--- title: "GSM BCH and CCCH operation" subject: "Wireless Communication" chapter: 8 key: "semester_7/wireless_communication:gsm-bch-ccch" source: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-bch-ccch site: BCT Engineering --- # GSM BCH and CCCH operation Wireless Communication · Chapter 8: Wireless Systems and Standards - Asked in: 2080 Bhadra, 4+4 marks - Video: [Episode6 - Five Minutes GSM Systems, Um - Control Channels](https://www.youtube.com/watch?v=Hxi9HHfe0NI) — Telecom Mentor - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-bch-ccch - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-8 ## Question Explain the operation of the Broadcast control channel (BCH) and Common control channel (CCCH) in GSM. ## Short answer BCH are downlink broadcast channels on TS0: FCCH gives frequency correction, SCH gives synchronization and BSIC, BCCH carries system information. CCCH are shared: PCH pages mobiles, RACH carries uplink access requests, AGCH grants dedicated channels. They let idle mobiles camp and initiate calls. ## Answer #### Broadcast Control Channels (BCH) BCH are **downlink-only, point-to-multipoint** channels sent on TS0 of the beacon carrier (C0) in the 51-frame multiframe. They let an idle mobile find the cell, synchronise and read cell information. ```text 51-MF on TS0 (downlink), frames: 0 1 2-5 6-9 10 11 ... 40 41 ... 50 F S BCCH CCCH F S ... F S ... idle (F=FCCH, S=SCH) ``` 1. **FCCH (Frequency Correction Channel)** – an all-zero burst which, with GMSK, gives a pure tone 67.7 kHz above the carrier. The mobile detects it to find the beacon and lock its oscillator frequency. Sent in frames 0, 10, 20, 30, 40. 2. **SCH (Synchronization Channel)** – the frame after each FCCH. Carries the **BSIC** (base station identity code) and the TDMA frame number, so the mobile aligns its timing with the frame/multiframe structure. 3. **BCCH (Broadcast Control Channel)** – carries system information: cell identity, location area identity, network code, neighbour cell BCCH list, CCCH configuration, cell selection and access parameters. Operation: on switch-on the mobile scans carriers, finds FCCH, then reads SCH, then BCCH, and "camps" on the best cell. In idle mode it keeps reading BCCH and measuring neighbours listed there. #### Common Control Channels (CCCH) CCCH are shared by all mobiles in the cell and are used to **set up a connection**. They also sit on TS0 of C0 (frames not used by BCH). 1. **PCH (Paging Channel)** – downlink. The network pages a mobile (by IMSI or TMSI) for an incoming call or SMS. Each mobile listens only to its paging group, saving battery (discontinuous reception). 2. **RACH (Random Access Channel)** – uplink. The mobile sends a channel request (short **access burst** with long guard time since timing advance is not yet known) using **slotted ALOHA**. Used to answer a page, start a call, update location or send SMS. 3. **AGCH (Access Grant Channel)** – downlink. The network replies to the RACH request and assigns a dedicated channel (SDCCH), also giving the initial timing advance. ```text Network --PCH: page--> MS Network <--RACH: request-- MS Network --AGCH: SDCCH no.--> MS then signalling continues on SDCCH ``` (CBCH, the cell broadcast channel, also uses CCCH resources for broadcast text messages.) ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In what order does a mobile on switch-on use the broadcast channels? A. FCCH, then SCH, then BCCH B. BCCH, then SCH, then FCCH C. SCH, then FCCH, then PCH D. RACH, then AGCH, then BCCH Answer: A. It finds FCCH for frequency, reads SCH for sync and BSIC, then BCCH for cell info before camping. (Key point: Mobile switch-on order) 2. Why does RACH use a short access burst with a long guard time? A. To carry more data B. To avoid ciphering C. Because it is downlink D. Timing advance is not yet known when the mobile first accesses Answer: D. The long guard time tolerates unknown propagation delay. (Key point: Access burst) 3. How does the PCH save mobile battery? A. Each mobile listens only to its own paging group (discontinuous reception) B. Mobiles never listen to the PCH C. Paging is sent on the uplink only D. The PCH is sent once a day Answer: A. Listening only to its paging group lets the mobile sleep otherwise. (Key point: Paging groups) 4. What does the network send on AGCH? A. The BSIC and frame number B. The assigned SDCCH and initial timing advance in reply to RACH C. A neighbour cell list D. A handoff command Answer: B. AGCH answers the RACH request and assigns a dedicated channel. (Key point: AGCH role) ## Related in Wireless Communication - [GSM broadcast control channel](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-bcch) — Wireless Communication, chapter 8 - [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 channel structure and TDMA frame efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-channel-structure-efficiency) — Wireless Communication, chapter 8 - [GSM traffic and control channels](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-traffic-control-channels) — Wireless Communication, chapter 8, asked 4× - [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 other subjects - [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