--- title: "Signaling system and reason for SS7" subject: "Telecommunications" chapter: 5 key: "semester_8/telecommunications:signaling-system-ss7-reason" source: https://www.bctengineer.com/q/semester_8/telecommunications/signaling-system-ss7-reason site: BCT Engineering --- # Signaling system and reason for SS7 Telecommunications · Chapter 5: Signaling Systems - Asked in: 2074 Asoj, 2+6 marks - Video: [SS7 Tutorial](https://www.youtube.com/watch?v=ccy3eUG03yA) — TechLair - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/signaling-system-ss7-reason - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5 ## Question What is signaling system? What is the reason behind the development of SS7? ## Short answer A signaling system exchanges control information between subscriber and exchange and between exchanges for setup, supervision, charging and release. SS7 was developed because older in-channel systems were slow, wasted trunk capacity, had limited codes, allowed fraud, and could not access databases or signal during calls; SS7 is fast, secure, flexible and reliable. ## Answer #### Signaling system A **signaling system** is the set of signals, codes and procedures used to exchange control information between subscriber and exchange and between exchanges, so that calls can be set up, supervised, charged and released. Signals are of three kinds: **supervisory** (seize, answer, clear), **address** (dialled digits) and **call-progress** (dial tone, ringing, busy tone). #### Reasons behind the development of SS7 Older systems (DC loop, single-frequency 2600 Hz, MF R1/R2 and CAS on E1 TS16) sent signals inside or alongside each voice channel. As networks became digital and larger, these methods had serious limits, which SS7 (ITU-T Q.700 series) was designed to remove. 1. **Slow call set-up:** MF digits are sent one by one with acknowledgements, taking several seconds per link. SS7 sends the whole address in one IAM message at 64 kb/s; set-up falls to well under a second. 2. **Wasted trunk capacity:** with in-channel signaling the speech trunk is seized for the whole set-up even if the called line is busy. SS7 checks first and seizes the trunk only when needed. 3. **Limited signal repertoire:** tone systems have only about 15 codes. SS7 messages are flexible and new messages and parameters can be added (ISDN, calling line identification, call forwarding). 4. **Fraud:** in-band tones could be imitated by users ("blue box" with 2600 Hz). SS7 links are out of reach of subscribers. 5. **No database access:** modern services (toll-free numbers, number portability, credit-card calling, mobile roaming via HLR/VLR, SMS) need queries to remote databases. SS7 with SCCP/TCAP supports these non-circuit transactions. 6. **Signaling during the call:** CAS cannot easily exchange information once speech starts; SS7 can at any time (supplementary services). 7. **Efficiency:** one 64 kb/s SS7 link can serve thousands of trunks, replacing per-trunk signaling equipment. 8. **Reliability and management:** SS7 has CRC error control, retransmission, redundant STP pairs and automatic re-routing. 9. **Digital and ISDN compatibility:** a message-based system suits stored-program-control digital exchanges and ISDN/mobile (GSM) networks. 10. **International standard:** one common system makes interworking between operators and countries simple. #### Result SS7 became the "nervous system" of the PSTN and GSM networks: an out-of-band packet network of SSPs, STPs and SCPs that is fast, secure, flexible and supports intelligent network services. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why was set-up slow in older MF systems compared with SS7? A. Digits were sent one by one with acknowledgements, taking seconds per link B. The whole address was sent at once C. Trunks were too many D. Exchanges were digital Answer: A. SS7 sends the whole address in one IAM at 64 kb/s, giving set-up well under a second. (Key point: Slow set-up) 2. Why is SS7 safer against fraud than in-band tones? A. It uses louder tones B. It uses 2600 Hz tones C. It uses rotary dials D. Its links are out of reach of subscribers, so tones cannot be imitated Answer: D. Blue-box fraud imitated 2600 Hz in-band tones; SS7 signaling is on separate links. (Key point: Blue box fraud) 3. Which feature of SS7 supports toll-free numbers, number portability and roaming? A. Flag delimitation B. SCCP/TCAP queries to remote databases C. DTMF dialling D. Ring-back tone Answer: B. SCCP and TCAP support non-circuit transactions with databases, which CAS cannot. (Key point: Database services) ## 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 - [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 - [Importance of signaling; SS7 protocol stack](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-importance-ss7-stack) — 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 ## Related in other subjects - [Expert system development steps](https://www.bctengineer.com/question.md?key=semester_6%2Fartificial_intelligence%3Aexpert-system-development-steps) — Artificial Intelligence, chapter 7, asked 3× - [Activities in OO system development](https://www.bctengineer.com/question.md?key=semester_6%2Fobject_oriented_analysis_and_design%3Aactivities-oo-system-development) — Object Oriented Analysis and Design, chapter 1 - [Water supply system and historical development](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aws-historical-development) — Water Supply Engineering, chapter 1 - [System development methodologies](https://www.bctengineer.com/question.md?key=semester_7%2Fict_project_management%3Asystem-development-methodologies) — ICT Project Management, chapter 4