GSM Frame Structure
& Timing Study Guide

An interactive study guide for understanding the TDMA hierarchy, burst types, and synchronization mechanisms in Global System for Mobile Communications.

Launch Simulator Read Theory

Fundamentals of GSM TDMA

GSM utilizes Time Division Multiple Access (TDMA) to share the radio spectrum. This means multiple users share the same frequency channel but are allocated different time slots.

  • 1

    Frequency Division Duplex (FDD)

    Uplink and Downlink use different frequencies (45MHz separation in 900MHz band).

  • 2

    Carrier Spacing

    Each carrier has a bandwidth of 200 kHz.

  • 3

    Modulation

    Gaussian Minimum Shift Keying (GMSK) is used for spectral efficiency.

Key Timing Parameters

Bit Duration (T) 3.692 µs
Time Slot Duration 576.9 µs (156.25 bits)
TDMA Frame Duration 4.615 ms (8 slots)
Multiframe (Traffic) 120 ms (26 frames)
Multiframe (Control) 235.4 ms (51 frames)

GSM Hierarchy Structure

From a single bit to the Superframe

The Burst (Slot)

The fundamental unit of transmission. One Time Slot lasts 0.577ms and carries 156.25 bits.

Bits: 156.25
Duration: 576.9 µs

TDMA Frame

Consists of 8 Time Slots. One user typically uses one slot per frame (1/8th of the time).

Slots: 8
Duration: 4.615 ms

Multiframe

Two types: 26-frame (Traffic) for voice/data and 51-frame (Control) for signaling.

26-Frame: 120 ms
51-Frame: 235.4 ms

Hierarchical Breakdown

Bit (3.69 µs)
Burst (Slot)
Tail 3 Data 57 Stealing 1 Training 26 Stealing 1 Data 57 Tail 3 Guard 8.25
TDMA Frame (4.615 ms)
0
1
2
3
4
5
6
7

Interactive Frame Simulator

Visualize the TDMA frame structure and burst composition.

Time: 0.000 ms

Burst Composition

Animation Controls

Knowledge Check

Test your understanding of GSM timing.

1. How many time slots are there in a single GSM TDMA frame?

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