GSM Architecture and working

Global system for mobile communication abbreviated as “GSM” is widely used because of its vast coverage. The GSM is the cellular norm of second developed generation in order to adapt to the delivery of vocal services and data that use the digital modulation. The big advantage of GSM is that it is flexible.

GSM overview

GSM was developed by Group Special Mobile which was contained by the CEPT (Conference of European Post and Telecommunication). Its growing percentage is increasing with the passage of time. Currently there are more than 3 billion subscribers in all around the world. Even after a survey it seems that some of the countries have more mobile phones rather than fixed phones.

gsm subscribers rate
gsm subscribers rate

GSM normally uses the Time Division Multiple Access technique (TDMA) because the common frequency is shared by the multiple users. For the normal GSM modem it uses the frequency which is 900MHz.

GSM Services

Tele Services

Services of telecommunications that allow the communication in phone by means of the portable telephones, the opened services, the movable telephony and urgency of call. It usesThe equipment of a service of carrier to move data, defining possibilities is demanded what and how they settle.

Earer of Data Services

It includes different services of data for the transference of the information between the GSM and of other networks like the PSTN, the RNIS etc to the types from 300 to 9600 bps. One also takes care of SMS (services of short messages), UMS (services “unified messages”)

Supplementary Services

The additional services of GSM are the most of the time vocal services and are first normally in being accessible by an operator of the network after provision of the basic service of GSM.

GSM Architecture diagram

Here is the flow chart of GSM architecture:

GSM architecture
GSM architecture diagram

GSM architecture and working

Follwings are the main components of GSM architecture

Mobile Station (MS)

MS grant access to the GSM network. It is main component of GSM architecture.It contains the following two components.

  1. Mobile Equipment (ME)
  2. Subscriber Identity Module (SIM)

Mobile Equipment (ME)

 There are number of mobile equipment being used for this architecture. These can be dispositive portable, mounted on vehicle, held in the hand. Every device has the sound only identified from a IMEI. These mobile devices are responsible for voice and data transmission simultaneously. It works at power level 0.8 – 20 W.

 Subscriber Identity Module (SIM)

It permit the user whom it sends and who receives calls and to receive others of the subscribed services. It is main part of gsm GSM architecture. Number details of definition of network like main Ki, algorithms kc, and A3, A5, and A8. A word of order or a PIN is protecting near. It can move of the telephone so that it is called by telephone and it contains the key information to activate the telephone.

 Base Transceiver Station (BTS)

 The station of receiving transmitter lowers code, calculates multiplex, modulates and introduces the signals of RF to the antenna. It makes the method of the frequency jump, communicates with the movable station and BSC. It consists of the units of receiving transmitter (TRx). The LOW SULFUR CONTENT provides the insurance by radio of GSM in a cell. It implies the radio that it transmits and that receives equipment and the treatment of the signals associate in GSM architecture.

Base Station Controller (BSC)

 The controller of basic station controls the resources by radio for the LOW SULFUR CONTENT. He assigns to the frequency and the sections of time for all the ` s of thousandth in his sector. He also directs the installation of call, functionality of adaptation. He provides to the delivery for each thousandth and the communication CAM and the LOW SULFUR CONTENT. BSC also provides the management of the resources by radio. It assigns and it releases to frequencies and sections of time for all the thousandth in his sector. One takes control frequencies between cells again.

Base Station Subsystem (BSS)

With air interface BSS provides MS and NSS (Network Station Subsystem). BSS consists of following elements.

  • One or more BTS
  • One BSC
  • One TRAU (Transcoding Rate and Adaptation Unit)

Main Switching Center (MSC)

 The center of main commutation is the heart of the net. The communication between the GSM and of other nets controls. Draft the function of system of call and the commutation, the advance of call, the information of invoicing and the collection of base. It is also plays important role in GSM architecture.

 Gateway MSC

The MSC with an interface to other network like Public Switched Telephone (PSTN) is called gateway main switching center.

Home Location Register (HLR)

 A HLR contains given customer as given of account, account position; the preferences customer, devices have undersigned with to the customer, the current situation of the customer, etc the data conserved in HLR for the several types of nets are similar but they differ in some details.

Visitor Location Register (VLR)

 A VLR is a disc, similar to a HLR, that it is used by the nets you furnish to the profiles of taken of temporality for the customers (those are out of their central field). These data of VLR are low on the searched data customer of a HLR.

 Authentication Center (AUC)

 AUC look after not in favor of robber or thief during air interface. It keeps the authentication keys and granted the security triplets. It is normally linked with the HLR.

 Equipment Identity Register (EIR)

GSM MS is possible to operate any valid GSM SIM. An opportunity exists for black market and stolen equipment. To combat this problem EIR is introduce and track such equipment.

GSM Frequency ranges

GSM network operates in four different frequency ranges. But most GSM networks operate in 900 or 1800 MHz bands. GSM 900 operates between 880-960MHz and 1800 operates between 1710-1880 MHz.

 GSM Link


The radio uplink is the transmission path from the Mobile Station (Cell Phone) to a Base Station (Cell Site). In the 900 MHz bank the uplink frequency bank is 890-915 MHz.


This is the broadcast path originating from a Base Transceiver Station to the Mobile Station. In the 900 MHz bank the downlink frequency band 935-960 MHz.Waves    the    ASCENDING    CONNECTION    (25    MHz)    and    the DESCENDANT CONNECTION (25 MHz) are divided in 124 carrier or hollow radio frequency of distant each 200 drips kilohertz spaced. The used access method is multiple accesses by distribution in frequency (FDMA). Each channel of radio frequency in 8 hour strips is divided.

The users broadcast it in the fast series, one after each other, every user using their individual hour strip. This permits the numerous locations to allocate the same means of communication (for example radio frequency channel) at the same time using only the part of its band width that they demand. Thus the advantage of the GSM on the system of similar portable telephone (AMPS) is, numerous users are 8 times greater than amperes on the simple channels of a radio frequency. The channel data rate is 270.833 Kbit/s, and the frame duration is 4.615ms. The modulation used in GSM is Gaussian minimum shift keying (GMSK), a kind of continuous-phase frequency shift keying.

In GMSK, the signal to be modulated onto the carrier is first smoothed with a Gaussian low- pass filter prior to being fed to a frequency modulator, which greatly reduces the interference to neighboring channels (adjacent channel interference).

 Gsm Cellular Concept

 Base Stations: BS implements space division multiplexing.

  • Each BS covers a certain transmission area called a
  • Each BS is allocated a portion of the total number of channels
  • Cluster is a group of nearby Bs’s that together use all available

The movable station only communicates by a basic station. The cell is a basic unit of the system. It classifies the sector on which the news article by radio is given by a basic station. According to the volume of traffic, each cell uses the one or more RFC. Whereas the network capacity is limited, the RFC several times must be used. A safe distance is necessary between the cells UNDER SULFUR CONTENT that use the same RFC. They are reused, but some frequencies in neighboring cells are not used due to the interference.

Reusability of frequency – the capacity to use the same frequencies in successive occasions through a cellular, possible system by the approach of basic conception for cellular. Since each cell is conceived to only use radio frequencies in its limits, the same frequencies can be reused not far in other cells with little potential for the interference. The reusability of the frequencies is what allows a cellular system to deal a great call numeral with a limited number of channels.

Security in GSM

GSM is quite for a safe communication media. On air interface, GSM normally uses encryption and TMSI instead of IMSI. SIM is provided 4-8 digits PIN to validate the ownership of SIM. For secure communication 3 algorithms are specified that are A3 algorithm for authentication, A5 algorithm for encryption, A8 algorithm for key generation.

Advantages of GSM over Analog system 

It has reduced RF transmission power and longer battery life. It provides International roaming capability. Security against fraud (through terminal validation and user authentication) is an added feature of GSM. It has encryption capability for information security and privacy and compatibility with ISDN, leading to wider range of services.

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