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Text Box: S. Krishnan
General Manager, BSNL,
Assam Telecom Circle





Text Box: This paper provides a useful introduction to the Architecture of GSM mobile system





1.  Introduction


One of the finest things that happened in 20th century is the use of radio channels for personal mobile communication systems.  Considered to be a revolution of sorts in telecommunications.  Considered to be a revolution of sorts in telecommunications, mobile communication is the fastest growing market segment and the field of intense research.  Mobile communication today is, perhaps, the most powerful catalyst for change in lifestyle of the people all over the world.  Mobile communication slowly,  but surely is evolving as the backbone for business transactions, efficiency and success silently taking over the role of the elder cousin PSTN.


The first mobile telephone service started in 1946 in St. Louis, Missouri, USA as a manually operated system.  Between 1950 and 1960, it evolved as an automatic system with reduced cost and increased, but small subscriber base.  Mobile telephony service in its useful form appeared in 1960s.


The first generation mobile communication systems appeared in 1970s and remained till 1980s.  They used analog transmission techniques for the radio link and confined its users to their respective system areas for which the mobile phone was designed.  Capacity of the system was very limited and roaming between the coverage areas of the different systems was impossible.  Apart from being very expensive, these systems provided poor QoS and supported only voice communication.


The second generation mobile communication systems grew out of the limitations of the 1st generation systems.  They supported large subscriber base, carried both voice and data and have capability to design and deliver new value added services.  The radio link became digital enabling use of versatile signaling capabilities and cross-network roaming.  However, multiple standards made seamless roaming across all the networks impossible.  GSM and CDMA emerged as the trend setting technologies.  The domination of the 2G systems in the mobile communication market became apparent in second half of 1990s.


The design objectives of the third generation mobile systems are to provide high functionality with seamless global roaming.  Apart from providing very high data rates, 3G systems seek to integrate the wire lines systems with mobile systems.  3G would provide users consistent voice, data, graphical, multi-media and video-based information service regardless of their location in the network (Cordless, Cellular, Satellite, Fixed/Wire line and so on….).  3G systems would also integrate the Intelligent Network (IN) capabilities into mobile systems.


Basic to the design of a mobile communication system is the ability of the user to liberate himself from the confines of a wired line and therefore his freedom to move anywhere he wants.  Mobile communication systems therefore are required to provide seamless service while the customers are on move transcending geographical and network borders.  Provision of service under such environment throws a number of challenging issues and the way these issues are addressed and resolved makes each system different from all others.


2.  Mobility Issues


A comparison of a typical PSTN and mobile call scenarios is shown in figure 1(a) and 1(b).



The wired 2W subscriber line is replaced with a wireless mobile link.  Once the customer is liberated from the confines of the wire and made free to move, the following issues arise:


·         Demand on the scarce radio resources

·         Authentication of the customer

·         Security and Privacy on the radio

·         Provide unique service profile

·         Keep track of the user as they move

·         Providing service across networks

·         Billing the customer whenever and wherever he makes and receives calls from


The bandwidth of the RF spectrum available for the mobile communication is limited.  However, service has to be provided for large number of subscribers notwithstanding the limitations in the availability of the frequency spectrum.  This issue is resolved through a two-pronged attack. In the first place, no permanent RF channel is assigned for any customer and second, the same RF channels are reused at geographical locations that are considerably separated so as not to interfere with each other’s communication.  These techniques of demand assignment and frequency re-use obviate the limitations posed by the limited RF spectrum in providing service to large number of customers.


Another important issue in mobile communication is the need to authenticate the genuineness of the customer whenever he receives or attempts to make communication.  Since the media being open space, it is necessary to verify whether the customer is the one whom he claims to be before resource allocation.  Security of the mobile account is to be ensured to prevent unauthorized use and also misuse of one’s subscription.  Privacy of the communication over radio is to be ensured though the radio signals are available everywhere for interception.


One of the important issues for the customer is the availability of seamless service profile irrespective of his location.  This is an essential feature of the mobile communication, particularly when the customer visits a service area served by an operator different from his own.  Different dialing codes for accessing the same service in different networks can play havoc in realizing the service by the customer.


Yet another  challenging issue in mobile communication is the need to keep track of the customer’s location so that an incoming call can be connected to him.  Equally important is the need to main established calls as the customer moves across areas covered by different radio transmitter / receiver. 


Since no service provider’s network can provide coverage throughout the world, there is a need to offer service to one’s customers through different other networks across the world to enable him to move freely.  This necessitates inter network compatibility and connectivity with strict access safeguards.


Finally, since the customer is on the move and can avail service from any permissible  network across the globe, there should be a foolproof billing and accounting mechanism that not only records the customer’s liability for his consumption, but also that between the service providers.




Like any other technology, mobile communication too has a set of unique terminology.  A few often sued ones are given below:


PLMN - Public Land Mobile Network.  A PLMN is the network set up by a cellular licensee in his licensed area.  Each network of each of the licensees in a lincensed area is considered to be a separate PLMN.  Thus as per the new licensing policy of Government of India, there will a maximum of 4 PLMNs in each of the licensed area.  A licensed area, typically, is coterminus with a telecom circle.  In India there are 58 private PLMNs working and another 22 PLMNs are launched by BSNL and two by MTNL.


HOME PLMN - Every mobile customer has to register himself with a service provider in a licensed area.  The PLMN with which he registers is known as his Home PLMN (HPLMN).  All the administrative data of his subscription will be available in the Home PLMN.


VISITED PLMN - When a mobile subscriber enters a PLMN area other than his Home PLMN, then he is said to be in the area of a Visited PLMN (VPLMN).


ROAMING - Whenever a customer, with an intention of availing service, enters a Visited PLMN area then he is said to Roaming.


HANDOVER or HANDOFF - This is the process of maintaining an established call when the customer moves from one radio transmitter / receiver area to another.  This process involves re-routing the customer’s communication path through a different area to avoid dropping of call and / or deterioration of quality.


CELL - The area served by a radio transmitter/receiver station.  The radio signal emanated by the transmitter is expected to be confined within the boundaries of the cell.


LOCATION AREA  -  An area covered by one or more radio transmitter/receiver stations used by the system to keep track of the user location for paging him on an incoming call.




The central concept that made mobile communication as a usable commercial proposition is the cellular principle.  BELL Laboratories, US in 1970 first introduced cellular principleUnder cellular concept, the service area is divided into a number of CLUSTERS,  each cluster consisting of a number of CELLS and each cell is assigned as many CARRIERS as required by the traffic in that cell.  There is a one-to-one correspondence between the  cells in each of the clusters that these cells use the same carrier frequencies. 


Since the frequency used being the same, the principle of frequency reuse demands that the interference between them when serving different cells and therefore different customers should be kept within permissible limits.  Figure-2 below shows the cluster and frequency re-use.


Figure-2 shows a 7-CELL reuse pattern.  The 7 cells, shown as seven different hexagons that are tagged in contiguity are called a cluster.  The cluster can be repeated any number of times to provide coverage for the service area using the very same frequencies F1-F7.  This is called a 7-CELL cluster.  There are different types of reuse pattern each governed by interference and capacity requirements of individual systems.




The cluster is repeated in such a manner that the distance between the cells using the same frequency is kept as far as possible.  Implicit in this arrangement is that the signal strength emanating from a cell is so engineered that it will be confined within the designated area of that cell and that its interference with the cells using the same frequency in the other clusters will be within prescribed limits so as not to interfere with the communication of those cells.  This reuse of frequency will remove the restrictions imposed by the limited bandwidth on the capacity of the system.  There are several interference reduction techniques that help improve the system capacity and performance.


The number of RF Channels in each cell is decided on the basis of the traffic load that is expected to be handled in the area served by the cell.  BSNL has been allotted 31 frequencies each designed to handle 8 simultaneous call totaling to just 248 voice/data channels.  It is the cellular concept that enables us to build a network of millions of subscribers with these meager 248 channels.




Oe of the important objectives of GSM group is to evolve a unified standard to provide seamless roaming across Europe.  Another landmark decision taken by the group is to standardize a digital radio interface for the communication between the mobile handset and the radio transmitter / receiver.  The design objectives of the GSM system can be briefly states as below:


  • Excellent speech quality
  • High security and privacy
  • Low module terminal cost
  • Low service and facilities cost
  • Design of sleek and handled mobile terminals
  • International roaming
  • Wide range of services and facilities
  • Ability to adopt to new and innovative features
  • Narrowband ISDN compatibility
  • Digital Radio
  • High Spectral efficiency


The digital radio uses the 900/1800 MHz  band.  The mobile terminal vary in power class 20 watts to as low as 0.8 watts.  A GSM cell can cover a maximum distance of up to 30 Kms.  The system can provide service to mobile customers traveling up to a maximum speed of 250 Km/hr.




  • Mobile Station (MS)
  • Base Station Subsystem (BSS)
  • Network Switching Subsystem (NSS)
  • Operation and Maintenance Subsystem (OMS)
  • Enhanced Services Subsystem (ESS)
  • Billing and Customer Care System (B&CCS)




Mobile Station, in fact, consists of two distinct entities.  They are:


  • Mobile Terminal or Equipment (MT)
  • Subscriber Identity Module (SIM) Card






The mobile terminal is the actual hardware and is almost anonymous.  Moibile equipment is being manufactured by a number of vendors and a number of make and models are available in the market.  Each MT is identified by a number embedded in it by the manufacturer called International Mobile Equipment Identity (IMEI).  IMEI is useful to locate those mobile phonesthat are reported stolen and also those manufactured without proper approval.  IMEI is also useful to route calls from MTs without SIM to emergency services.  Mobile terminals are distinguished mainly by their power class and application.


The SIM card is given by the service provider (PLMN Operator) when a customer enrolls himself as a subscriber of that PLMN.  SIM makes the MT operational and provides the subscriber access to all his subscribed services.  The subscriber information and provides personal mobility.  Each SIM is identified by a unique identifier called International Mobile Subscriber Identity (IMSI).  The SIM holds such vital information such as Authentication Key, various algorithms, Cipher Key etc to ensure security of the subscription and privacy of the conversation.


Sicne sIM is such an important element of the personal subscription and can be used in conjunction with any MT,  it can be protected with a password  of 4 to 8 digits long to prevent misuase.  Wrong entry of the password consecutively for 3 times would block the SIM cards are classified on the basis of their operating voltage and the capacity of the Random Access Memory (RAM).




Base Station subsystem connects the MS to the rest of the network elements in the PLMN.  It provides all the digital radio interface functions.  BSS comprises of three parts:


  • Base transceiver Station (BTS)
  • Base Station Controller (BSC)
  • Transcoding and Rate Adoption Unit (XCDR/TRAU)


Base transceiver Station (BTS) :  BTS houses the radio transeivers that define a cell. It transmits to and receives signals from the MS.  It handles the radio-link protocols with the Mobile Station for call and mobility related activities on one side and with the BSC on the other side.  Each BTS can support a number of transceivers depending on the capacity of subscribers in the cell.  The BTS needs to be rugged, reliable, and portable to ensure reliable service and coverage.


The Base Station Controller (BSC) manages the radio resources of one or a group of BTSs.  BSC handles radio channel setup, performance improvement techniques such as control of the RF power levels, etc and handovers.  It establishes connection between the mobile station and the MSC.


Transcoding and Rate adoption Unit (DCDR/TRAU) converts the transmission rate on the land line to that compatible for transmission over the digital radio to and from the MS.




NSS manages the communication among the mobile users of the same PLMN and also with other PLMN/PSTN users.  It provides all the functionality needed to handle a mobile subscriber such as:


  • Registration
  • Authentication and security
  • Location updating
  • Handovers and
  • Routing to roaming subscribers


The central component in the NSS is the Mobile Switching Centre (MSC) that performs all the switching functions of the network.  Those MSCs that interface with other networks are called Gateway MSC (GMSC).  MSC realizes the above functions in conjunction with four intelligent databases called:


  • Home Location Register (HLR)
  • Visitor Location Register (VLR)
  • Authentication Centre (AuC) and
  • Equipment Identity Register (EIR)


HLR contains the administrative information of each of the subscriber registered in the network and is the permanent copy of the subscriber data.  Logically there will be one HLR per PLMN, implemented as a centralized or distributed database.  HLR maps each IMSI with a unique mobile phone number called Mobile Subscriber ISDN (MSISDN).  This is the number dialed by others to access the mobile customer.  HLR also holds most of the information held by the SIM and also more importantly contains the pointer to the current location of the mobile customer in order to gain access to him on an incoming call.


Each MSC has a VLR to holds the data relevant for handling calls from and to the MSs that are currently located in its area..  The relevant data is downloaded from the home HLR when the mobile subscriber switches on the mobile handset in the area of the visited MSC thereby initiating the process of registration.  VLR holds the exact location of the MS and keeps on updating the location as the mobile move across its jurisdiction.


Authentication Centre is a protected database used for security purposes.  It is considered as part of HLR and provides all the parameters needed for authentication of the subscriber and encryption of voice/data over radio channel.


Equipment Identity Register is a database that contains a list of all valid mobile equipment in the network.  It generates Valid, Suspect & Fraudulent lists (also called white list, grey list and black list) of Mobile Hndsets through the use of International Mobile Equipment Identity (IMEI).  It forbid calls from unauthorized mobile terminals (non-standard and stolen terminals).




OMS is used to configure, control and monitor the GSM network.  It comprises of two parts:


  • Operation and Maintenance Centre - Switch
  • Operation and Maintenance Centre - Radio


While OMC-S is used in conjunction with the NSS, OMCs-R is used in conunction with the BSS.  The OMC-R controls the traffic load on the various cells and performs automatic reconfiguration of the transceivers to cope with the fluctuation of traffic, load caused due to the mobility of the customers.


The OMCs also provide traffic data measurements, reporting and analysis.  OMCs also store all the data and software for the network elements and perform as the central maintenance control point for all the network elements.




The ESS includes such elements as Unified Messaging System (UMS), Wireless Application Protocol (WAP) System, Interlligent Network (IN) system, Content and Location Based Services (C&LBS) system etc.


The Unified Messaging System provides a variety of voice and data services such as:


  • Voice Mail
  • Fax Mail
  • E-Mail
  • Visual Mail
  • Short Message Service (SMS)


All the above services can be subscribed by the customer and can be provided from a single platform.


The WAP system allows wireless access to Internet sites that are enabled for customized access from mobile handsets.  Thus a customer, on move, can surf the web from his WAP enabled handset through the WAP gateway.  The WAP system also manages telephony events such as incoming call etc when a subscriber connected to the Internet.


Intelligent Network system provides a host of value added features and services such as:


  • Pre-paid service (PPS)
  • Free Phone Service (FPH)
  • Premium Rate Services (PRM)
  • Mobile Virtual Private Network (MVPN)
  • Universal Access Number (UAN) etc


The pre-paid service provided through IN system in conjunction with Voucher Management System (VMS) and Over-the-Air Charging (OTAC) system can provide national and international roaming and recharging through a variety of payment instruments such as Credit/Debit card, ECS etc.  It can also support a number of tariff and promotional plans that can be chosen by the customer himself/herself.


Content and Location Based Services can offer a variety of PUSH and PULL services ranging from advising the customer of the discounts being offered in the shops near his current ‘location’ to verification of flight details etc.




Billing and Customer Care System is responsible for obtaining the call details of each of the customers from the HPLMN as well as from all the VPLMNs for raising the invoice.  B&CCS incorporates a powerful and flexible rating engine that would enable the service provider to offer innovative and competitive tariff packages.


The Customer Care module includes an automatic Service Provisioning Module that will activate / deactivate a customer account from a Customer Care terminal without the need for any manual intervention.  The CC module also would handle the Trouble Tickets generated on faults reported by the customer and routes such tickets automatically to appropriate terminal for action.


The B&CCS also incorporates a sophisticated Printing Subsystem for distributed printing of the customer invoices.



GSM System incorporates a number of performance enhancement techniques to ensure excellent quality of service under different load and transmission environment.  Some of the important techniques as given below:


  • Discontinuous Transmission (DTX)
  • Discontinuous Reception (RTX)
  • Frequency Hopping (FH)
  • Power Control (PC)


Discontinuous Transmission (DTX) is a technique that improves the performance of the system from the point of view of quality as well as capacity.  The concept of DTX is based on the fact that speech activity in any voice connection exists only fro 40% of time.  Rest of the 60% time, the channel carries “silence”.  DTX attempts  to de-assign the channels from an active connection during such “silence” period and assigns them to needy subscribers.  Speech channels are reassigned to the subscribers the moment they resume the speech activity.  Thus a given number of voice channels can be used to cater to more number of simultaneous conversations resulting in increased capacity of the system.  DTX, however, would need a mechanism to detect Voice activity with precision.  Failure to discriminate between voice activity and background noise would result in annoying clicks and would render the DTX inefficient.  It is also required to generate Comfort Noise at receiver to avoid the feeling of the originating set being dead.


DTX can also be used a mechanism  to improve the quality of the connection by controlling the “crowd” in the air.  The interference in RF channels increase with the traffic load and DTX under such situation brings down the load on the RF thereby improving the quality of the connection.  Since signal transmission is avoided during inactive period, DTX also saves the battery power in the mobile handset.


Like DTX, Discontinuous Reception (RTX) is another technique used to conserve mobile station’s battery power.  This is achieved by assigning specific time slot to each mobile station to watch for paging on incoming call.  The MS listen to the paging channel during the specified time slot only and go to sleep until the occurrence of next time slot.  Thus, between two time slots, the MS consumes   very little power as it remains in sleep mode during the intervening period.


Frequency Hopping (FH) is a technique to randomize the interference across the active mobiles by constantly changing RF channel for each of the connection during the conversation.  This will prevent a interference prone RF channel being assigned to a customer for the whole period of his connection thereby increasing the quality of the speech.


Power Control (PC) is another performance enhance techniques aimed at both saving the power of the battery in the MS as well as to regulate the power transmitted in the air to levels just sufficient to ensure appropriate speech quality.  The BTS performs the measurement of the power level of each MS and sends Power control commands to adjust the transmit power level of each mobile station so as to ensure that BTS receives signals of just the right power.


There are a number of other performance and capacity improvement technioques that are quite complicated and are beyond the scope of this article.



The setting up of a mobile call involves exchange of a number of messages between the various network elements in the system.  A highly simplified sequence is given below just to appreciate the process involved in setting up a mobile call.  The example deals with two situations:

  • Terminating call when the MS is in the HPLMN
  • Terminating call when the MS is roaming


Figure-4 depicts the different network elements involved in the call set up.  The call set up broadly involves the following steps:


  • The PSTN subscriber dials the MSISDN (Mobile telephone number)
  • The call is routed by the PSTN network to the GMSC of the HPLMN of the dialed mobile subscriber.


  • The GMSC interrogates the HLR for verification of the access privileges/service profile and for obtaining the location details if the call is permitted.
  • HLR directs the call to the VLR of the MSC area in which the mobile customer is currently located.
  • MSC interrogates the VLR to obtain the exact location of the MS.
  • VLR provides the LAC (Location Area Code) to the MSC.
  • MSC translates the LAC code into BTS identities.
  • BSS pages all the BTSs identified by the MSC within which the MS is located using IMSI.
  • MS responds to the paging.
  • Call is connected.
  • In case of a roaming call, the MS will be located in a VPLMN.  In such case, the following additional steps would be involved.
  • On registration with the VPLMN, the HLR will place a pointer in the database for the MS indicating the current VLR address.
  • On interrogation by the GMSC of the HPLMN, the HLR will, in turn interrogate the VLR of the VPLMN using the already stored pointer.
  • The VLR in the VPLMN will assign a Roaming Number called MSRN to enable the HPLMN to route the call to the VPLMN.
  • Using the MSRN call is routed back from the HPLMN to the VPLMN and the VMSC interrogates the VLR.
  • Now the VLR provides the LAC and call proceeds exactly in the same manner as for the call in HPLMN.



GSM is a 2nd generation mobile communication systems.  The data transmission speed of GSM was only 9.6 Kbps and this speed is considered to be too inadequate for many applications.  The data speed of GSM can be increased to 171.2 Kbps using GPRS and to 384 Kbps using EDGE.  3G system, which is already operational in Japan, is capable of delivering bandwidth of up to 2 Mbps to mobile terminal.

Convergence of Mobile and Internet is opening up new vistas and mobility would be the newest ingredient in the converged IT era.



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