--- title: "GSM frame structure and channels during a call" subject: "Wireless Communication" chapter: 8 key: "semester_7/wireless_communication:gsm-frame-call-channels" source: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-frame-call-channels site: BCT Engineering --- # GSM frame structure and channels during a call Wireless Communication · Chapter 8: Wireless Systems and Standards - Asked in: 2070 Bhadra, 4+4 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-call-channels - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-8 ## Question Draw and explain the frame structure for GSM. Describe how various traffic and control channels are used while making a call in GSM system. ## Short answer GSM's TDMA frame has 8 slots of 4.615 ms each, with a normal burst containing tail bits, 57-bit data halves, a 26-bit training sequence, and a guard period. For a mobile-originated call, the MS synchronizes via FCCH/SCH/BCCH, requests a channel on RACH, gets an SDCCH for authentication and setup, then traffic channel TCH with SACCH reports and FACCH for handover. ## 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. #### Use of traffic and control channels while making a call **Mobile-originated call (user dials a number):** ```text MS Network |<--- FCCH, SCH, BCCH (sync, info) ----| |---- RACH: channel request ---------->| |<--- AGCH: SDCCH assigned ------------| |<--> SDCCH: authentication, ciphering,| | setup (called number) | |<--- SDCCH: TCH assignment -----------| |<==> TCH: speech (+ SACCH reports) | |<--- FACCH: handoff / release --------| ``` 1. **FCCH** – the mobile finds the beacon carrier and corrects its frequency. 2. **SCH** – it reads BSIC and frame number and synchronises. 3. **BCCH** – it reads cell/network information and camps on the cell. 4. **RACH** – it sends a channel request (access burst, slotted ALOHA). 5. **AGCH** – the network assigns an SDCCH. 6. **SDCCH** – authentication (RAND/SRES), ciphering start, and call setup messages with the dialled number; the network then assigns a TCH. 7. **TCH** – speech flows at 13 kbps (22.8 kbps coded); **SACCH** runs with it carrying measurement reports (uplink) and power control/timing advance (downlink). 8. **FACCH** – steals TCH bursts for urgent signalling such as handoff commands and call disconnection. For a **mobile-terminated call**, the only change is that the network first sends a page on **PCH**; the mobile answers on RACH and the rest is the same. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In a mobile-originated call, what is the order of channels used? A. RACH, FCCH, SCH, AGCH, TCH, SDCCH B. PCH, RACH, AGCH, SDCCH, TCH C. FCCH, SCH, BCCH, RACH, AGCH, SDCCH, TCH D. BCCH, SDCCH, RACH, AGCH, TCH Answer: C. Sync and info first, then access, assignment, signalling, then speech. (Key point: Call order) 2. How does a mobile-terminated call differ? A. It starts with a FACCH B. It skips RACH and SDCCH C. The network first pages the mobile on the PCH D. It uses PCH only Answer: C. After the page, the mobile answers on RACH and the rest is the same. (Key point: MT call) 3. What happens on the SDCCH during setup? A. Speech transmission B. Handoff only C. Frequency correction D. Authentication, ciphering start and call setup, then TCH assignment Answer: D. SDCCH carries signalling before a TCH exists. (Key point: SDCCH use) 4. What do SACCH and FACCH do during a call? A. SACCH steals bursts; FACCH sends measurements B. Both carry speech C. Both carry paging D. SACCH: measurement reports and power control/timing advance; FACCH: urgent handoff/release by stealing Answer: D. SACCH is slow and continuous; FACCH is fast and steals TCH. (Key point: During the call) ## Related in Wireless Communication - [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 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 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