The Sim Card Generation We Were Born In : GSM




GSM:- GLOBAL SYSTEM FOR MOBILE COMMUNICATION


GSM is a digital mobile network that is widely used for mobile phone users. It is a set of ETSI STANDARDS (European Telecommunications Standards Institute)

used in approximately 85 countries in the world including EUROPE, JAPAN, AUSTRALIA. It is most popular in 2G digital cellular networks used by mobile devices and tablets. The 2G networks developed as a replacement of 1G ANALOG CELLULAR NETWORKS. SIM cards were introduced in the early 1990s. They were introduced after Europe adopted the GSM network protocols.



It's initially developed as "GROUPE SPECIAL MOBILE". GSM services follow

ISDN (Integrated Services Digital Network) guidelines and are classified as TELESERVICES (includes mobile telephony and mobile-oriented traffic) or DATA SERVICES (includes a computer to computer communication and packet-switched traffic).


GSM services originally described a digital, circuit-switched network optimized for duplex voice telephony (which is a point-to-point system composed of two or more network-connected devices that communicate with one another in both directions).



The GSM services expanded over time to include data communication. First by circuit-switched transport then, by Packet data transport via General Packet radio service (GPRS) and Enhanced data rates for GSM Evolution (EDGE). Subsequently, 3GPP developed 3G UMTS (Universal mobile telecommunications system) standards followed by 4G LTE (Long Term Evolution) ADVANCED standards.


It was estimated in 2011 that technology defined in GSM standards served 80% of the mobile network. It uses TDMA (time-division multiple access). It does not include CDMA and 4gLTE.


DISCONTINUATION

Telstra in Australia shut down its 2g GSM network on DEC 1 2016, the first mobile network operator to decommission a GSM network.


Working of GSM




We can divide this given flowchart into three parts:-

  • Include a connection MS and BTS in FRONT- END LOGIC.

  • Include a connection between BTS and BSC in MIDDLE LOGIC.

  • Include a connection between BSC and MSC in BACK-END LOGIC.


FRONT-END LOGIC COMPONENTS


MS- It stands for MOBILE STATION (our smartphone)

We call other mobile stations through our mobile station. The mobile station connects to BSS through an interface of UM (air interface).


MIDDLE - LOGIC COMPONENTS


BSS- It stands for the BASE STATION SUBSYSTEM.

BTS ( BASE TRANSCEIVER STATION) is the part of BSS. This station both transmits and receives the signal. BTS can be visualized as the tower near our house. Through 1 BTS many MS are connected. BTS sends the signal provided by MS. Inside BSS, there are many BTS but only 1 BSC which stands for BASE STATION CONTROLLER.

BSC acts like a parent and BTS like a child. BSC takes care and control all the BTS inside a BSC. FOR EXAMPLE: If there is too much traffic in a BTS then BSC's job is to rotate the traffic to some other BTS.

Within BSS, the interface between BTS and BSC has an ABIS interface.


BACK-END LOGIC


NSS- It stands for NETWORK AND SWITCHING SUBSYSTEM.

MSC- It stands for MOBILE SWITCHING CENTER.

BSC is connected to MSC which acts as a parent to BSC and takes care of all the requirements of BSC. Through 1 MSC, many BSC can be connected and therefore it has too much work to do and it needs some help to do this type of work efficiently. Therefore, they use registers:- VLR, HLR, AuC, EIR.

The interface between BSC and MSC is an A-interface.


EXPLANATION OF THE TERMS AND THEIR FUNCTIONS.


# BASE STATION SUBSYSTEM (BSS) AND IT'S COMPONENTS BSC AND BTS.


FUNCTIONS OF BSC

  • It allocates the channel for the duration of the call.


FOR EXAMPLE:- (see the diagram given below) A called his friend B. If A talk to him then he needs a medium to make his voice reach B. This medium is the channel that is allocated by BSC.




  • handles handover

  • maintains the call and channel and it's quality

  • control the power transmitted by BTS and MS.


FUNCTIONS OF BTS


  • It transmits and receives the signal.

  • helps in signal processing and control equipment.

  • Antennas are connected to the BTS through co-axial cables and are used to receive/transmit data from/for the nearby users (mobile phones).

BTS and MSC are connected by an interface named:- ABIS.


FUNCTIONS OF INTERFACES IN GSM SYSTEM.


# UM INTERFACE:-

When many users use only one channel to send information and data, then there will be a collision, and all data will be lost and destroyed, but if a mobile station uses TDMA then that channel will be divided into many slots and will be assigned to each user differently.

FOR EXAMPLE:- If there are n users, then divide the channel into n time slots.

# ABIS-INTERFACE:-

It exists between BTS and BSC and transmits information related to signals. It uses LINK ACCESS PROTOCOL (LAP) to perform its work.


# A-INTERFACE:-

  • helps in call handling

  • bandwidth and connectivity provided by it is maximum 2Mbps.

  • manages all the BSS and its mobility.


FUNCTIONS OF REGISTERS


# HLR:- HOME LOCATION REGISTER

  • HLR has entries for every subscriber and every mobile ISDN number which has it's home in the respective network.

  • It stores permanent subscriber data and relevant temporary data of subscribers.

  • also stores the current location of MS.

  • stores subscription information of registered devices

# VLR:- VISITER LOCATION REGISTER


  • stores the data of all mobile stations which are currently staying in the administrative area of the associated MSC.

  • A VLR can be responsible for areas of more than 1 MSC.

  • The MS roaming freely may be registered in a VLR of its home network or the foreign network, depending upon its current location.

  • contains subscription information of every visiting user in the area.

  • After VLR authenticates one roaming mobile, MSC sends the necessary information to the visiting subscriber's HLR so that call may be routed.

# AuC:- AUTHENTICATION CENTER


  • Protects from different types of fraud.

  • It is a strongly protected database that handles the authentication and encryption keys for every single subscriber in the HLR and VLR.

  • It also holds information about EIR.

# EIR:- EQUIPMENT IDENTITY REGISTER


  • Database which holds all the list of all valid mobile on the network.

  • IMEI ( International mobile equipment identity ) used to identify each MS. It is marked as invalid in case of stolen or fraud mobiles.













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