Global System for Mobile Communications

Interactive Virtual Laboratory for Understanding GSM Channel Structure, Burst Types, and TDMA Frame Architecture

01 Learning Objectives

Understand the GSM TDMA frame structure and hierarchy (Frame → Multiframe → Superframe → Hyperframe)

Differentiate between Traffic Channels (TCH) and Control Channels (BCCH, SDCCH, PCH, RACH, AGCH)

Analyze the five GSM burst types: Normal, Frequency Correction, Synchronization, Dummy, and Access

Examine the 26-frame and 51-frame multiframe structures for traffic and control channels

Understand channel mapping and physical layer organization in GSM

Analyze timing relationships and guard periods in burst structures

02 Theoretical Background

GSM TDMA Hierarchy

Slot
0.577ms

156.25 bits

→
Frame
4.615ms

8 time slots

→
Multiframe
120ms/235ms

26/51 frames

→
Superframe
6.12s

51/26 multiframes

26-Frame Multiframe (120ms): Used for Traffic Channels (TCH) and Slow Associated Control Channels (SACCH). Structure: 24 frames for TCH, 1 for SACCH, 1 idle.
51-Frame Multiframe (235ms): Used for Control Channels (BCCH, CCCH, SDCCH). Dedicated to signaling and control functions.

GSM Logical Channels

Traffic Channels (TCH)

TCH/F (Full Rate): 22.8 kbps, carries speech or data

TCH/H (Half Rate): 11.4 kbps, two half-rate channels per slot

Control Channels (CCH)
BCCH: Broadcast Control Channel - downlink only, broadcasts cell parameters
SDCCH: Stand-alone Dedicated Control Channel - call setup, authentication
PCH: Paging Channel - downlink, alerts mobile of incoming call
RACH: Random Access Channel - uplink, mobile requests channel access
AGCH: Access Grant Channel - downlink, assigns SDCCH to mobile

GSM Burst Types

Normal Burst (NB)
TCH, SDCCH, BCCH
[3 tail][58 data][26 training][58 data][3 tail][8.25 guard]

Used for traffic and dedicated control channels. Contains 2×58 data bits, 26-bit training sequence for equalization.

Frequency Correction Burst (FB)
FCCH
[3 tail][142 all zeros][3 tail][8.25 guard]

Allows mobile to correct its local oscillator frequency. Pure sine wave transmission.

Synchronization Burst (SB)
SCH
[3 tail][39 encrypted][64 sync][39 encrypted][3 tail][8.25 guard]

Provides TDMA frame number (FN) and Base Station Identity Code (BSIC). Longer sync sequence for timing.

Dummy Burst
BCCH carrier

Transmitted on unused time slots of BCCH carrier to maintain power envelope.

Access Burst (AB)
RACH
[8 extended guard][41 sync][36 encrypted][3 tail]

Shorter data, longer guard period (68.25 bits) for unknown timing advance.

Interactive Channel Simulator

Channel Information

Traffic Channel (TCH)
  • • Bit rate: 22.8 kbps (full rate)
  • • Uses Normal Burst
  • • 26-frame multiframe
  • • Frame 12: SACCH
  • • Frame 25: Idle
Timing Parameters
Bit duration: 3.69 μs
Time slot: 0.577 ms
TDMA frame: 4.615 ms
26-Multiframe: 120 ms
51-Multiframe: 235 ms
Lab Tip

Click on any time slot in the visualization to see detailed burst structure and bit allocation.

03 Laboratory Procedure

Experiment 1: TDMA Frame Structure Analysis

Objective: Understand the basic TDMA frame structure and timing relationships.

  1. Set the View Mode to "TDMA Frame Structure"
  2. Select Channel Type as "Traffic Channel (TCH)"
  3. Observe the 8 time slots (TS0-TS7) in the frame
  4. Note the duration: 4.615 ms per frame, 0.577 ms per slot
  5. Click on individual slots to see burst type allocation
  6. Record the bit allocation for each burst component

Observation Point: Calculate the total bits per frame (8 × 156.25 = 1250 bits) and verify the frame duration mathematically.

Experiment 2: 26-Frame Multiframe Analysis

Objective: Examine the traffic channel multiframe structure and associated control channels.

  1. Set View Mode to "26-Frame Multiframe (TCH)"
  2. Observe the sequence of 26 TDMA frames
  3. Identify Frame 12: Slow Associated Control Channel (SACCH)
  4. Identify Frame 25: Idle frame (no transmission)
  5. Analyze the repetition pattern: 24 TCH + 1 SACCH + 1 Idle
  6. Calculate the total multiframe duration: 26 × 4.615 ms = 120 ms

Analysis: Why is the SACCH sent every 26th frame? How does this affect speech transmission continuity?

Experiment 3: 51-Frame Control Channel Analysis

Objective: Understand the organization of common control channels.

  1. Set View Mode to "51-Frame Multiframe (Control)"
  2. Select Channel Type as "BCCH + CCCH"
  3. Identify the FCCH (Frequency Correction) bursts in TS0
  4. Locate the SCH (Synchronization) bursts following FCCH
  5. Analyze the BCCH broadcast content
  6. Observe PCH (Paging) and AGCH (Access Grant) distribution
  7. Note: 51-frame multiframe duration = 235.4 ms

Experiment 4: Burst Structure Comparison

Objective: Compare different burst types and their applications.

  1. Set View Mode to "Burst Structure Detail"
  2. Select different channels to see associated burst types:
    • TCH → Normal Burst
    • BCCH → Normal Burst (with specific training seq.)
    • FCCH → Frequency Correction Burst
    • SCH → Synchronization Burst
    • RACH → Access Burst
  3. Compare the guard period lengths (Normal: 8.25 bits vs Access: 68.25 bits)
  4. Analyze why Access Burst needs extended guard period

Experiment 5: Channel Mapping and Configuration

Objective: Understand how logical channels map to physical channels.

  1. Set Channel Type to "Mixed Configuration"
  2. Observe TS0: Typically carries BCCH + CCCH
  3. Observe TS1: Often configured for SDCCH
  4. TS2-TS7: Usually allocated to TCH
  5. Analyze the frequency spectrum view to see channel spacing (200 kHz)
  6. Understand the concept of channel combination (e.g., BCCH + CCCH + SDCCH/4)

04 Laboratory Report Guidelines

Report Structure

1
Title Page

Experiment title, student name, ID, date, course code

2
Objectives

Clear statement of learning goals (3-4 points)

3
Theory Summary

Brief explanation of GSM channel concepts (max 1 page)

4
Experimental Data

Screenshots, measurements, tables of observed values

5
Analysis & Discussion

Interpretation of results, answers to lab questions

6
Conclusion

Key findings, learning outcomes, practical implications

Key Questions to Address

Q1: Timing Calculations

Calculate the exact duration of a 26-frame multiframe. If a speech frame is transmitted every 4.615 ms, how many speech frames are sent in one multiframe?

Q2: Channel Capacity

A cell has 8 frequencies with 8 time slots each. How many full-rate traffic channels can be supported if TS0 of each frequency is reserved for control channels?

Q3: Burst Comparison

Why does the Access Burst have a 68.25-bit guard period while the Normal Burst only has 8.25 bits? Show the calculation for maximum cell radius.

Q4: SACCH Timing

Explain why SACCH is transmitted every 26th frame in a traffic multiframe. What happens if signaling information needs to be sent more frequently?

Q5: Frequency Planning

If BCCH is always on TS0, how does a mobile phone know which frequency to tune to when first powered on?

Submission Requirements
  • • Report length: 8-12 pages maximum
  • • Include at least 4 screenshots from the simulation
  • • Show all mathematical calculations clearly
  • • Cite GSM specifications (ETSI/3GPP) where applicable
  • • Submit PDF format only
  • • Deadline: One week from lab session