Mobile Telephony: Cellular Systems, GSM, and Roaming

Posted on Jan 23, 2025 in Communication

Mobile Telephony

What is a Cell System?

A cell system consists of dividing the service area into cells, each containing a radio station that restricts its coverage area.

Elements of a Cellular System

  • Mobile Stations (MS)
  • Base Stations (BS)
  • Mobile Switching Center (MSC)
  • Public Switched Telephone Network (PSTN)

Understanding Hand-Over and Hand-Off

These are the necessary changes to keep communication channels uninterrupted when moving from one cell to another.

Technical Characteristics of the GSM System

  • Frequency Bands:
    • Mobile Station Transmission: 890-915 MHz
    • Base Station Transmission: 935-960 MHz
  • Duplex Communication: 45 MHz distance between transmission and reception.
  • Modulation: GMSK (Gaussian Minimum Shift Keying).
  • Nominal Mobile Station Power: 2, 5, 8, and 20 W.
  • Dual-Band Phones: Currently, there are mobile phones that operate on two bands (900 MHz and 1800 MHz).

Blocks of a Mobile Phone

Transmitter, Receiver, Control Circuit, Antenna, Battery System.

What is Roaming?

Roaming is the ability to send and receive calls on mobile networks outside your local service area. This can be within the service area of another company in the same country or during a stay in a different country, using the network of a foreign company.

What is the MSC?

The Mobile Switching Center (MSC) handles all call-related activities, whether initiated by or directed to mobile stations.

Advantages of the GSM System over Analog

  • Auto-tracking capability, both nationally and internationally.
  • Great traffic capacity.
  • Single telephone number for the user.
  • Coexistence with earlier systems.
  • Improvements in service quality and better facilities than other existing mobile systems.
  • Possibility of interconnection with the ISDN (Integrated Services Digital Network).
  • Inclusion of non-telephony services.
  • Possibility of using small user terminals.
  • More secure voice transmission.
  • More efficient mobile phone batteries.
  • Use of advanced signaling systems.
  • No increased cost to the user.

Mission of the Control Section in a Mobile Phone

It coordinates system devices such as the keyboard, microphone, speaker, and display. It also includes a micro-regulator that governs the overall functionality of the phone and coordinates signal processing and communication functions (transmission and reception).

Functional Checks Performed on a Mobile Phone

  • Call Processing: A base station emulator is used to check if the calling process is successful and to detect faults, such as the device not receiving calls. It is also helpful for testing radio frequency processes.

  • Hopping and Encryption: These are specific tests. Some phones have self-checks that occur immediately after ignition, providing information about the terminal’s internal state.