--- title: "GSM traffic and control channels" subject: "Wireless Communication" chapter: 8 key: "semester_7/wireless_communication:gsm-traffic-control-channels" source: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-traffic-control-channels site: BCT Engineering --- # GSM traffic and control channels Wireless Communication · Chapter 8: Wireless Systems and Standards - Asked in: 2078 Chaitra, 8 marks; 2075 Bhadra, 8 marks; 2072 Asoj, 8 marks (asked 3 times) - Video: [Overview of GSM RF Channels](https://www.youtube.com/watch?v=ZfTC9s_P_pg) — ALTTC Ghaziabad - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-traffic-control-channels - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-8 ## Question Explain the working of all traffic and control channels used in GSM. ## Short answer GSM channels consist of traffic channels (TCH/F, TCH/H) for user data and control channels for signaling. Broadcast channels (FCCH, SCH, BCCH) provide frequency correction, synchronization, and cell parameters. Common control channels (PCH, RACH, AGCH) handle paging and access requests. Dedicated channels (SDCCH, SACCH, FACCH) support call setup, measurements, and urgent signaling like handoff. ## Answer GSM logical channels are mapped onto the physical TDMA time slots. They are of two kinds: **traffic channels (TCH)** that carry user speech/data, and **control channels (CCH)** that carry signalling for synchronisation, access, call setup and mobility. #### Traffic channels (TCH) Carry user speech or data; they use the **26-frame multiframe** (24 TCH frames, frame 12 for SACCH, frame 25 idle). - **Full-rate TCH (TCH/F)**: 22.8 kbps gross. Speech at 13 kbps (TCH/FS); data at 9.6, 4.8, 2.4 kbps (TCH/F9.6, F4.8, F2.4). - **Half-rate TCH (TCH/H)**: 11.4 kbps gross, uses alternate frames so two users share one slot. Speech at about 6.5 kbps (TCH/HS); data at 4.8 and 2.4 kbps. #### Control channels (CCH) Use the **51-frame multiframe**, mostly on time slot 0 of the beacon carrier (C0). **(a) Broadcast Channels (BCH)** – downlink only, point-to-multipoint: - **BCCH (Broadcast Control Channel)**: broadcasts cell identity, location area code, network code, list of neighbour cell frequencies, control channel structure, access parameters. - **FCCH (Frequency Correction Channel)**: a burst of all zeros that gives a pure tone (carrier + 67.7 kHz); the mobile uses it to correct its oscillator frequency. Sent in frames 0, 10, 20, 30, 40. - **SCH (Synchronization Channel)**: sent in the frame after each FCCH; carries the BSIC (base station identity code) and the TDMA frame number so the mobile can synchronise with the frame structure. **(b) Common Control Channels (CCCH)** – shared, for call setup: - **PCH (Paging Channel)**, downlink: pages a mobile for an incoming call or SMS. - **RACH (Random Access Channel)**, uplink: mobile requests a channel (to answer a page or start a call) using slotted ALOHA and the short access burst. - **AGCH (Access Grant Channel)**, downlink: base station replies to RACH and assigns an SDCCH. - (CBCH: cell broadcast messages.) **(c) Dedicated Control Channels (DCCH)** – bidirectional, for one user: - **SDCCH (Standalone DCCH)**: signalling during call setup: authentication, ciphering, location update, SMS, before a TCH is assigned. - **SACCH (Slow Associated CCH)**: tied to a TCH or SDCCH. Downlink: power control level and timing advance; uplink: signal strength/quality reports of serving and neighbour cells (for handoff). - **FACCH (Fast Associated CCH)**: urgent signalling (e.g. handoff command) sent by **stealing** TCH bursts; stealing flags mark these bursts. #### How they work together (mobile-terminated call) ```text MS BTS/Network | FCCH, SCH, BCCH (sync, cell info) <-----| | PCH: page (incoming call) <-----| |-----> RACH: channel request | | AGCH: SDCCH assigned <-----| |<====> SDCCH: authentication, ciphering, | | call setup messages | | TCH assigned (on SDCCH/FACCH) <-----| |<====> TCH: speech; SACCH alongside | | FACCH: handoff command / release <-----| ``` 1. On power-on, the mobile finds FCCH (frequency lock), then SCH (frame sync), then reads BCCH. 2. An incoming call is announced on PCH. 3. The mobile sends a request on RACH; the network assigns an SDCCH on AGCH. 4. Authentication and call setup are done on SDCCH. 5. A TCH is assigned; speech flows, with SACCH carrying measurements and power control. 6. Handoff and release commands go on FACCH. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the difference between TCH and CCH? A. TCH carries signalling; CCH carries speech B. TCH carries user speech/data; CCH carries signalling for sync, access, setup and mobility C. Both carry speech D. Both are uplink only Answer: B. Traffic channels carry user information; control channels carry signalling. (Key point: Channel types) 2. What do FCCH and SCH do for a newly powered-on mobile? A. FCCH gives paging; SCH gives access B. FCCH assigns SDCCH; SCH sends handoff C. FCCH lets the mobile correct its frequency; SCH gives BSIC and frame number for frame synchronisation D. FCCH and SCH carry speech Answer: C. Mobile first locks frequency using the FCCH tone, then synchronises using SCH, then reads BCCH. (Key point: FCCH and SCH) 3. How does a mobile get a channel for a call? A. Sends a request on RACH; the network assigns an SDCCH on AGCH B. Sends on PCH; reply on RACH C. Sends on BCCH; reply on SACCH D. Sends on FACCH; reply on FCCH Answer: A. RACH is the uplink random-access request; AGCH is the downlink grant. (Key point: RACH and AGCH) 4. What does SACCH carry? A. Urgent handoff commands by stealing bursts B. Paging for incoming calls C. Frequency correction tone D. Power control and timing advance (downlink), measurement reports (uplink) Answer: D. SACCH is slow, associated with a TCH, and carries measurements and control; FACCH steals TCH bursts for urgent signalling. (Key point: SACCH function) 5. How does FACCH send urgent signalling such as handoff commands? A. On a separate carrier B. By stealing TCH bursts, marked by stealing flags C. On the BCCH D. In the idle frame 25 Answer: B. FACCH replaces TCH bursts temporarily, flagged by stealing flags. (Key point: FACCH) ## Close variants set in other exams - [GSM frame structure, traffic and control channels](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-frame-traffic-control-channels) — Wireless Communication, chapter 8, asked 4× ## Related in Wireless Communication - [GSM control channels and their purposes](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-control-channels) — Wireless Communication, chapter 8, asked 2× - [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 - [GSM broadcast control channel](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-bcch) — Wireless Communication, chapter 8 ## Related in other subjects - [Traffic control devices and traffic signs](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atraffic-control-device-requirements-signs) — Transportation Engineering II, chapter 1, asked 2× - [Control type of hydropower governor and traffic lights](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Ahydro-governor-traffic-type) — Control Systems, chapter 1 - [Traffic control devices; traffic signal advantages](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atraffic-control-devices-signal-pros-cons) — Transportation Engineering II, chapter 1