--- title: "Advantage and working of SS7" subject: "Telecommunications" chapter: 5 key: "semester_8/telecommunications:ss7-advantage-working" source: https://www.bctengineer.com/q/semester_8/telecommunications/ss7-advantage-working site: BCT Engineering --- # Advantage and working of SS7 Telecommunications · Chapter 5: Signaling Systems - Asked in: 2070 Chaitra, 2+5 marks - Video: [what is SS7 ? - Signalling System Number-7 Architecture](https://www.youtube.com/watch?v=anfSFQvE8RE) — HackIsOn - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/ss7-advantage-working - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5 ## Question What is the advantage of common channel signaling system #7 (SS7)? Explain its working principle. ## Short answer SS7 provides fast call setup, seizes trunks only after the called party is free, and supports ISDN, mobile, and database services. It operates by transmitting digital signaling messages over a separate common-channel network, with ISUP for call control and TCAP for database queries, while voice trunks carry only speech. ## Answer #### Advantages of SS7 - Fast call set-up (whole number sent in one message at 64 kb/s). - Trunks are seized only after the called party is free, so trunk efficiency is higher. - One signaling link serves thousands of trunks; less per-circuit equipment. - Large, extendable message set: ISDN, caller ID, call forwarding, mobile (MAP), IN services. - Signaling possible during conversation; no fraud by tone imitation. - Database access (toll-free, number portability, HLR) through SCCP/TCAP; high reliability with CRC and redundant STPs. #### Working principle SS7 sends all control information as **digital messages** on a separate signaling network, while the voice trunks carry only speech. ```text Tel-A Tel-B | | SSP-A ---A link--- STP pair ---A link--- SSP-B | | | +====== voice trunks (CIC) ==============+ | SCP (database) ``` 1. **Message creation:** the originating SSP's user part (ISUP) builds a message (e.g. IAM) containing called/calling number and the **CIC** of the chosen trunk. 2. **Routing label:** MTP-3 adds DPC (destination), OPC (origin) and SLS. MTP-2 puts it in an MSU with sequence numbers and CRC; MTP-1 sends it on the 64 kb/s link. 3. **Transfer:** the STP reads the DPC and forwards the message on a link toward SSP-B; MTP-2 at each hop checks the CRC and acknowledges the message or asks for retransmission. 4. **Delivery:** SSP-B's MTP-3 sees its own point code and passes the message (by the service indicator) to ISUP. 5. **Call set-up sequence:** IAM → ACM (called party ringing) → ANM (answer; charging starts) → conversation on the trunk → REL → RLC (trunk freed). 6. **Non-circuit services:** for an 800 number, SSP-A first sends a TCAP query through SCCP/STP to the SCP, which returns the real routing number; then the normal ISUP sequence follows. 7. **Network management:** if a link fails, MTP-3 changes traffic over to another link or STP automatically. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does SS7 improve trunk efficiency? A. Trunks carry data at higher speed B. Trunks are seized only after the called party is found free C. Signaling tones are sent over the trunk D. Each trunk has its own signaling link Answer: B. Since signaling is separate, the trunk is reserved only after the called party is known to be free, so no trunk is held for busy calls. (Key point: Trunk efficiency) 2. What is the correct ISUP message sequence for a normal call? A. ACM, IAM, ANM, RLC, REL B. IAM, ANM, ACM, REL, RLC C. IAM, ACM, ANM, REL, RLC D. REL, IAM, ACM, ANM, RLC Answer: C. IAM starts the call, ACM means ringing, ANM means answer and charging starts, REL and RLC release the trunk. (Key point: Call set-up order) 3. What does the Circuit Identification Code (CIC) in the IAM tell the far end? A. Which voice trunk the message refers to B. The point code of the STP C. The charging rate of the call D. The CRC of the frame Answer: A. Because signaling is separate, the CIC names the voice trunk the message is about. (Key point: CIC) 4. How is a toll-free (800) number call handled? A. The SSP sends a TCAP query via SCCP/STP to an SCP, which returns the real routing number B. The STP translates it using ISUP only C. The called SSP charges the caller directly D. The trunk is seized first and the SCP is asked after answer Answer: A. The SSP queries an SCP database through SCCP/TCAP and then runs normal ISUP with the returned number. (Key point: 800 number) ## Close variants set in other exams - [CCS and SS7 working principle](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Accs-ss7-working-principle) — Telecommunications, chapter 5, asked 2× ## 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 - [Signaling system and reason for SS7](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-system-ss7-reason) — 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 - [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 - [N-channel enhancement MOSFET construction, working](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Amosfet-n-channel-enhancement) — Basic Electronics Engineering, chapter 3, asked 9× - [N-channel depletion MOSFET construction, working](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Amosfet-n-channel-depletion) — Basic Electronics Engineering, chapter 3, asked 9× - [N-channel JFET construction and working](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Ajfet-construction-working) — Electronics Devices and Circuit, chapter 3, asked 9× - [n-channel E-MOSFET construction and working](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3An-emosfet-construction-working) — Electronics Devices and Circuit, chapter 3, asked 9×