--- title: "CCS7 network architecture and terminology" subject: "Telecommunications" chapter: 5 key: "semester_8/telecommunications:ccs7-architecture-terminologies" source: https://www.bctengineer.com/q/semester_8/telecommunications/ccs7-architecture-terminologies site: BCT Engineering --- # CCS7 network architecture and terminology Telecommunications · Chapter 5: Signaling Systems - Asked in: 2080 Bhadra, 4+4 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/ccs7-architecture-terminologies - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5 ## Question What is common channel signaling 7 (CCS7) network architecture? Explain the CCS7 terminologies. ## Short answer CCS7/SS7 is a separate packet-switched signaling network alongside the speech network, with SSPs connected to mated STP pairs by A-links, STP pairs interconnected by B/D/C links, and SCPs as databases. Key terms: signaling point, point code, link set, route, A-F link types, signaling modes, OPC/DPC/SLS, CIC, signal units MSU/LSSU/FISU, MTP, ISUP, SCCP, TCAP, MAP, INAP, global title. ## Answer #### CCS7 network architecture **Common Channel Signaling No. 7 (CCS7 / SS7)** uses a separate packet-switched signaling network to carry call-control and database messages between exchanges. The speech network and signaling network are separate but linked through the exchanges. ```text Signaling plane [SCP] [SCP] | A | A [STP]--------B/D--------[STP] | \ C / | | [STP]--------[STP] | A| |A [SSP]=======trunks=======[SSP] Speech (transport) plane ``` - Exchanges (**SSPs**) are connected to a **mated pair of STPs** by A-links. - STP pairs are joined to each other by B- or D-links and internally by C-links. - **SCPs** (databases) hang off STPs. - Heavy routes may have direct F-links between SSPs. - Everything is duplicated so that a single link or node failure does not stop signaling. #### CCS7 terminologies - **Signaling Point (SP):** any node that sends or receives SS7 messages; identified by a **point code** (14 bits in ITU). - **SSP (Service Switching Point):** an exchange with SS7 capability; originates and terminates calls. - **STP (Signal Transfer Point):** routes messages between links without being their origin or destination. - **SCP (Service Control Point):** database answering queries (toll-free, prepaid, HLR). - **Signaling link:** a 64 kbps bidirectional data link between two SPs. - **Link set:** group of links between the same two SPs (up to 16). - **Route / route set:** the sequence of link sets used to reach a destination. - **Link types:** A (access), B (bridge), C (cross), D (diagonal), E (extended), F (fully associated). - **Signaling modes:** associated (direct link alongside trunks), quasi-associated (via STPs on fixed route), non-associated (via STPs, route may vary). - **OPC / DPC / SLS:** originating point code, destination point code and signaling link selection, forming the **routing label**. - **CIC (Circuit Identification Code):** identifies the speech trunk to which an ISUP message refers. - **Signal units:** **MSU** (message signal unit, carries user data), **LSSU** (link status), **FISU** (fill-in, keeps link alive). - **MTP (Message Transfer Part):** levels 1–3 giving reliable transport and routing. - **User parts:** **ISUP** (call control), **TUP**, **SCCP** (global title translation), **TCAP** (transactions), **MAP**, **INAP**. - **Global Title (GT):** an address such as a dialled number that the STP translates into a point code and subsystem. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How are exchanges connected in the CCS7 architecture? A. Each SSP connects by A-links to a mated pair of STPs, with everything duplicated B. SSPs connect only to SCPs C. Exchanges are connected only by trunks D. Each SSP connects to a single STP without redundancy Answer: A. SSPs use A-links to a mated STP pair, and duplication ensures a single failure does not stop signaling. (Key point: CCS7 architecture) 2. What forms the routing label of an SS7 message? A. CIC, GT and TCAP B. OPC, DPC and SLS C. FISU, LSSU and MSU D. ISUP, TUP and SCCP Answer: B. The routing label has the originating point code, destination point code and signaling link selection. (Key point: Routing label) 3. What is the FISU used for? A. Carrying user data such as IAM B. Filling the link when idle and keeping it alive C. Reporting link status D. Translating a global title Answer: B. FISU is the fill-in signal unit. MSU carries user messages and LSSU carries link status. (Key point: Signal units) ## Related in Telecommunications - [Common channel signaling and SS7 call set-up](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Accs-ss7-call-setup) — Telecommunications, chapter 5, asked 2× - [Signaling; CAS vs common channel](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-cas-vs-ccs) — Telecommunications, chapter 5 - [CCS7 stack; why CCS over in-channel](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Accs7-stack-ccs-preferred) — Telecommunications, chapter 5 - [Signaling activities and SS7 architecture](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-activities-ss7-architecture) — Telecommunications, chapter 5 - [In-channel vs CCS; SS7 architecture](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ainchannel-ccs-ss7-architecture) — Telecommunications, chapter 5 - [In-channel multi-frequency signaling design](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ainchannel-mf-design) — Telecommunications, chapter 5 ## Related in other subjects - [Meaning of network architecture](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Anetwork-architecture-meaning) — Computer Networks and Security, chapter 1 - [Terminology used in stair construction](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Astair-terminology) — Building Technology, chapter 5 - [Earthquake terminology and seismic construction](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aearthquake-terminology-and-seismic-construction) — Engineering Geology I, chapter 2 - [Earthquake occurrence, terminology, causes, effects](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aearthquake-occurrence-terminology-causes-effects) — Engineering Geology I, chapter 2