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
AMPS, MTS and IMTS
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!
