GSM Implementation

GSM Implementation in various places

Transaction terminals: SMS messaging can be used by machines like the EDC, POS terminals to conform transaction from the main server.

Supply Chain Management: The GSM M2M technology can be used to fulfil the needs of SCM’s huge IT infrastructures with leased lines, data centre, workstations.

Multiple Remote Data Collection Points: With the help of the GSM modem connected to PCs, GSM electronic terminals PC’s one can send and receive data from all over the world. This help client to receive data from branch offices, business associates, warehouses, and Mobile phones.

Large Transaction Volumes: GSM SMS messaging is capable of handling large number of transaction data in a very small period of time as it can receive large number of messages on your server without internet connectivity. With the help of GSM at a time the shopkeeper can handle hundreds of transaction.

High Uptime: GSM mobile network have high uptime as compared to other communication networks so it helps business. It can be used in communication industries as it doesn’t require any kind of wires for setting up the network so there is no fear of cutting of wires.

Implementation of GSM Technology

There are three main parts of the GSM Network

* Mobile Station (MS):-The subscriber carries out this part.

* Base Station Subsystem (BSS) :- It controls radio link with mobile station.

* Network and Switching Subsystem (NSS):- This part keeps track of the call switching between two mobiles and also looks over Memory management and also between fixed network users and mobile.

Network structure

All of the elements when combined together produce many GSM services such as voice calls and SMS.

Mobile Station Consists of:

* Mobile Equipment (ME):- These consist of equipment such as hand portable and vehicle mounted unit

* Subscriber Identity Module (SIM):- It is a chip size object which contains the information about the user like its identification, secret keys.


In the above diagram the Base Station Subsystem (BSS) is shown containing the Base Station Controller (BSC) and the Base Transceiver Station (BTS) which are connected together on to A-bis interface. The Packet Control Unit (PCU) is also shown connected to the BTS although the exact position of this depends on the vendor’s architecture. The Air Interface or Um is used to connect the mobile to the BSS which is connected by the A interface to the NSS.


The Network and Switching Subsystem(NSS)comprises of the MSC connected via the SS7 network to the HLR. The AUC and EIR, although function differently from the HLR are shown together as combining them is a standard in all Vendor’s networks. The NSS is connected by the A interface to the BSS. It is directly connected to the PSTN from the MSC. There is also a connection to the Packet Core (called the Gs) although this is optional and not always implemented.

The GPRS Core Network

The GPRS Core Network shown here consists of SGSN which is connected to the BSS by the Gb interface and the GGSN. These two devices are connected together to a private IP network called the GPRS backbone shown as the Gn Reference Point.


Modulation is the technique of embedment of a signal into the carrier frequency (wave). There may be analog (includes FM, AM) or digital modulation. In digital modulation Binary values 0 and 1 is embedded in the carrier frequency by changing its frequency, amplitude or phase. In this way digital modulation may be ASK (Amplitude shift keying), FSK (Frequency shift keying) or PSK (Phase shift keying).

In GSM, GMSK (“Gaussian Minimum shift keying”, also known as “Gaussian filtered Minimum Shift Keying”) Modulation technique is used which is a type of FSK that continuously uses Phase modulation.

Advantages of GMSK over other techniques->

* Being able to carry digital modulation and efficiently using spectrum.

* Sidebands do not extend outwards from main carrier like other phase shift keying and thus, it avoids interference.

* No discontinuities as change in frequency occur at “carrier zero” crossing points.

* Being continuous frequency shift keying modulation scheme.

It uses the frequency hopping technique, which involves switching the call frequency many times per second for security.

GMSK Fundamentals

The unique factor of MSK (Maximum Shift Keying) is that frequency difference between the logical 1 and 0 states always equals to half of data rate, expressed in modulation index terms and always being equal to 0.5.

MSK modulation

Signal using MSK modulation

Sidebands extend can be easily traced in the plot drawn for the spectrum of an MSK signal. Therefore before applying it to the carrier modulating signal is passed through a low pass filter (Gaussian filter). Before filtering it must have narrow bandwidth, sharp cut-off and impulse response should have no overshoot. Finally MSK signal gets converted to GMSK modulation.

Spectral density of MSK and GMSK signals

Spectral density of MSK and GMSK signals

Generation of MSK Modulation

There are mainly two ways of to generate MSK modulation.

Description of First way- The filtering of Modulating signal is done using Gaussian filter and then it is applied to frequency Modulator to set modulation index to 0.5. It is the simplest way. Its limitation lies in the fact that Modulation index must exactly equal to 0.5, which is practically not suitable as drifts are experienced by the components (which is not possible to be set exactly).

Generating GMSK using a Gaussian filter and VCO

Generating GMSK using Gaussian filter & VCO

Description of second way- Quadrature Modulator (or I-Q Modulator) is one of the ways. Here the phase of a signal lies in quadrature or 90 degrees to other one. It uses one signal, known as to be in-phase and another being quadrature to the first one. Use of Modulator of this type the Modulator index is exactly maintained to 0.5 without any further setting. Thus it is much easier to use and also it provides the required level of performance.

Block diagram of I-Q modulator used to create GMSK

Block diagram of I-Q modulator used to create GMSK

For Demodulation the above techniques can be used in reverse.



The three systems of GSM are–

* Switching system (SS)

* Base station system (BSS)

* Operation and support system (OSS)

The Switching System- It is responsible for all call processing and other functions with following described units.

GSM Network Elements

Home Location Register (HLR)-

It is the most important database of an operator storing every important details of subscribed customer. Whenever a customer buys a new connection his registration is made in HLR.

Mobile services switching centre (MSC)-

It performs telephony switching functions of system including controlling calls to and from other telephone and data systems.

Visitor Location registers (VLR)-

When a cell phone roams into a new MSC area, VLR connected to that MSC requests for data about mobile station from HLR. Later VLR will not need to interrogate HLR whenever customer makes any call.

Authentication centre (AUC)-

It verifies each caller’s identity and ensures confidentiality of each call.

Equipment Identity Register (EIR)-

This database stores information of Mobile devices thus preventing calla from unauthorised or stolen ones.

Base Station System- It consists of

Base Station Controllers (BSC)-

It is the switch serving as physical links between MSC and BTS also providing functions like handover, cell configuration data, and control of Radio frequency (RF) power levels in BTS.

Base Transceiver Stations (BTS)-

It is the radio equipment which serves each cell in Network. A group of BTS are controlled by BSC.

GSM Telecommunication Tower

Operation and Support System (OMC)-

This entity (connected to all equipment in switching System and to BSC) helps the operator to govern the whole system and offer customer cost-effective support and maintenance activities need for GSM network.

Additional Elements-

Message centre (MXE)-

It Handles SMS, voice mail, fax mail, email, and cell broadcast.

Mobile Service Node (MSN)-

It governs Mobile intelligent network (IN) services.

Gateway Mobile Services Switching Centre (GMSC)-

It interconnects two networks. MSC then known as GMSC.

GSM Interworking Unit (GIWU)-

It consists of Hardwires and Software both and acts as an interface to various network for data communication.

Problems in Implementation of Technology:-

Ø Implementation of GSM technology requires a number of equipment. Many equipment need to be installed like tall towers. These towers are fixed at proper area in line of sight. Also satellites functionality also requires attention.

Ø Installation of Several equipment and devices require huge amount of money. Thus GSM architecture is very costly.

Ø Maintenance of equipment used in GSM is practically not affordable time and again.

Ø There are several protocols applied in GSM Technology, resulting problem in implementation.

Ø There is possibility of breaking into GSM networks that use cipher text-only cryptanalysis encrypted communication. These attacks are based on security flaws of the GSM protocols, and work whenever the mobile phone supports A5/2.

Ø The available radio frequencies need to be re-used closer together and continuously within the network so that spectral efficiency can be unlimited.

Ø There is huge consumption of power in GSM implementation, which is not convenient.

Ø GSM is also not secure as the sent signal or message is not so much hard to be decoded. So the third party may avail of it somehow using some technology.


GSM (Global System for Mobile communications: originally from Groupe Special Mobile) is the most widely used digital cellular technology for transmitting mobile voice and data services in the world.

It uses SIM (SubscriberIdentityModule) cards, which acts as digital identity, and is tied to the cellular network by searching for cells towards the towers (bts) in the nearby location.

GSM is a second-generation (2G) cell phone system used by over 3 billion people across more than 212 countries and territories. In over twenty years of development, GSM has been continuously upgraded to provide better signal, speech quality and thus fulfil the mass demands. It also facilitates to roam, switch carriers and the operators without replacement of handsets.


The GSM technology originally originated by the European Conference of Postal and Telecommunications Administrations (CEPT) in 1982 when it firstly created Group Special Mobile (GSM) for the purpose of designing a pan-European mobile technology. European Telecommunications Standardization Institute (ETSI) later in 1991 adopted the GSM standard. GSM technology was firstly commercially lunched in 1991 in Finland, although it has been for testing purposes since 1980.


Newer versions of the standard are much compatible with the original GSM system. For example,Release ’97,the General Packet Radio Service (GPRS) added the packet data capabilities to GSM technology. Release ’99 introduced higher speed data transmission usingEnhanced Data Rates for GSM Evolution(EDGE).

It uses variation of Time Division Multiple Access (TDMA) and is the most widely used of the three digital wireless telephone technologies (TDMA, GSM, and CDMA). Data is digitized and compressed and, sent down a channel with two other streams of user data, each in its own time slot. Most 2G GSM networks operates in the 900MHz or 1800MHz bands. It operates on four different frequencies worldwide. GSM calls are either based on data or voice. Voice calls use audio codecs called half-rate, full-rate and enhanced full-rate. Data calls can turn the cell phone into a modem operating at 9600 bps having ability to carry 64 kbps to 120 Mbps of data rates.

GSM technology even keeps people in touch while roaming also as it home operators are having roaming collaboration with foreign operators.


GSM is the most popular, reliable and the fast growing wireless technology for mobile phones in the world. It has approximately covered the 89% share market of the global mobile telecom in more than 219 countries and territories with more than 3.8 billion people as per May 2009 study. Its signalling and speech channels being digital differs it from its and so pronounced as a second generation (2G) mobile phone system. This has also meant that data communication was easy to build into the system.

It also provides the integrated high-speed data, fax, paging, voice mail and short messages services (SMS) capabilities and thus offering communication to be secure, private and fraud prevented.

Many operators provide access to ISDN services to transmit data fast. Several offer GPRS facility to connect to high speed data communication channel (9600 bps).

Global GSM Growth

GSM market share has grown at exponent rate over recent years. However the target to achieve 1 billion customers took 12 years (Feb 2004) but later it was a surprising fact that it crossed the 2 billion mark only in another 2.5 years (By June 2006) and 3 billion GSM customers in another less than 2 years. Now the Telecoms and Media hope to surpass 5 billion targets by 2013.

GSM is the legacy network of the evolution to the third generation (3G) technologies Universal Mobile Telecommunication System, (UMTS) also known as WCDMA, and High Speed Packet Access (HSPA).

Maximum Mobile phones manufacturer develop their products based on GSM technology and all mobile companies provide their customers GSM networks.

The Social Impact of Mobile Telephony

Mobile telephony is undoubted one of the most superb developments telecommunications industry. The number of subscribers is rapidly increasing in tremendous way. More and more people are being connected to the telecom.

GSM technology has greatly affected the social life. The various operators have been proving a number of offers to fascinate their customers. Due to aspects like reliability, security, voice and data transmission facilities GSM is much popular among the users. People are shifting to wireless communication from their fixed phones.

Rapid increase in GSM market

Popularity Reasons of GSM Technology

* Improved spectrum efficiency.

* High speech quality.

* International roaming.

* Better signal quality.

* Offer voice, data and fax.

* Offer secure communication and privacy.

* Low-cost mobile sets and base stations (BSs).

* Flexibility SIM.

* High share market capture.

* Low terminal and service cost.

* ISDN compatibility.

* Compatibility with Integrated Services Digital Network (ISDN) for data transmission.

* Support for new services like VAS.

Backbone for 3G and other evaluating technologies

GSM customer growth in India

Health Issues

It is the recent report that use of GSM mobile technology poses Health Hazards. Its regular and continuous use for hours may create heath problems like irritations, allergies, headache, ears pain etc.

It is fact proved by scientists that radio waves of frequencies that it utilises can create electric fieldsin human cells and brain. Long term exposure may have adverse health effects too.

Many research institutions controlled by government and independent bodies are continuously studying over it. Also research results are being revived by ICNIRP (International Commission on Non-Ionizing Radiation Protection) and the WHO (World Health Organisation) at global level.

Use of cellular phone is highly restricted to be turned off while in flight as the radio frequencies may interfere. The same case happens at hospitals as the radio signal may create interference with electronic medical devices at short rang or cause uneasiness among patients.

While driving use of cell phone is advised to avoid and in petrol pumps as the radio waves used in GSM technology may cause interference.



As we know that security is the essential feature in any service. Hence, GSM network also follows certain level of security in the services which it provides. Hence, there are mainly two security purposes which are kept in mind when it is concerned of the GSM network:-

First of all, to protect the Accessibility to the mobile services and secondly to prevent disclosure of any crucial information/data at the radio path to achieve the privacy regarding that particular data.


There are several features of the GSM service for the security of its customers. Some features are stated below: —

1. Authentication is being provided to the registered customers only.

2. The process of encryption is being used for secure data transfer.

3. The identity of customer is being verified.

4. The cellular phones are useless without a SIM card.

5. A secret key Ki is being used to achieve authentication.

3. Representation of the GSM Security:-

GSM security is represented in two ways:-

a. Authentication

b. Encryption

A. Authentication:—

The process of authentication is being categorized in to various stages which are stated below:-

1. First of all an IMSI (International mobile subscriber Identity) is being send to the network by the mobile station.

2. After that the IMSI is being received by the network and the KI corresponding to that IMSI is found.

3. A 128 bit random number (RAND) gets generated by the corresponding network and is send to the mobile station through the medium of air.

4. Through the use of the algorithm A3, the network (AuC) and the MS (SIM) use KI and RAND to produce a signed result (SRES).

5. The SRES is being sent to the network by the MS.

6. For the validity purpose network performs the testing on the received SRES.

B. Encryption:—

The process of encryption gets started by ciphering mode request (i.e.: secretly executed request) command from the GSM network. After receiving the request the process of encryption and decryption of data gets performed by the mobile station. Different key stream is involved in the encryption of each frame over the air traffic. Different key stream is generated for each frame as the A5algorithm which is used to encrypt the data is initialised with the KC and the number of the frame to be executed resulting in the generation of different key stream.

The same KC gets executed as long as it does not get verified by the MSC (Mobile services switching Centre) through the MS(mobile station) which results in the generation of the new KC. The Authentication of MS is an optional process and is usually not performed in the beginning of a call. Hence it is assured that the KC will not get changed during a particular call. As it gets switched off IMSI (International mobile subscriber identity) gets stored by the mobile station on the SIM card to assure that is gets available when it is switched on again. In the hardware of the mobile phone the A5 algorithm gets executed as data is being encrypted and decrypted through the air.

4. The Algorithm used in the GSM Security:–

There are three algorithms which are used in the GSM security:-

1> A3: The MS Authentication Algorithm:—-

In the GSM model, A3 acts as the authentication algorithm. This algorithm generates a 32 bit output as it gets the RAND (128 bit random number) from the MSC (Mobile service switching Centre) and the secret key KI as input from the SIM. The size of KI and RAND is of 128 bits. The A3 algorithm is represented as a one-way hash function. This function produces an output which is of fixed length as a random input is given to it. A COMP 128 algorithm is used for the A3 algorithm by every GSM operator in the world. This algorithm acts as the reference algorithm for the various tasks which has been pointed out by the GSM association.

2> A8, The ciphering Key Generation Algorithm:—

In the GSM network, A8 algorithm acts the session key algorithm. The A8 algorithm generates 64 bit output by taking two 128 bit input. The output generated is 64 bit session key KC. COMP128 acts the reference algorithm for the A8 algorithm also as it is there in the A3 algorithm. At one run, COMP128 generates both the session key KC as well as the SRES response.

3> A5, the stream-ciphering algorithm:—–

In this algorithm encryption of data over the air occurs. For secure data, there are three possibilities in the case of GSM; it includes the A5/0, A5/1 and the A5/2 algorithm. This algorithm came into existence because the GSM standard was designed for Europe and as per the export regulations it did not allow the implementation of the technology outside Europe.

f> Advantage of GSM Services:-

Following are the Advantage of GSM service: ——-

1>First of all the GSM network is fully developed which means that it contains stability with proper features and services. Hence it has over 450 million subscribers worldwide.

2> The other advantage of GSM network is that it contains better voice quality and low- cost alternatives to make calls, such as Short message service (SMS) . In the case of GSM network the quality of voice is fine inside the buildings.

3) Another advantage of this service is that the equipment’s are easily available from the vendors who execute the following service.

4> GSM services provide the roaming facility to its customers so that they can use the service anywhere in the world.

5) GSM networks provide higher talk time due to its pulse nature of transmission and its ability to use routers.

6) GSM provides the facility of International Roaming which is highly beneficial in context to its customers.

7> The presence of Smart Cards which acts as subscriber Identity Modules through which encryption of data occurs in a secure manner which is highly advantageous with respect to commercial background.

8> lastly the common advantage of the GSM service is that it provides the user the freedom to choose a mobile set of his/her choice and the SIM card is also being selected by user of their choice. They also have the option of making the changes to both the equipment’s accordingly.

Disadvantage of GSM Services:-

The GSM service has following Disadvantages:-

1> The first disadvantage is that the algorithms which are being used in this service for the security purpose is not available to the public.

2> In the GSM service only Access security is being provided and the different communications occurs only in a fixed network.

3> The major disadvantage of the GSM service is that it lacks the access to the American market.

4> For the usage of this service, the user needs the requirement of higher power and the coordination between cell to cell.

5> The network coverage of this technology is not better in the rural areas as compared to its competitor service of the CDMA technology.

Reference: —

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