--- title: "SS7 signaling points: STP, SSP, SCP" subject: "Telecommunications" chapter: 5 key: "semester_8/telecommunications:ss7-stp-ssp-scp" source: https://www.bctengineer.com/q/semester_8/telecommunications/ss7-stp-ssp-scp site: BCT Engineering --- # SS7 signaling points: STP, SSP, SCP Telecommunications · Chapter 5: Signaling Systems - Asked in: 2081 Bhadra, 4+4 marks - Video: [Signaling System 7 (SS7): MTP, TCAP, ISUP, SCCP, and Network Elements](https://www.youtube.com/watch?v=BP2gMEkE0wc) — Typhon’s Workshop - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/ss7-stp-ssp-scp - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5 ## Question What are the main signaling points in the SS7 network and what roles do they play in the signaling process? Discuss the functions of Signal Transfer Points (STPs), Signal Switching Points (SSPs), and Signal Control Points (SCPs). ## Short answer SSP is an exchange that originates/ends call signaling, converting subscriber events into ISUP messages and sending TCAP queries. STP is a packet router that forwards messages by point code, performs global title translation and is deployed in mated pairs. SCP is a database answering queries for toll-free, prepaid, number portability and mobile services. ## Answer #### Main signaling points in an SS7 network A **signaling point (SP)** is any node that can send, receive or forward SS7 messages. Each has a unique **point code** (14 bits in ITU). There are three main types: **SSP**, **STP** and **SCP**. ```text [SCP] [SCP] databases |A |A [STP]=====C====[STP] mated pair | \ / | A \--B/D--/ A | | [SSP] ======trunk===== [SSP] exchanges | | phones phones ``` Roles in the signaling process: - **SSPs** start and end call signaling (they are the "users"). - **STPs** relay messages (they are the "routers"). - **SCPs** answer queries (they are the "databases"). #### Signal Switching Point (SSP) (Standard ITU-T/ANSI name: **Service Switching Point**; similarly the SCP is the **Service Control Point**.) - A telephone exchange (local, tandem or mobile switch) with SS7 software. - Converts subscriber events (off-hook, digits, answer, on-hook) into **ISUP messages** (IAM, ACM, ANM, REL, RLC) for trunk set-up and release. - Detects calls needing special handling (e.g. 1660/800 numbers, prepaid) and sends **TCAP queries** to an SCP, then routes the call using the reply. - Is the origin or end point of signaling messages; it does not normally relay messages for others. #### Signal Transfer Point (STP) - A high-capacity **packet switch/router** for SS7 messages; it does not switch speech. - Receives messages on one link and forwards them on another using the **destination point code** (MTP level 3 routing). - Performs **global title translation** (SCCP) to find which SCP should get a query (e.g. translate a toll-free number to the SCP point code). - Provides **screening** (gateway security), measurement of traffic, and network management such as rerouting around failed links. - Deployed in **mated pairs** joined by C-links so that one failure does not cut the network. #### Signal Control Point (SCP) - A **database server** connected to STPs by A-links. - Answers TCAP queries from SSPs and gives instructions for the call. - Supports **Intelligent Network** services: toll-free and premium-rate number translation, calling card and prepaid validation, number portability, virtual private networks. - In mobile networks, the HLR/VLR databases play the SCP role through MAP. - Usually duplicated (mated SCPs) for reliability. #### Example: toll-free call 1. The caller dials a toll-free number; the **SSP** sends a TCAP query. 2. The **STP** performs global title translation and forwards it to the right **SCP**. 3. The **SCP** returns the real routing number. 4. The SSP sends an ISUP **IAM** through the STP to the far SSP to set up the trunk. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which describes the roles correctly? A. SSP is the database, STP is the exchange, SCP is the router B. SSP starts and ends signaling, STP relays messages, SCP answers queries C. SSP relays, STP answers queries, SCP originates calls D. All three only relay messages Answer: B. SSPs are the users (exchanges), STPs the routers and SCPs the databases. (Key point: Roles of the three SPs) 2. What does an STP do with a toll-free number in a TCAP query? A. Rings the called party B. Performs SCCP global title translation to find the SCP point code C. Charges the caller D. Stores the real routing number permanently Answer: B. The STP translates the dialled number (global title) into the point code of the SCP, then forwards the query. The SCP returns the real number. (Key point: STP global title translation) 3. Why are STPs deployed in mated pairs joined by C-links? A. To double the speech capacity B. So one failure does not cut the signaling network C. To translate numbers faster D. To avoid using point codes Answer: B. Redundancy keeps signaling working if one STP or link fails. (Key point: Why mated pairs) 4. What does an SSP do when it detects a toll-free or prepaid call? A. Sends a TCAP query to an SCP and routes the call using the reply B. Sends REL immediately C. Forwards the call to an STP without any message D. Rings the called party directly Answer: A. SSP queries the SCP via TCAP, gets the real number, then sends ISUP IAM to set up the trunk. (Key point: SSP detection of special calls) ## 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× - [Importance of signaling; SS7 building blocks](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-importance-ss7-blocks) — Telecommunications, chapter 5 - [SS7 message transfer part levels](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-mtp-levels) — Telecommunications, chapter 5 - [Importance of signaling; ITU-T SS7](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-importance-itu-ss7) — 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 - [Signaling system and reason for SS7](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-system-ss7-reason) — Telecommunications, chapter 5 ## Related in other subjects - [Context switching; distributed vs network OS](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Acontext-switch-dos) — Distributed System, chapter 3 - [4×4 omega switching network](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Aomega-network) — Computer Organization and Architecture, chapter 8 - [Process vs thread; context switching](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Athread-context-switching) — Operating System, chapter 2 - [Control memory vs main memory](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Acontrol-vs-main-memory) — Computer Organization and Architecture, chapter 3