How GSM works ?

GSM stands for Global System for Mobile communication. Today, GSM is used by more than 800 million end-users spread across 190 countries which represent around 70 percent of today’s digital wireless market. So, let’s see how it works. 

In GSM, the geographical area is divided into hexagonal cells whose side depends upon the power of the transmitter and load on the transmitter (number of end-user). At the center of the cell, there is a base station consisting of a transceiver (combination of transmitter and receiver) and an antenna. 

Architecture :
Image of GSM Architecture : 

Function of Components :

1. Mobile station (MS) : It refers for mobile station. Simply, it means a mobile phone.
2. Base transceiver system (BTS) : It maintains the radio component with MS.
3. Base station controller (BSC) : Its function is to allocate necessary time slots between the BTS and MSC.
4. Home location register (HLR) : It is the reference database for subscriber parameters like subscriber’s ID, location, authentication key, etc.
5. Visitor location register (VLR) : It contains a copy of most of the data stored in HLR which is temporary and exists only until the subscriber is active.
6. Equipment identity register (EIR) : It is a database that contains a list of valid mobile equipment on the network.
7. Authentication center (AuC) : It performs authentication of subscribers.

Working :
GSM is combination of TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access) and Frequency hopping. Initially, GSM use two frequency bands of 25 MHz width : 890 to 915 MHz frequency band for up-link and 935 to 960 MHz frequency for down-link. Later on, two 75 MHz band were added. 1710 to 1785 MHz for up-link and 1805 to 1880 MHz for down-link. up-link is the link from ground station to a satellite and down-link is the link from a satellite down to one or more ground stations or receivers. GSM divides the 25 MHz band into 124 channels each having 200 KHz width and remaining 200 KHz is left unused as a guard band to avoid interference. 

Control channels : These are main control channels in GSM : 

1. BCH (Broadcast Channel) : It is for down-link only. It has following types – 
   1. BCCH (Broadcast Control Channel) : It broadcasts information about the serving cell.
   2. SCH (Synchronization channel) : Carries information like frame number and BSIC (Base Station Identity Code) for frame synchronization.
   3. FCCH (Frequency Correction Channel) : Enable MS to synchronize to frequency.
2. CCCH (Common Control Channel) : It has following types – 
   1. RACH (Random Access Channel) : Used by MS when making its first access to network. It is for up-link only.
   2. AGCH (Access Grant Channel) : Used for acknowledgement of the access attempt sent on RACH. It is for down-link only.
   3. PCH (Paging Channel) : Network page the MS, if there is an incoming call or a short message. It is for down-link only.
3. DCCH (Dedicated Control Channel) : It is for both up-link and down-link. It has following types – 
   1. SDCCH (Stand-alone Dedicated Control Channel) : It is used for call setup, authentication, ciphering location update and SMS.
   2. SACCH (Slow Associated Control Channel) : Used to transfer signal while MS have ongoing conversation on topic or while SDCCH is being used.
   3. FACCH (Fast Associated Control Channel) : It is used to send fast message like hand over message.