--- title: "GSM channel structure and TDMA frame efficiency" subject: "Wireless Communication" chapter: 8 key: "semester_7/wireless_communication:gsm-channel-structure-efficiency" source: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-channel-structure-efficiency site: BCT Engineering --- # GSM channel structure and TDMA frame efficiency Wireless Communication · Chapter 8: Wireless Systems and Standards - Asked in: 2074 Magh, 6+2 marks - Video: [Frame structure of GSM||Time Slot||TDMA Frame||Hyperframe||Superframe](https://www.youtube.com/watch?v=XY8aKR9mdgU) — Engineering Basics - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-channel-structure-efficiency - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-8 ## Question Explain briefly channel structure of GSM. Show that TDMA frame efficiency cannot reach 100% in GSM. ## Short answer A GSM slot has 156.25 bits; overhead is 6 tail bits plus 26 training bits plus 8.25 guard bits, totaling 40.25 bits per slot. Per 8-slot frame, efficiency is 1 minus 322/1250, about 74.24%, so it cannot reach 100% because these essential overhead bits cannot be removed. ## Answer #### Channel structure of GSM GSM has **124 carriers** of 200 kHz in GSM-900 (890–915 MHz uplink, 935–960 MHz downlink). Each carrier is divided into **8 time slots**; one slot on one carrier is a **physical channel** (a burst of 156.25 bits every 4.615 ms frame). **Logical channels** are mapped onto these physical channels. ```text Logical channels +-- Traffic (TCH): TCH/F 22.8 kbps, TCH/H 11.4 kbps +-- Control (CCH) +-- BCH : FCCH, SCH, BCCH (down) +-- CCCH: PCH, AGCH (down), RACH (up) +-- DCCH: SDCCH, SACCH, FACCH (both) ``` - **TCH**: carry speech (13 kbps full rate, ~6.5 kbps half rate) or data; organised in the **26-frame multiframe** (24 TCH, 1 SACCH, 1 idle; 120 ms). - **BCH**: FCCH (frequency correction), SCH (frame sync and BSIC), BCCH (cell and network information). - **CCCH**: PCH (paging), RACH (random access request, slotted ALOHA), AGCH (channel assignment). - **DCCH**: SDCCH (call setup, authentication, location update, SMS), SACCH (measurement reports, power control, timing advance), FACCH (urgent signalling by stealing TCH bursts). - Control channels use the **51-frame multiframe** (235.4 ms) on TS0 of the beacon carrier. #### Why TDMA frame efficiency cannot reach 100% Frame efficiency is the fraction of transmitted bits that carry user data: ```text η_f = (1 − b_OH / b_T) × 100% ``` Each normal burst must contain overhead bits that carry no data: 3 + 3 tail bits, 26 training bits and 8.25 guard bits (stealing flags are counted with the data here). ```text Bits per slot = 156.25 Overhead/slot = 6 + 26 + 8.25 = 40.25 Per frame: b_T = 8 × 156.25 = 1250 bits b_OH = 8 × 40.25 = 322 bits η_f = (1 − 322/1250) × 100% = 74.24 % ``` (If the 2 stealing flags are also treated as overhead, η = 114/156.25 = 72.96 %.) Since training bits (for equalization), tail bits (for the decoder) and guard time (for timing differences and power ramping) are essential, b_OH can never be zero, so **η < 100%; for GSM η ≈ 74%**. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a GSM physical channel? A. A whole carrier of 8 slots B. One time slot on one carrier C. A logical set of TCH and BCH D. A 51-multiframe Answer: B. Logical channels are mapped onto physical channels (slots). (Key point: Physical channel) 2. Which bits count as overhead in a normal burst for frame efficiency? A. Data bits (114) B. Tail (6), training (26) and guard (8.25) bits C. Only the guard bits D. Only the stealing flags Answer: B. Overhead = 6 + 26 + 8.25 = 40.25 bits per slot. (Key point: Overhead bits) 3. What is the TDMA frame efficiency computed in the answer? A. 74.24 % B. 100 % C. 85.6 % D. 90 % Answer: A. 1 - 322/1250 = 0.7424. (Key point: Efficiency value) 4. Why can efficiency never reach 100%? A. Because 8 slots cannot fit in a frame B. Because GMSK wastes power C. Training, tail and guard bits are essential, so overhead is never zero D. Because of FDMA only Answer: C. Equalization, decoder start-up and timing differences require those bits. (Key point: Why below 100%) ## 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 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 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 - [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 broadcast control channel](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-bcch) — Wireless Communication, chapter 8 - [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 - [TDM frame, time slot and 24-channel system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atdm-frame-24-channel) — Telecommunications, chapter 3 - [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 - [Frame SFD, BMD (100 kN, 20 kN/m, hinge)](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Aframe-sfd-bmd-100kn-hinge-60deg) — Strength of Materials, chapter 1 - [Frame AFD, SFD, BMD (100 kN, inclined member)](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Aframe-afd-sfd-bmd-100kn-20knm) — Strength of Materials, chapter 1