Question 1: GSM Frequency Allocation

A GSM network operator has been allocated 25 MHz in the 900 MHz band. The system uses FDD with 200 kHz channel spacing and 8 time slots per carrier.

Calculate:

  1. The total number of available duplex channels
  2. The maximum number of simultaneous users per cell

Analysis Approach:

To solve this problem, we need to consider:

  • Total bandwidth = 25 MHz
  • Channel spacing = 200 kHz (0.2 MHz)
  • Time slots per carrier = 8
  • FDD requires paired spectrum (uplink and downlink)
Question 2: TDMA Frame Structure

A GSM TDMA frame has a duration of 4.615 ms and contains 8 time slots. Each time slot carries 156.25 bits, including guard periods and control information.

Determine:

  1. The bit duration
  2. The raw data rate per user channel

Analysis Approach:

Key parameters to consider:

  • Frame duration = 4.615 ms
  • Time slots per frame = 8
  • Bits per time slot = 156.25
  • Each user gets one time slot per frame
Question 3: GSM Network Architecture

Consider a GSM network with the following components: MSC, BSC, BTS, HLR, VLR, AuC, EIR.

Analyze the signaling flow for a mobile-terminated call when the called subscriber is roaming in a different network.

GSM Network Architecture

MSC: Mobile Switching Center | BSC: Base Station Controller | BTS: Base Transceiver Station

HLR: Home Location Register | VLR: Visitor Location Register

AuC: Authentication Center | EIR: Equipment Identity Register

Question 4: Handover Analysis

A mobile station is moving from Cell A (served by BSC1) to Cell B (served by BSC2). Both BSCs are connected to the same MSC.

Identify:

  1. The type of handover occurring
  2. The network elements involved in the handover decision

Handover Types in GSM:

  • Intra-BTS: Handover between sectors of same BTS
  • Inter-BTS Intra-BSC: Handover between BTSs under same BSC
  • Inter-BSC Intra-MSC: Handover between BSCs under same MSC
  • Inter-MSC: Handover between different MSCs
Question 5: Security Mechanisms

In GSM security, the A3 algorithm is used for authentication and A5 for encryption. The A8 algorithm generates the session key.

Explain:

  1. How the Ki (Subscriber Authentication Key) is stored and used
  2. Why the encryption key (Kc) is not transmitted over the air

GSM Security Architecture:

  • Ki is stored in the SIM card and AuC
  • Authentication uses challenge-response mechanism
  • Encryption key Kc is derived from Ki and RAND
  • A5 stream cipher encrypts data between MS and BTS
Question 6: GPRS Data Transmission

A GPRS network uses a coding scheme with a rate of 2/3 and transmits at 270.833 kbps per timeslot.

Calculate:

  1. The effective data rate per timeslot
  2. The maximum data rate for a mobile station using 4 timeslots

GPRS Coding Schemes:

  • CS-1: 9.05 kbps (rate ≈ 1/2)
  • CS-2: 13.4 kbps (rate ≈ 2/3)
  • CS-3: 15.6 kbps (rate ≈ 3/4)
  • CS-4: 21.4 kbps (rate ≈ 1)

Each timeslot has a raw data rate of 270.833 kbps

Quiz Results

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Review your answers below to reinforce your understanding of GSM concepts.