--- title: "SS7 building blocks and links" subject: "Telecommunications" chapter: 5 key: "semester_8/telecommunications:ss7-building-blocks-links" source: https://www.bctengineer.com/q/semester_8/telecommunications/ss7-building-blocks-links site: BCT Engineering --- # SS7 building blocks and links Telecommunications · Chapter 5: Signaling Systems - Asked in: 2070 Asar, 5+3 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/ss7-building-blocks-links - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5 ## Question Explain the major building blocks or parts used in SS7 network. Show the different links used in SS7 network also. ## Short answer SS7 building blocks are Service Switching Points (SSPs), Signal Transfer Points (STPs) in mated pairs, Service Control Points (SCPs), and signaling link sets. Links are A (access), B (bridge), C (cross), D (diagonal), E (extended), and F (fully associated), plus link sets between nodes. ## Answer #### Major building blocks of an SS7 network SS7 is a packet-switched signaling network separate from the voice network. Its nodes are called **signaling points**, each identified by a **point code**. 1. **Service Switching Point (SSP):** a telephone exchange (local, tandem or MSC) with SS7 software. It originates and terminates ISUP call-control messages, and sends TCAP queries to SCPs when a call needs database information (e.g. toll-free number). 2. **Signal Transfer Point (STP):** a packet switch that receives SS7 messages and routes them to the correct destination by point code or **global title translation**. STPs are installed in **mated pairs** so that the failure of one does not stop signaling. They may also screen messages (gateway STP between operators). 3. **Service Control Point (SCP):** a database server that answers TCAP queries: 800/free-phone translation, local number portability, calling-card validation, prepaid, HLR in mobile networks. Also usually deployed in pairs. 4. **Signaling links and link sets:** 64 kb/s bidirectional channels connecting the nodes; parallel links between two nodes form a link set (up to 16 links) and share load. #### Links used in SS7 ```text A A SSP1 ---> STP-a <==C==> STP-b <--- SSP1 || \ / || B/D|| B B ||B/D || / \ || STP-c <==C==> STP-d ---A--- SSP2 SSP1 -------------F------------------ SSP2 SSP1 ----E---- STP-c (alternate pair) SCP ----A---- STP-a / STP-b ``` | Link | Connects | Purpose | |---|---|---| | A (Access) | SSP/SCP to its home STP pair | Normal entry to the network | | B (Bridge) | Two mated STP pairs at the same level | Carry traffic between regions | | C (Cross) | The two STPs of one mated pair | Used only when a link fails | | D (Diagonal) | Local STP pair to higher-level (regional) pair | Hierarchical routing | | E (Extended) | SSP to a remote (non-home) STP pair | Backup route | | F (Fully associated) | Directly between two SSPs | Heavy traffic between two switches, no STP | The "quad" arrangement of B/D links between two mated pairs gives each message several alternative paths, giving very high availability. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the main job of a Signal Transfer Point (STP) in SS7? A. Store subscriber databases and answer TCAP queries B. Route SS7 messages to the destination using point code or global title translation C. Originate and terminate ISUP call-control messages D. Carry the voice of the call between exchanges Answer: B. An STP is a packet switch that routes signaling messages by point code or global title. Databases are SCPs and call origination is done by SSPs. (Key point: STP role) 2. Why are STPs installed in mated pairs? A. To double the 64 kb/s link speed B. So that failure of one STP does not stop signaling C. To carry voice and signaling separately D. To allow SSPs to talk without any link Answer: B. Mated pairs give redundancy: if one STP fails the other carries the signaling. (Key point: Mated pairs) 3. Which link connects the two STPs of one mated pair and is used only when a link fails? A. A link B. B link C. C link D. F link Answer: C. C (Cross) links join the mated STPs and are used for rerouting on failure. (Key point: C link) 4. When is an F (fully associated) link used? A. For direct signaling between two SSPs with heavy traffic, without an STP B. To connect an SCP to its home STP pair C. To connect local STPs to regional STPs D. As a backup from an SSP to a non-home STP pair Answer: A. F links go directly between two SSPs, bypassing STPs, when traffic between them is heavy. (Key point: F link) ## Related in Telecommunications - [Importance of signaling; SS7 building blocks](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-importance-ss7-blocks) — Telecommunications, chapter 5 - [In-channel vs CCS; SS7 link types](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ainchannel-ccs-ss7-links) — Telecommunications, chapter 5 ## Related in other subjects - [Direct mapping with 32-block memory, 10 blocks used](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Adirect-mapping-example-10) — Computer Organization and Architecture, chapter 6, asked 3× - [Direct mapping with 64-block memory, 10 blocks used](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Adirect-mapping-example-64) — Computer Organization and Architecture, chapter 6, asked 3× - [IRC, FoIP; e-commerce building blocks](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Airc-foip-ecommerce-blocks) — Internet and Intranet, chapter 7 - [Problem statement and UML building blocks](https://www.bctengineer.com/question.md?key=semester_6%2Fobject_oriented_analysis_and_design%3Aresult-management-problem-statement-uml-blocks) — Object Oriented Analysis and Design, chapter 2