In the new generation, everyone uses mobiles, and the call connects within a second.

We will discuss in this article the process of each and every call.
Mobile communication started with 1G

First-generation mobile network:

• It is called by different names such as AMPS, MTS, IMTS.
• 1st generation mobile network was first established in the 1980s.
• It supports voice services over limited ranges.
• It uses FDMA with analog modulation.
• The 1st generation mobile handset is bulkier and heavy with an antenna on the top of the mobile device.

Second-generation mobile network:

• GSM is a 2nd generation mobile network.
• The limitations of 1G are replaced by GSM.
• GSM is (Global system of mobile communication).
• First established in the year 1991 by European telecommunication.
• GSM supports both voice and data services.
• Uses TDMA, with digital modulations.
• Whereas 2G mobiles are of low cost and the size is less.
• GSM first developed the use of SIM cards.
• GSM specifications are:
  Uplink, downlink, transfer rate, no of carriers, carrier separation, etc to carry the information in the signal.
• GSM uses TDMA and GMSK digital modulation.
• GSM has a data speed of approx 14.4 kbps.
• It uses different frequency signal bands in different regions or locations.
• GSM frequency in India is 900 and 1800 MHz.

GSM architecture is the basic architecture in the mobile network.

pic source: pexels

It consists of 4 subsystems

1. Mobile station
2. Base station subsystem
3. Network switching subsystem
4. Operation support subsystem

GSM architecture has the representation of all the subsystems present in it.
There is some component in the subsystem.

Mobile Station (MS)

It consists mainly of 2 components:
Mobile Equipment (ME):

• Subscribers use hardware to access the network.
• Which is identified by the IEMI number ( international mobile equipment identity).

Subscriber identity module (SIM):

• It is a removable smart card with consists of the IMSI (International mobile subscriber identity).
• It main use is to send and receive calls.

Base station subsystem (BSS)

It contains 2 components:
Base transceiver station (BTS)

• It sends and receives signals from the mobile station.
• It does many functions like encoding, modulation, encryption, and multiplexing.

Base station controller (BSC)

• It controls the group of BTS.
• The base station controller allocates radio channels.
• It handover from one BTS to another BTS.

Network switching sub-system

It consists of 5 components
Mobile switching center( MSC)

• The main component of the network switching subsystem.
• MSC is known as the heart of the GSM mobile network.
• It helps in the management of mobile services.
• Registration, authentication, etc.
• It performs call routing, call setup, and call switching.
• MSC also communicate with other components such as HLR, VLR, AUC, and EIR.

Home location register

• It is a master database that stores subscriber’s current location and information.

Visitor location register (VLR)

• It is a subset in the HLR that is mainly hold in the local database.
• In the local database, it stores the current new location in another domain.
• If the user lives in Delhi, then it is HLR and the HLR works here.
• If the user is visiting Kolkata, then it is his visiting network and VLR is working.

Equipment identity register (EIR)

• It is a database that contains the mobile handset information with the IEMI number.
• It marks IEMI invalids when the handset is stolen.

Authentication Center (AUC)

• It is a protected database that contains the IMSI number.
• Does authentication and encryption.
• Protects from different types of fraud in the network.

Operation support subsystem

• It connects to all equipment in the switching system.
• Used for administration and commercial operation.
• It performs security, operation, and performance management.
• Used for network configuration and maintenance tasks.
• The mobile station sends a signal to the base station sub-system through an interface.
• BSS routes the call in-network sub-system which various tasks.
• BSS and NSS support OSS.
• Finally, the signal from NSS is sent to an external wired network.
• like PSDN, ISDN, etc.

The GSM call system can be explained as

• The user calls some number with handset ie MS.
• The mobile tower called BTS receives the signal.
• The call is routed to BSC for allocating radio channels.
• The call reaches MSC for tracking location and other checks.
• Lastly signal reaches the telephone network ie the PSTN and ISDN.
• On the receiver side, the signal transmits in the reverse direction.