--- title: "GSM control channels and their purposes" subject: "Wireless Communication" chapter: 8 key: "semester_7/wireless_communication:gsm-control-channels" source: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-control-channels site: BCT Engineering --- # GSM control channels and their purposes Wireless Communication · Chapter 8: Wireless Systems and Standards - Asked in: 2082 Baisakh, 2+4 marks; 2081 Bhadra, 8 marks (asked 2 times) - Video: [GSM Control Channels](https://www.youtube.com/watch?v=tHlPoH0nXNQ) — Sheetal Borde - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-control-channels - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-8 ## Question List the various types of control channels available in GSM and explain the operation of each with their purposes. ## Short answer GSM control channels are BCH, CCCH, and DCCH. BCH (FCCH, SCH, BCCH) gives frequency sync, frame sync, and cell info downlink. CCCH (PCH, RACH, AGCH) handles paging, random access, and access grant. DCCH (SDCCH, SACCH, FACCH) carries dedicated signalling, measurement/power control, and urgent handoff messages. ## Answer GSM control channels carry signalling between the mobile and the network for synchronisation, access, paging, call setup and handoff. They are grouped into three types: **Broadcast channels (BCH), Common control channels (CCCH) and Dedicated control channels (DCCH).** ```text Control channels +----------------+------------------+ BCH CCCH DCCH FCCH,SCH,BCCH PCH,RACH,AGCH SDCCH,SACCH,FACCH ``` #### (a) Broadcast channels – downlink only, to all mobiles - **FCCH (Frequency Correction Channel)**: an all-zero burst that gives a pure sine wave 67.7 kHz above the carrier. *Purpose*: the mobile finds the beacon carrier and corrects its local oscillator frequency. - **SCH (Synchronization Channel)**: sent in the frame after FCCH; carries the **BSIC** and the reduced TDMA frame number. *Purpose*: frame synchronisation and identification of the base station. - **BCCH (Broadcast Control Channel)**: carries cell identity, location area identity, network code, neighbour cell list, control channel configuration and access parameters. *Purpose*: lets an idle mobile camp on the cell and monitor neighbours. #### (b) Common control channels – shared, used for access - **PCH (Paging Channel)**, downlink. *Purpose*: alerts a particular mobile (by IMSI/TMSI) of an incoming call or SMS. - **RACH (Random Access Channel)**, uplink. *Purpose*: the mobile requests a dedicated channel, using slotted ALOHA and the short access burst (to answer a page or make a call). - **AGCH (Access Grant Channel)**, downlink. *Purpose*: answers a RACH request by assigning an SDCCH (or TCH). #### (c) Dedicated control channels – one per user, bidirectional - **SDCCH (Stand-alone Dedicated Control Channel)**. *Purpose*: signalling before a traffic channel exists – authentication, ciphering, location updating, call setup, SMS. - **SACCH (Slow Associated Control Channel)**: always paired with a TCH or SDCCH (one frame in each 26-multiframe). *Purpose*: uplink measurement reports (signal level/quality of serving and neighbour cells); downlink power control and timing advance commands. - **FACCH (Fast Associated Control Channel)**: steals bursts from the TCH (stealing flags set). *Purpose*: urgent messages such as handoff commands and call release. | Group | Channel | Direction | |---|---|---| | BCH | FCCH, SCH, BCCH | Downlink | | CCCH | PCH, AGCH | Downlink | | CCCH | RACH | Uplink | | DCCH | SDCCH, SACCH, FACCH | Both | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which are the three groups of GSM control channels? A. BCH, CCCH, DCCH B. TCH, BCH, CCCH C. BCCH, SDCCH, TCH D. FCCH, SCH, RACH Answer: A. Control channels are grouped as broadcast (BCH), common control (CCCH) and dedicated control (DCCH). (Key point: Control channel groups) 2. What is the purpose of the FCCH? A. Lets the mobile find the beacon carrier and correct its oscillator frequency B. Carries the BSIC for identification C. Pages the mobile for an incoming call D. Carries handoff commands Answer: A. FCCH is an all-zero burst giving a pure tone 67.7 kHz above the carrier for frequency correction. (Key point: FCCH purpose) 3. Which statement about RACH is correct? A. It is downlink and assigns an SDCCH B. It is downlink and pages mobiles C. It is uplink, uses slotted ALOHA, and requests a dedicated channel D. It is dedicated to one user and carries measurement reports Answer: C. RACH is the uplink common channel where the mobile requests access; AGCH is the reply. (Key point: RACH) 4. How does FACCH carry urgent messages such as handoff commands? A. By stealing bursts from the TCH B. By using its own dedicated carrier C. By using one frame in each 26-multiframe D. By using the paging channel Answer: A. FACCH steals TCH bursts, marked by stealing flags; SACCH is the one using a frame in the 26-multiframe. (Key point: FACCH) ## Related in Wireless Communication - [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× - [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 BCH and CCCH operation](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-bch-ccch) — 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 - [GSM system architecture](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-architecture) — Wireless Communication, chapter 8, asked 4× ## Related in other subjects - [PID control operation with block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Apid-operation-block-diagram) — Embedded System, chapter 7, asked 4× - [Four quadrant operation in DC motor control](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Afour-quadrant-dc-motor-control) — Utilization of Electrical Energy, chapter 3