---
title: "Short note: Specifications of GSM"
subject: "Wireless Communication"
chapter: 8
key: "semester_7/wireless_communication:gsm-specifications"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-specifications
site: BCT Engineering
---

# Short note: Specifications of GSM

Wireless Communication · Chapter 8: Wireless Systems and Standards

- Asked in: 2074 Bhadra, 5 marks
- Video: [GSM History, GSM Introduction](https://www.youtube.com/watch?v=M8h2OBo6NAw) — Telecom learning
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-specifications
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-8

## Question

Write a short note on specifications of GSM.

## Short answer

GSM-900 uses uplink 890-915 MHz, downlink 935-960 MHz, 200 kHz carrier spacing, 8 slots, 270.833 kbps, GMSK BT=0.3, RPE-LTP 13 kbps speech, 4.615 ms frames and 35 km max cell radius. Security uses SIM and A3/A5/A8 algorithms.

## Answer

GSM (Global System for Mobile communications) is the 2G digital cellular standard developed by ETSI. Its main specifications:

| Parameter | Specification |
|---|---|
| Frequency band (GSM-900) | Uplink 890–915 MHz, downlink 935–960 MHz |
| Other bands | DCS-1800 (1710–1785 / 1805–1880 MHz), PCS-1900 |
| Duplex distance | 45 MHz (GSM-900) |
| Carrier spacing | 200 kHz |
| Number of carriers | 124 (GSM-900) |
| Multiple access | FDMA/TDMA, FDD |
| Time slots per carrier | 8 (16 with half rate) |
| Channel data rate | 270.833 kbps |
| Modulation | GMSK, BT = 0.3 |
| Frame duration | 4.615 ms (slot 576.9 µs) |
| Speech coder | RPE-LTP, 13 kbps (full rate) |
| Gross TCH rate | 22.8 kbps after coding |
| Channel coding | Rate 1/2 convolutional + CRC, interleaving over 8 bursts |
| Equalization | Adaptive, 26-bit training sequence |
| Frequency hopping | Slow, 217 hops/s |
| Max cell radius | 35 km (timing advance limit) |
| Data services | 9.6 kbps CSD; GPRS/EDGE in 2.5G |
| Security | SIM, A3 (authentication), A5 (ciphering), A8 (key) |
| Mobile power | 0.8–8 W (GSM-900 classes) |

**Other features**: SIM-based subscriber identity and roaming, SMS, mobile-assisted hard handoff, discontinuous transmission (DTX) with voice activity detection, and power control.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the uplink/downlink spacing in GSM-900?
   A. 20 MHz
   B. 200 kHz
   C. 95 MHz
   D. 45 MHz (890-915 up, 935-960 down)
   Answer: D. Uplink 890-915 MHz and downlink 935-960 MHz are 45 MHz apart. (Key point: Duplex distance)

2. How many 200 kHz carriers does GSM-900 have?
   A. 8
   B. 25
   C. 124
   D. 200
   Answer: C. 25 MHz / 200 kHz gives 125, with 124 used. (Key point: Carrier count)

3. Which modulation does GSM use?
   A. QPSK
   B. 64-QAM
   C. GMSK with BT = 0.3
   D. OQPSK
   Answer: C. GMSK has constant envelope; BT = 0.3 gives 270.833 kbps. (Key point: Modulation)

4. What limits the maximum GSM cell radius to about 35 km?
   A. Transmit power of 8 W
   B. The timing advance range
   C. The 200 kHz carrier spacing
   D. Frequency hopping rate
   Answer: B. Timing advance covers a limited delay range. (Key point: Cell radius)