--- title: "GSM frame structure, traffic and control channels" subject: "Wireless Communication" chapter: 8 key: "semester_7/wireless_communication:gsm-frame-traffic-control-channels" source: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-frame-traffic-control-channels site: BCT Engineering --- # GSM frame structure, traffic and control channels Wireless Communication · Chapter 8: Wireless Systems and Standards - Asked in: 2077 Chaitra, 4+6 marks - Video: [GSM Frame Structure - 2G Technologies - Mobile Communication System](https://www.youtube.com/watch?v=VYZCzWS7AV4) — Ekeeda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-frame-traffic-control-channels - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-8 ## Question Draw and explain the frame structure for GSM. Explain the working of all traffic and control channels used in GSM. ## Short answer GSM frame hierarchy: hyperframe, superframe, 26/51 multiframes, TDMA frame of 8 slots (4.615 ms), slot of 156.25 bits. Traffic uses 26-multiframe with 24 TCH, one SACCH, one idle. Control uses 51-multiframe carrying FCCH, SCH, BCCH, CCCH and SDCCH on TS0. ## Answer #### Frame structure of GSM GSM uses **FDMA + TDMA**: each 200 kHz carrier is divided into **8 time slots**, gross bit rate 270.833 kbps (bit period 3.69 µs). ```text Hyperframe = 2048 superframes = 3 h 28 min 53.76 s | Superframe = 26 x 51-MF = 51 x 26-MF = 6.12 s | +------------------+--------------------+ 26-multiframe 51-multiframe (traffic) 120 ms (control) 235.4 ms | TDMA frame = 8 slots = 4.615 ms | TS0 | TS1 | TS2 | TS3 | TS4 | TS5 | TS6 | TS7 | | Time slot = 156.25 bits = 576.9 µs ``` **Normal burst (one time slot):** ```text |T | Data |S| Training |S| Data |T |Guard| |3 | 57 |1| 26 |1| 57 |3 |8.25 | ``` - **Tail bits (3+3)**: all zeros; let the equalizer/Viterbi decoder start and end in a known state, and give time for power ramp. - **Data (2 × 57)**: coded speech/data bits. - **Stealing flags (1+1)**: show whether the data half-burst carries traffic or has been "stolen" for FACCH signalling. - **Training sequence (26)**: known pattern in the middle of the burst, used by the adaptive equalizer to estimate the channel. - **Guard period (8.25 bits ≈ 30 µs)**: prevents overlap of bursts from mobiles at different distances. Other bursts: frequency correction burst (FCCH, all zeros), synchronisation burst (SCH, long 64-bit training), access burst (RACH, short, long guard of 68.25 bits), dummy burst. - **26-multiframe (120 ms)**: frames 0–11 and 13–24 TCH, frame 12 SACCH, frame 25 idle. - **51-multiframe (235.4 ms)**: carries FCCH, SCH, BCCH, CCCH and SDCCH control channels on TS0 of the beacon carrier. #### 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. **Working during a call**: the mobile syncs using FCCH/SCH and reads BCCH; an incoming call is paged on PCH; the mobile requests access on RACH and gets an SDCCH on AGCH; authentication and setup run on SDCCH; then speech flows on TCH with SACCH reports, and handoff commands come on FACCH. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How many bits are in a GSM normal burst slot and how long does it last? A. 114 bits, 4.615 ms B. 156.25 bits, 576.9 µs C. 156.25 bits, 4.615 ms D. 120 bits, 120 ms Answer: B. A time slot of 156.25 bits lasts 576.9 µs; eight make a 4.615 ms frame. (Key point: Normal burst size) 2. In the 26-frame traffic multiframe, which frame carries the SACCH? A. Frame 12 B. Frame 0 C. Frame 25 D. Frame 13 Answer: A. Frames 0-11 and 13-24 carry TCH, frame 12 SACCH, frame 25 is idle. (Key point: 26-multiframe) 3. What is the gross rate of a half-rate TCH and why? A. 22.8 kbps; one user uses two slots B. 11.4 kbps; it uses only 4 slots per frame C. 11.4 kbps; two users share one slot using alternate frames D. 6.5 kbps gross; it uses speech coder only Answer: C. TCH/H uses alternate frames so two users share a slot, giving 11.4 kbps gross each. (Key point: Half-rate TCH) 4. During a mobile-originated call, which channel assigns the SDCCH after the RACH request? A. PCH B. AGCH C. BCCH D. FACCH Answer: B. AGCH answers the RACH and assigns an SDCCH where authentication and setup run. (Key point: Call flow) ## Close variants set in other exams - [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× ## Related in Wireless Communication - [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 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; GSM vs CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-frame-vs-cdma) — Wireless Communication, chapter 8 - [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 - [CDMA merits/demerits; GSM frame efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amultiple-access-cdma-frame-efficiency) — Wireless Communication, chapter 7 - [GSM NSS components and frame hierarchy](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-nss-frame-hierarchy) — 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× - [Work breakdown for frame structure quantities](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Awork-breakdown-quantity-frame) — Estimating and Costing, chapter 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