--- title: "Sequence of call steps in SS7" subject: "Telecommunications" chapter: 5 key: "semester_8/telecommunications:ss7-call-steps-sequence" source: https://www.bctengineer.com/q/semester_8/telecommunications/ss7-call-steps-sequence site: BCT Engineering --- # Sequence of call steps in SS7 Telecommunications · Chapter 5: Signaling Systems - Asked in: 2076 Chaitra, 7 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/ss7-call-steps-sequence - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5 ## Question List various sequence of steps used during call steps in SS7 system. ## Short answer Caller off-hook; SSP-A collects digits, selects trunk, sends IAM to SSP-B. SSP-B rings called party, returns ACM (ringback). When answered, SSP-B sends ANM, speech path connects, charging starts. After conversation, hanging-up side sends REL; other side replies RLC, trunk released. ## Answer In SS7 a call is set up and released by **ISUP messages** exchanged between the originating and terminating Service Switching Points (SSPs) through Signal Transfer Points (STPs), while the voice trunk itself carries only speech. #### Network arrangement ```text Caller--SSP-A ----- STP ----- SSP-B--Called | (signaling link) | +======= voice trunk ==+ ``` #### Sequence of steps for a successful call 1. **Off-hook and dial tone:** calling subscriber lifts the handset; SSP-A detects it and gives dial tone. 2. **Digit collection and analysis:** SSP-A receives the dialled number, finds that the called party is on SSP-B, and selects an idle trunk (identified by its **CIC**, circuit identification code). 3. **IAM (Initial Address Message):** SSP-A sends IAM via the STP to SSP-B. It contains the called and calling numbers, the CIC and the nature of connection. The trunk is reserved. 4. **Called line check:** SSP-B checks that the called line is free, applies **ringing** to it and connects ring-back towards the trunk. 5. **ACM (Address Complete Message):** SSP-B returns ACM to SSP-A through the STP, telling that all digits are received and the called party is being alerted. Caller hears **ring-back tone** over the trunk. 6. **ANM (Answer Message):** when the called party lifts the handset, SSP-B sends ANM; the speech path is through-connected in both directions and **charging starts**. 7. **Conversation** on the voice trunk; no signaling traffic on it. 8. **REL (Release):** when either party hangs up, its switch sends REL with a cause value (e.g. normal clearing). 9. **RLC (Release Complete):** the other switch frees the trunk and replies RLC; the CIC becomes idle, charging stops. #### Message flow ```text SSP-A STP SSP-B |---IAM-------->|---IAM-------->| ringing |<--ACM---------|<--ACM---------| | (ring-back) | |<--ANM---------|<--ANM---------| answer |====== conversation (trunk) ===| |---REL-------->|---REL-------->| clear |<--RLC---------|<--RLC---------| ``` #### Unsuccessful call If the called line is busy, SSP-B sends **REL** with cause "user busy" instead of ACM; SSP-A gives busy tone to the caller and returns RLC. If the trunk or route is congested, SSP-A itself gives a congestion tone. #### Points to note - Signaling and speech use separate paths, so the trunk is held only after IAM and is released immediately on failure. - Because the trunk is not seized for ringing on a busy line, call set-up is fast (under about 1 s) and trunk usage is efficient. - Database-type steps (free-phone 1660 numbers, number portability, mobile location) use **TCAP** queries to an **SCP** before the IAM is sent. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does the Initial Address Message (IAM) sent from SSP-A to SSP-B contain? A. Only the answer indication B. Called and calling numbers, the CIC and nature of connection C. The speech samples D. A release cause value Answer: B. IAM carries the numbers, CIC and connection nature, and the trunk is reserved. (Key point: IAM contents) 2. What does the Address Complete Message (ACM) tell the originating switch? A. The called party has answered and charging begins B. The call is being released C. The trunk is congested D. All digits are received and the called party is being alerted, so ring-back is given Answer: D. ACM is sent when ringing is applied; ANM later signals the answer. (Key point: ACM meaning) 3. When is charging started in an SS7 ISUP call? A. When ANM is sent after the called party lifts the handset B. When IAM is sent C. When ACM is received D. When REL is sent Answer: A. ANM through-connects the speech path and starts charging. (Key point: ANM effect) 4. If the called line is busy, what does SSP-B send? A. ACM followed by ANM B. REL with cause user busy instead of ACM C. Nothing, the trunk stays held D. A second IAM Answer: B. SSP-B releases with cause 'user busy'; SSP-A gives busy tone and replies RLC, so the trunk is freed quickly. (Key point: Busy line handling) ## Related in Telecommunications - [SS7 call set-up and release](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-call-setup-release) — Telecommunications, chapter 5 - [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 system and reason for SS7](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-system-ss7-reason) — Telecommunications, chapter 5 - [Poisson call arrival probabilities](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Apoisson-call-arrivals) — Telecommunications, chapter 6 - [SS7 building blocks and links](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-building-blocks-links) — Telecommunications, chapter 5 - [Advantage and working of SS7](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-advantage-working) — Telecommunications, chapter 5, asked 2× ## Related in other subjects - [GSM frame structure and channels during a call](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-frame-call-channels) — Wireless Communication, chapter 8 - [Sequence of steps when interrupt occurs](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Ainterrupt-occurrence-steps) — Microprocessor, chapter 5, asked 2× - [System call; monolithic and VM architecture](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Asystem-call-architecture) — Operating System, chapter 1 - [System call; read() execution](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Asystem-call-read-example) — Operating System, chapter 1