How GSM Works

We live in a world where most people cannot survive a day without their mobile phones. That is understandable, many now are on the move and business has to be transacted on the move. Gone are the days of exclusive use landlines and desk phones for making callings. Every day, we walk across cell phone mast towers, have you ever wondered what goes on inside there?



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Definition and Brief History of GSM

A GSM is the Global System for Mobile communication, originally known as Groupe Spécial Mobile. It is an open digital cellular technology used for transmitting mobile voice and data services. Mobile services based on GSM technology were first launched in Finland in 1991.

The first GSM call was made by the Finnish Prime Minister, Harri Holkeri, in Helsinki (Capital of Finland) to the Mayor of Tampere (Kaarina Suonio) who was in front of the Rosendahl Hotel in Tampere in Sothern Finland. This call was made over a private operator network on the July 1st, 1991.

Today there are about 5.093 billion GSM users worldwide ¹ in 219 countries.

How GSM Works

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The GSM Architecture is made up of three parts. These are

1. The Mobile System(MS): This the system made up of your mobile phone and SIM that communicates with the Base Transceiver (BTS) through the air interface.

2. Base Station Subsystem (BSS) or Access network
   
   
   A Base Station Subsystem(BSS) consists of a Base Station Controller (BSC) and one or more Base Transceiver Station. It is known as access network because here is where the mobile system accesses the GSM network which is made up of BTS, BSC, and TRAU.

BTS( Base Transceiver Station) The towers or masts you see. This is the only visible part of the GSM network to a user. These BTSs are usually many, scattered across towns and villages. Each BTS makes a cell, the distances between each BTS could range between 100m to 35KM dependent on terrain and topography of the place the BTS is located. People locally refer to them as Mast in Nigeria.



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BSC (Base Station Controllers)

They control a certain number of BTSs in a network. The BTSs are made to have little functionality besides receiving and transmitting. The bulk of the control function resides in the BSC.

3. Network Subsystem (NSS) or Core Network


This provides a link between the data or cellular network with Public Switched Telecommunication Network. These switching systems consists of the following

• MSC Mobile Switching Center: Main Switching System controls the BTS.
  
  
• HLR Home Location Register: It is the main database where the documents or information of a user is stored. The entire documents that you give during SIM card purchase are stored in this HLR.
  
  
• VLR Visitor Location Register: This register takes connects to your HLR to retrieve information when you are roaming.
  
  
• AUC Authentication Center: Authenticates users on a network; checks the plan, and International Mobile Equipment Identity (IMEI) and whether you have an active plan before allowing the call to go through
  
  
  
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• EIR Equipment Identity Register: Identifies if a device is supposed to be given access to a network.
  When you initiate a call, HLR and AUC will see if you are a genuine Mobile user with valid IMEI number and Plan, and then the call is set up from source to the destination device.
  This is only the basic information of a call setup