameters that may or may not appear; each coded as name + length + value, ending with an end-of-optional-parameters octet. #### SCCP address (called/calling party) ```text | Address indicator | SPC | SSN | Global title | ``` - **Address indicator:** shows whether PC, SSN and GT are present and whether routing is on GT or on PC+SSN. - **SPC:** signaling point code; **SSN:** subsystem (e.g. HLR = 6, VLR = 7, MSC = 8). - **Global title:** digits (e.g. E.164 or E.212 number) with translation type and numbering plan. #### Example A UDT carrying a MAP "send routing info" query from an MSC to an HLR uses connectionless class 0; the called party address holds the subscriber's MSISDN as global title, which the STP translates to the HLR's point code and SSN 6. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does SCCP add to MTP-3? A. Addressing of subsystems by SSN and global title translation B. Flag delimitation C. Error correction on a link D. Physical 64 kb/s channel Answer: A. MTP routes only by point code; SCCP allows routing by number and addressing of subsystems. (Key point: SCCP purpose) 2. Which parameters are in the mandatory variable part of an SCCP message? A. Message type code only B. Protocol class and local references C. Parameters like called/calling party address and data, located using pointers and lengths D. The routing label Answer: C. Variable-length mandatory parameters are found using pointers and each begins with its length. (Key point: Mandatory variable part) 3. Which SCCP classes are connectionless? A. Classes 2 and 3 B. Classes 0 and 1 C. Classes 1 and 2 D. Only class 3 Answer: B. Classes 0 and 1 are connectionless; 2 and 3 are connection-oriented. (Key point: Protocol classes) 4. What identifies the target application (e.g. HLR) inside a node in an SCCP address? A. SPC B. CIC C. Subsystem number (SSN) D. SLS Answer: C. SSN identifies the subsystem, e.g. HLR = 6, VLR = 7, MSC = 8. (Key point: SSN) ## Related in Telecommunications - [SS7 message transfer part levels](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-mtp-levels) — 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 - [SS7 signaling points: STP, SSP, SCP](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-stp-ssp-scp) — Telecommunications, chapter 5 - [SS7 protocol structure](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-protocol-structure) — Telecommunications, chapter 5 ## Related in other subjects - [GSM frame structure, traffic and control channels](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-frame-traffic-control-channels) — Wireless Communication, chapter 8, asked 4× - [Failure mechanisms of bolted connection](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Abolted-failure-mechanisms) — Design of Steel and Timber Structure, chapter 4 - [Bolted bracket connection, 200 kN at 250 mm](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Abracket-bolted-200kn-250mm) — Design of Steel and Timber Structure, chapter 4 - [Splice connection for ISHB 250, 900 kN](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Acolumn-splice-ishb250-900kn) — Design of Steel and Timber Structure, chapter 6