How the cellular network works? - GSM Architecture | 1G & 2G | Arun

Sharing buttons:

In today's world, everyone uses mobile phones and

each call gets connected within a second

Does anyone know about the process behind a each and every call?

Let's see in this video

Initially, it was 1G

1G is a first generation cellular standard called by different names like


It was introduced in 1980s and

it supports only voice services over limited range

It employs analog modulation technique with FDMA

1G handsets were bulkier with the antenna on top of device

The cons of 1g are replaced by GSM

Global System for Mobile Communication

GSM is a second generation cellular standard and

developed in 1991 by European telecommunications

to support voice and data services

It uses digital modulation with TDMA

While 2g handsets were cheaper and less in size

GSM was the first generation to introduce a sim cards

GSM had certain specifications like

Uplink, downlink, transfer rate, etc to carry information

Mainly, it has GMSK digital modulation and TDMA access method

GSM data speed was approximately 14.4 kilobytes per second

GSM uses a different frequency bands in different regions

In India, it was 900 MHz and 1800 MHz

GSM architecture is the basic architecture in cellular network

It contains four subsystems

Mobile station, Base station subsystem,

Network switching subsystem, Operation support subsystem

Here it is the representation of each subsystem in architecture

It is noticed that each subsystem contain some components

Let's discuss it detailly

First of all is mobile station

It has two components. Mobile equipment..

It is the hardware used by a subscriber to access network

identified by IMEI number (International Mobile Equipment Identity)

SIM- Subscriber Identity Module

It is a detachable smart card containing a IMSI number

International Mobile Subscriber Identity

It also allows user to send and receive calls

Second subsystem is a base station subsystem

It also contains two components

Based transceiver system. It is nothing but, Mobile Towers

It sends and receives signals from mobile phones

And performs various functions like encoding,

multiplexing, modulation and encryption

Then, Base station controller

It controls group of BTSs

It allocates radio channels

and handover from one BTS to another

The third sub system is a network switching subsystem

And it contains five components

The main element is a mobile switching center

It is the heart of GSM

It manages mobile services like registration, authentication, etc and

and performs call routing, call setup and call switching

It communicates with other NSS components like HLR, VLR, etc

The second element is a home location register

It is a central master database of subscribers IMSI,

current location and information

Third one is visitor location register

It is a subset of HLR and holds local database of users

currently visiting location in other domain

If the user lives in Chennai, then it is his

home network and HLR works here

When he travels to Delhi, then it is his

visiting network and VLR starts working

The fourth one is a Equipment Identity register

It is also a database that contains all valid handsets on network using IMEI number

It marks IMEI as a invalid if handset is stolen

Fifth element is authentication center

A protected database that has copy of IMSI number

for authentication and encryption task

It protects from a different types of frauds on network

The last subsystem is Operation support subsystem

It is connected to all equipments in the switching system on GSM network

It provides administration and commercial operation

and manages security & operation

Performs network configuration and maintenance task

Now, let's view the GSM structure again

Mobile station sends a signal to BSS through air interface

The BSS routes that the signal to an NSS

which performs various tasks as discussed earlier

BSS and NSS are supported by OSS finally

Finally, the signal from NSS is sent to external wired network

like PSDN, ISDN,etc

The GSM system can be a simplified as shown

The user calls a certain number using mobiles

The signal is received by the mobile tower which is BTS

Then, it is routed to be MSC for allocating the radio channel and

reaches MSC for tracking location under the degree

Finally, the signal is transferred to a telephone network

At the receiver side, the signal transmission occurs in reverse direction

Thanks for watching. This is Arun, signing off!