The mobile communications


GSM, which stands forGlobalSystem forMobile communications, reigns as the world's most widely used digital cellular technology for transmitting mobile voice and data services.

It uses SIM (SubscriberIdentityModule) cards, which acts as digital identity, and is tied to the cell phone service carrier's by searching for towers (bts) in the nearby area.

GSM is a second-generation (2G) cell phone system providing the world with mobile communications since 1991. In over twenty years of development, GSM has been continually enhanced to provide platforms that deliver an increasingly broad range of mobile services as demand grows Good subjective speech quality.

The origins of GSM can be traced back to 1982 when the Group Special Mobile (GSM) was created by the European Conference of Postal and Telecommunications Administrations (CEPT) for the purpose of designing a pan-European mobile technology.European Telecommunications Standardization Institute (ETSI) adopted the GSM standard in 1991.

GSM is a digital mobile telephone system that is widely used in different parts of the world. GSM uses a variation of Time Division Multiple Access (TDMA) and is the most widely used of the three digital wireless telephone technologies (TDMA, GSM, and CDMA). GSM digitizes and compresses data, then sends it down a channel with two other streams of user data, each in its own time slot. It operates at either the 900 MHz or 1,800 MHz frequency band.Several companies have adopted GSM and it is spreading fast among AT&T Wireless, T-Mobile.It has an ability to carry 64 kbps to 120 Mbps of data rates. GSM operates on 4 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. An extended GSM feature is high speed circuit switched data, allowing the phone to transmit up to around 40 kbps.

GSM is now used in 219 countries and territories serving more than three billion people and providing travellers with access to mobile services due to having roaming collaboration with foreign operators.

Importance Of Technology

In the past decade mobile communications has become one of the driving forces of the digital revolution. Every day, millions of people are making phone calls by pressing a few buttons. Little is known about how one person's voice reaches the other person's phone that is thousands of miles away. The complexity of the cell phone is increasing as people begin sending text messages and digital pictures to their friends and family. The cell phone is slowly turning into a handheld computer. All the features and advancements incell phone technology require a backbone to support it. The system has to provide security and the capability for growth to accommodate future enhancements. General System for Mobile Communications, GSM, is one of the many solutions out there. GSM has been dubbed the "Wireless Revolution" and it doesn't take much to realize why. GSM provides a secure and confidential method of communication.

GSM became popular very quickly because it provided improved speechquality and, through a uniform international standard, made it possible touse a single telephone number and mobile unit around the world.

Popularity Reasons of GSM

  • Improved spectrum efficiency.
  • International roaming.
  • Low-cost mobile sets and base stations (BSs).
  • Flexibility of changing SIM.
  • High-quality speech
  • Compatibility with Integrated Services Digital Network (ISDN) and other telephone company services.
  • Support for new services.

The Social Impact of Mobile Telephony

Mobile telephony is without doubt one of the most explosive developments ever to have taken place in the telecommunications industry. By the end of 1990 there were just 11 million cellular subscribers world-wide; eight years later that figure had jumped to 320 million and is now forecast by the ITU to exceed 550million by the end of next year.

For the world's poorest countries, cellular telephony and wireless local loop systems represent the best chance yet of bringing the power of telecommunications to economically disadvantaged or isolated communities. Cambodia, for example, is one of only half a dozen countries in the world where cellular subscribers already outnumber fixed-line subscribers.

Health Issues

While evidence remains inconclusive, there remain concerns about the frequent use of mobile phones on human health. The potential impact of the kind of electromagnetic fields generated by cellular phones on the human brain has received little attention until relatively recently, and it's probably still much too soon to pronounce on the possible adverse effects of long-term exposure.

Security Issues

GSM security is addressed in two aspects: - authentication and encryption. Authentication avoids fraudulent access of a cloned MS.

Encryption avoids unauthorized listening.

A secret key Ki is used to achieve authentication. Ki is stored in the AuC as well as in the SIM.The Ki value is unknown to the subscriber.

SecurityMeasures in GSM:-

  • Subscriber Identity Authentication.
  • PIN code (authentication of SIM = local security measure, network not involved)
  • User authentication (performed by network)
  • Ciphering of information sent over air interface
  • Usage of TMSI (Temporary Mobile Subscriber Identity) instead of IMSI (International Mobile Subscriber Identity) over air interface
  • Duplicate SIM are not allowed on the network.
  • User and Signalling Data Confidentiality.
  • Subscriber Identity Confidentiality.
  • Authentication and encryption algorithms were never made public.

PIN Code:-

Personal Identification Number. It is stored in SIM card

Asked when phone is switched on

If 3 faulty PIN inputs then longer Personal Unblocking Key (PUK) code is asked

If 10 faulty PUK inputs then SIM card is locked and we have to get new card from operator.

User Authentication:-

Authentication key (Ki) is never sent over radio interface

Ciphering in GSM:-

For each call a new ciphering key (Kc) is generated during authentication.

Application of TMSI instead of IMSI:-

IMSI uniquely identifies the subscriber

Rather than sending IMSI (International Mobile Subscriber Identity), TMSI (Temporary mobile subscriber identity) is sent which prevents the intruder from:-

  • Gaining information on the resources the user is using.
  • tracing the location of the user
  • matching the user and the transmitted signal IMSI is sent only when necessary, for example
  • when the SIM is used for the first time
  • when there is data loss at VLR

Use of SIM card:-

The smart card is like a microcomputer which has memory, CPU and operating system. By Programming the ROM, it can store the sensitive data with very high security level. So it provides agood way to store the Ki and IMSI and other sensitive user data.

Security Threats:-

GSM still is a reasonably secure cellular telecommunications system but still there are some concerns which cannot be avoided:-

  • End-to-end security is not provided
  • No open algorithms tested by engineering community
  • SIM cloning is a real threat.
  • Only air interface transmission is encrypted.

Modulation Technique Used

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 neighbouring channels (adjacent channel interference).

Gaussian Minimum Shift Keying (GMSK) is a modulation technique involving Gaussian filtering of the input data prior to its application to the phase modulator. This results in a narrow occupied spectrum and better adjacent channel interference performance.

GSM uses a digital modulation format called0.3 Gaussian minimum shift keying, or0.3 GMSK. The 0.3 describes the bandwidth of the Gaussian filter in relation to the bit rate.

GMSK is a special type of digital FM modulation. Ones and zeroes are represented by shifting the RF carrier by plus or minus 67.708 kHz. Modulation techniques that use two frequencies to represent ones and zeroes are calledfrequency shift keying(FSK). In the case of GSM, the data rate of 270.833 kbps is chosen to be exactly four times the RF frequency shift. This has the effect of minimizing the modulation spectrum and improving channel efficiency. FSK modulation where the bit rate is exactly four times the frequency shift is calledminimum shift keying(MSK). In GSM, the modulation spectrum is further reduced by applying a Gaussian pre-modulation filter. This slows down the rapid frequency transitions, which would otherwise spread energy into adjacent channels

It is a highly successful form of modulation, being used in GSM cellular technology, and as a result, its use is particularly widespread. It is also used in other radio communications applications because of its advantages in terms of spectral efficiency, resilience to noise and its ability to allow the use of efficient transmitter final amplifiers. Even though other radio communications systems utilise other forms of modulation, GMSK is an ideal choice for many applications.

GMSK modulation is based on MSK, which is itself a form of phase shift keying. One of the problems with standard forms of PSK is that sidebands extend out from the carrier. To overcome this, MSK and its derivative GMSK can be used.

MSK and also GMSK modulation are what is known as a continuous phase scheme. Here there are no phase discontinuities because the frequency changes occur at the carrier zero crossing points. This arises as a result of the unique factor of MSK that the frequency difference between the logical one and logical zero states is always equal to half the data rate. This can be expressed in terms of the modulation index, and it is always equal to 0.5.

The fundamental problem with MSK is that the spectrum has side-lobes extending well above the data rate.For wireless systems which require more efficient use of RF channel BW, it is necessary to reduce the energy of the upper side-lobes. So as a solution we use a pre-modulation filter known as Gaussian Filter (Gaussian filter, afilter whose impulse response. Gaussian filters are designed to give no overshoot to a step function input while minimizing the rise and fall time.)

Basic Requirements for Implementation

Hardware and Software:-

The GSM network consists mainly of the following functional parts:

  • MSC - the mobile service switching centre (MSC) is the core switching entity in the network.The MSC is connected to the radio access network (RAN); the RAN is formed by the BSCs andBTSs within the Public Land Mobile Network (PLMN). Users of the GSM network are registeredwith an MSC; all calls to and from the user are controlled by the MSC. A GSM network hasone or more MSCs, geographically distributed.
  • VLR - the visitor location register (VLR) contains subscriber data for subscribers registered inan MSC. Every MSC contains a VLR. Although MSC and VLR are individually addressable,they are always contained in one integrated node.
  • GMSC - the gateway MSC (GMSC) is the switching entity that controls mobile terminating calls. When a call is established towards a GSM subscriber, a GMSC contacts the HLR of thatsubscriber, to obtain the address of the MSC where that subscriber is currently registered. ThatMSC address is used to route the call to that subscriber.
  • HLR - the home location register (HLR) is the database that contains a subscription record foreach subscriber of the network. A GSM subscriber is normally associated with one particularHLR. The HLR is responsible for the sending of subscription data to the VLR (during registration)or GMSC (during mobile terminating call handling).
  • CN - the core network (CN) consists of, amongst other things, MSC(s), GMSC(s) and HLR(s).These entities are the main components for call handling and subscriber management. Othermain entities in the CN are the equipment identification register (EIR) and authentication centre (AUC). CAMEL has no interaction with the EIR and AUC.
  • BSS - the base station system (BSS) is composed of one or more base station controllers (BSC)and one or more base transceiver stations (BTS). The BTS contains one or more transceivers(TRX). The TRX is responsible for radio signal transmission and reception. BTS and BSC are connected through the Abis interface. The BSS is connected to the MSC through the A interface.
  • MS - the mobile station (MS) is the GSM handset.

GSM uses the following sub division of PLMN(Public Land Mobile Network):-

Network System Architecture

Advantages and Disadvantages


  1. Like other cellular standards, GSM allows network operators to offer roaming services so that subscribers can use their phones on GSM networks all over the world.
  2. In addition there are no replay attacks possible. The only possible attack is the denial of service attack.
  3. The advantage for network operators has been the ease of deploying equipment from any vendors that implement the standard.
  4. GSM is mature; this maturity means a more stable network with robust features.
  5. Less signal deterioration inside buildings.
  6. Talktimeis generally higher in GSM phones due to the pulse nature of transmission.
  7. The availability ofSubscriber Identity Modules (SIM)allows users to switch networks and handsets at will, aside from asubsidy lock.
  8. Ability to use repeaters. Talk time is generally higher in GSM phones due to the pulse nature of transmission. The availability of Subscriber Identity Modules allows users to switch networks and handsets at will.
  9. GSMcovers virtually all parts of the worldso internationalroamingis not a problem.
  10. The availability of Subscriber Identity Modules, which are smart cards that provide secure data encryption give GSM m-commerce advantages.
  11. Call quality is much more pure and secure in GSM. More and more value-added services like GPRS, EDGE etc. are coming every day.
  12. Power is less consumed in GSM handsets compared to CDMA handsets.
  13. The advantage for network operators has been the ease of deploying equipment from any vendors that implement the standard.
  14. International roaming -- is available through GSM technologies


  1. Implementation Call costs will still remain higher compared to CDMA. Roaming costs are much lower in CDMA compared to GSM.
  2. Intellectual property is concentrated among a few industry participants, creating barriers to entry for new entrants and limiting competition among phone manufacturers.
  3. GSM has a fixed maximum cell site range of 35 km, which is imposed by technical limitations.
  4. GSM phones can be tampered with. They have their unique IMEI number which is used to lock the phone permanently but nowadays software's are available which can tamper them too.
  5. If our SIM is lost then all data is lost unless we have it stored in the phone's memory.
  6. Higher power requirements and more cell-to-cell coordination needed by operators.
  7. It does not have extended reach - not beneficial to rural users situated far from cells



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