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GSM

1. INTRODUCTION

The first generations of cellular phones were analog, but the currentgeneration is digital, using packet radio. Digital transmission has severaladvantages over analog for mobile communication. First, voice, data andfax, can be integrated in to a single system. Second, as better speechcompression algorithms are discovered, less bandwidth will be needed perchannel. Third, error correcting codes can be used to improve transmissionquality. Finally, digital signals can be encrypted for security.

Although it might have been nice if the whole world had adopted the samedigital standard, such is not the case. The US system, IS-54, and theJapanese system, JDC, have been designed to be compatible with eachcountrys existing analog system, so each AMPS channel could be usedeither for analog or digital communication.

In contrast the European digital system, GSM (global system for mobilecommunication) has been designed from scratch as a fully digital system,

without any compromises for the sake of backward compatibility. GSM iscurrently in use in over 100 countries, inside and outside Europe, and thusserves as an example of digital cellular radio.GSM was originally designedfor use in the 900 MHz band. Later, frequencies were allocated at 1800MHz, and the second system, closely patterned on GSM, was setup there.The later is called DCS 1800, but it is essentially GSM.

A GSM system has up to a maximum of 200 full duplex channels per cell.Each cell consists of a downlink frequency (from base station to mobile

station) and uplink frequency (from mobile station to base station). Eachfrequency band is 200 KHz wide.

1.History of GSMDuring the early 1980s, analog cellular telephone systems wereexperiencing rapid growth in Europe, particularly in Scandinavia and theUnited Kingdom, but also in France and Germany. Each country developedits own system, which was incompatible with everyone else's in equipment

and operation. This was an undesirable situation, because not only was themobile equipment limited to operation within national boundaries, whichin a unified Europe were increasingly unimportant, but there was also a

very limited market for each type of equipment, so economies of scale andthe subsequent savings could not be realized.The Europeans realized this early on, and in 1982, the Conference ofEuropean Posts and Telegraphs (CEPT) formed a study group called the


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Group Special Mobile (GSM) to study and develop a pan-European publicland mobile system. The proposed system had to meet certain criteria:

Good subjective speech quality Low terminal and service cost Support for international roaming Ability to support handheld terminals Support for range of new services and facilities Spectral efficiency ISDN compatibility

In 1989, GSM responsibility was transferred to the EuropeanTelecommunication Standards Institute (ETSI), and phase I of the GSMspecifications were published in 1990. Commercial service was started in

mid-1991, and by 1993, there were 36 GSM networks in 22 countries.Although standardized in Europe, GSM is not only a European standard.Over 200 GSM networks (including DCS1800 and PCS1900) areoperational in 110 countries around the world. In the beginning of 1994,there were 1.3 million subscribers worldwide which had grown to morethan 55 million by October 1997. With North America making a delayedentry into the GSM field with a derivative of GSM called PCS1900, GSMsystems exist on every continent, and the acronym GSM now aptly standsfor Global System for Mobile communications.The developers of GSM chose an unproven (at the time) digital system, as

opposed to the then-standard analog cellular systems like AMPS in theUnited States and TACS in the United Kingdom. They had faith thatadvancements in compression algorithms and digital signal processors

would allow the fulfillment of the original criteria and the continualimprovement of the system in terms of quality and cost. The over 8000pages of GSM recommendations try to allow flexibility and competitiveinnovation among suppliers, but provide enough standardization toguarantee proper interworking between the components of the system. Thisis done by providing functional and interface descriptions for each of thefunctional entities defined in the system.

2.Services provided by GSMFrom the beginning, the planners of GSM wanted ISDN compatibility interms of the services offered and the control signaling used. However, radiotransmission limitations, in terms of bandwidth and cost, do not allow the


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standard ISDN B-channel bit rate of 64 kbps to be practically achieved.Using the ITU-T definitions, telecommunication services can be dividedinto bearer services, teleservices, and supplementary services. The most

basic teleservice supported by GSM is telephony. As with all othercommunications, speech is digitally encoded and transmitted through theGSM network as a digital stream. There is also an emergency service, wherethe nearest emergency-service provider is notified by dialing three digits(similar to 911). A variety of data services is offered. GSM users can sendand receive data, at rates up to 9600 bps, to users on POTS (Plain OldTelephone Service), ISDN, Packet Switched Public Data Networks, andCircuit Switched Public Data Networks using a variety of access methodsand protocols, such as X.25 or X.32. Since GSM is a digital network, amodem is not required between the user and GSM network, although anaudio modem is required inside the GSM network to interwork with POTS.

Other data services include Group 3 facsimile, as described in ITU-Trecommendation T.30, which is supported by use of an appropriate faxadaptor. A unique feature of GSM, not found in older analog systems, is theShort Message Service (SMS). SMS is a bidirectional service for shortalphanumeric (up to 160 bytes) messages. Messages are transported in astore-and-forward fashion. For point-to-point SMS, a message can be sentto another subscriber to the service, and an acknowledgement of receipt isprovided to the sender. SMS can also be used in a cell-broadcast mode, forsending messages such as traffic updates or news updates. Messages canalso be stored in the SIM card for later retrieval .Supplementary services

are provided on top of teleservices or bearer services. In the current (PhaseI) specifications, they include several forms of call forward (such as callforwarding when the mobile subscriber is unreachable by the network), andcall barring of outgoing or incoming calls, for example when roaming inanother country.

Worldwide GSM Networks in ServiceCountries with GSM serviceCountries without GSM service

4. Architecture of the GSM network

A GSM network is composed of several functional entities, whose functionsand interfaces are specified. Figure 1 shows the layout of a generic GSMnetwork. The GSM network can be divided into three broad parts. TheMobile Station is carried by the subscriber. The Base Station Subsystemcontrols the radio link with the Mobile Station. The Network Subsystem,the main part of which is the Mobile services Switching Center (MSC),performs the switching of calls between the mobile users, and between


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mobile and fixed network users. The MSC also handles the mobilitymanagement operations. Not shown is the Operations and MaintenanceCenter, which oversees the proper operation and setup of the network. TheMobile Station and the Base Station Subsystem communicate across theUm interface, also known as the air interface or radio link. The Base StationSubsystem communicates with the Mobile services Switching Center acrossthe A interface.

Figure 1. General architecture of a GSM network

4.1 Mobile Station

The mobile station (MS) consists of the mobile equipment (the terminal)and a smart card called the Subscriber Identity Module (SIM). The SIMprovides personal mobility, so that the user can have access to subscribedservices irrespective of a specific terminal. By inserting the SIM card intoanother GSM terminal, the user is able to receive calls at that terminal,make calls from that terminal, and receive other subscribed services.The mobile equipment is uniquely identified by the International MobileEquipment Identity (IMEI). The SIM card contains the InternationalMobile Subscriber Identity (IMSI) used to identify the subscriber to the

system, a secret key for authentication, and other information. The IMEIand the IMSI are independent, thereby allowing personal mobility. TheSIM card may be protected against unauthorized use by a password orpersonal identity number.

4.2 Base Station Subsystem


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The Base Station Subsystem is composed of two parts, the Base TransceiverStation (BTS) and the Base Station Controller (BSC). These communicateacross the standardized Abis interface, allowing operation between

components made by different suppliers.The Base Transceiver Station houses the radio transceivers that define a celland handles the radio-link protocols with the Mobile Station. In a largeurban area, there will potentially be a large number of BTSs deployed, thusthe requirements for a BTS are ruggedness, reliability, portability, andminimum cost.The Base Station Controller manages the radio resources for one or moreBTSs. It handles radio-channel setup, frequency hopping, and handovers,

as described below. The BSC is the connection between the mobile stationand the Mobile service Switching Center (MSC).

4.3 Network Subsystem

The central component of the Network Subsystem is the Mobile servicesSwitching Center (MSC). It acts like a normal switching node of the PSTNor ISDN, and additionally provides all the functionality needed to handle amobile subscriber, such as registration, authentication, location updating,handovers, and call routing to a roaming subscriber. These services are

provided in conjunction with several functional entities, which togetherform the Network Subsystem. The MSC provides the connection to thefixed networks (such as the PSTN or ISDN). Signaling between functionalentities in the Network Subsystem uses Signaling System Number 7 (SS7),used for trunk signaling in ISDN and widely used in current publicnetworks.


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The Home Location Register (HLR) and Visitor Location Register (VLR),together with the MSC, provide the call-routing and roaming capabilities ofGSM.

1. Home Location Register (HLR)A Home Location Register (HLR) is a database that contains semi-permanent mobile subscriber information for a wireless carriers' entiresubscriber base. HLR subscriber information includes the InternationalMobile Subscriber Identity (IMSI), service subscription information,location information (the identity of the currently serving Visitor LocationRegister (VLR) to enable the routing of mobile-terminated calls), servicerestrictions and supplementary services information.The HLR handles SS7 transactions with both Mobile Switching Centers

(MSCs) and VLR nodes, which either request information from the HLR orupdate the information contained within the HLR. The HLR also initiatestransactions with VLRs to complete incoming calls and to updatesubscriber data.Traditional wireless network design is based on the utilization of a singleHome Location Register (HLR) for each wireless network, but growthconsiderations are prompting carriers to consider multiple HLR topologies.. The location of the mobile is typically in the form of the signaling addressof the VLR associated with the mobile station. The actual routing procedure

will be described later. There is logically one HLR per GSM network,

although it may be implemented as a distributed database.

2. Visitor Location Register (VLR)A Visitor Location Register (VLR) is a database which contains temporaryinformation concerning the mobile subscribers that are currently located ina given MSC serving area, but whose Home Location Register (HLR) iselsewhere.

When a mobile subscriber roams away from his home location and into aremote location, SS7 messages are used to obtain information about the

subscriber from the HLR, and to create a temporary record for thesubscriber in the VLR. There is usually one VLR per MSC.The Visitor Location Register (VLR) contains selected administrativeinformation from the HLR, necessary for call control and provision of thesubscribed services, for each mobile currently located in the geographicalarea controlled by the VLR. Although each functional entity can beimplemented as an independent unit, all manufacturers of switching


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equipment to date implement the VLR together with the MSC, so that thegeographical area controlled by the MSC corresponds to that controlled bythe VLR, thus simplifying the signaling required. Note that the MSCcontains no information about particular mobile stations --- thisinformation is stored in the location registers.The other two registers are used for authentication and security purposes.The Equipment Identity Register (EIR) is a database that contains a list ofall valid mobile equipment on the network, where each mobile station isidentified by its International Mobile Equipment Identity (IMEI). An IMEIis marked as invalid if it has been reported stolen or is not type approved.The Authentication Center (AuC) is a protected database that stores a copyof the secret key stored in each subscriber's SIM card, which is used forauthentication and encryption over the radio channel.

3.Adding a Second HLR to the GSM Network

As a GSM wireless carrier's subscriber base grows, it will eventually becomenecessary to add a second HLR to their network. This requirement might beprompted by a service subscription record storage capacity issue, orperhaps an SS7 message processing performance issue. It might possibly beprompted by a need to increase the overall network reliability.Typically, when new subscribers are brought into service, the second HLR

will be populated with blocks of IMSI numbers that are allocated when newMSE equipment is ordered. As the following example shows, this grouping

of IMSI numbers within a single HLR simplifies the routing translationsthat are required within the SS7 network for VLR to HLR Location UpdateRequest transactions. Global Title Translation (GTT) tables will containsingle translation records that translate an entire range of IMSIs numbersinto an HLR address. Even if some individual records are moved betweenthe HLRs, as shown in the example, the treatment of IMSIs as blocksresults in a significant simplification of the Global Translation tables.Much more complicated SS7 message routing Global Title Translations arerequired for Routing Information Request transactions between the MSCsdistributed over the entire wireless carrier serving area and the two or moreHLRs. MSC Routing Information Requests are routed to the appropriateHLR based on the dialed MSISDN and not the IMSI. Unlike the IMSInumbers, the MSISDN numbers can not easily be arranged in groups toreside within a single HLR and therefore, the MSC must contain anMSISDN to HLR address association record for every mobile subscriberhomed on each of the MSCs. As the example illustrates, the MSC routing


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tables quickly grow much more extensive than the STP tables. The networkadministration becomes increasingly complex and prone to error.

4.7 Example: Simple Network with two MSCs and two HLRs

The example illustrates the issues relating to GSM network routing tableadministration with multiple HLRs. A simple GSM network is shown, withthe various routing tables following:

HLR DatafillHLR #1 is populated with IMSI Range 310-68-4451000 to 310-68-4451005and is populated with service subscribers from two different MSCs.

HLR #1IMSI MSISDN Other Subscriber Data

310-68-4451000

813-567-1234

~~~~~~~~~~~~

310-68-4451001

813-567-4355

~~~~~~~~~~~~

310-68-4451002

813-567-8479

~~~~~~~~~~~~

310-68-

4451003

415-457-

0238

~~~~~~~~~~~~

310-68-4451004

415-457-2332

~~~~~~~~~~~~

310-68-4451005

415-387-6325

~~~~~~~~~~~~

310-68- 415-387- ~~~~~~~~~~~~


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5568099 8884

New HLR#2 is populated with IMSI Range 310-68-5568095 to 310-68-5568100 and is populated with new service subscribers from the same twoMSCs. One subscriber has been moved from HLR #2 to HLR #1 (IMSI =

310-68-5568099).HLR #2

IMSI MSISDN Other Subscriber Data

310-68-5568095

415-457-1235

~~~~~~~~~~~~

310-68-5568096

415-387-4444

~~~~~~~~~~~~

310-68-5568097

415-457-1236

~~~~~~~~~~~~

310-68-5568098

415-457-4444

~~~~~~~~~~~~

310-68-5568100

813-567-0055

~~~~~~~~~~~~

STP DatafillThe STPs route SS7 messages to these HLRs based on the IMSI numbers

which are usually provisioned in blocks. In this case, the STPs (which haveidentical GTT tables) are provisioned to route one block of IMSIs to theeach HLR. Note that individual records can be moved between HLRs with

the addition of another record in the routing table which specifies theindividual IMSI. Individual records take precedence over IMSI blockentries.STP #1, #2IMSI HLR

310-68-4451XXX 1

310-68-5568XXX 2

310-68-5568099 1

MSC Datafill

When a GSM subscriber receives a phone call, the call attempt messagesare routed to the subscriber's MSC, based on the dialed numbers (theMSISDN). The MSC is provisioned with routing tables which relate eachMSISDN to an HLR. Note that the MSISDN numbers cannot be assigned inconvenient blocks like the IMSI numbers.MSC #1MSISDN HLR


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813-567-1234 1

813-567-4355 1

813-567-8479 1

813-567-0055 2

MSC #2MSISDN HLR

415-457-1235 2

415-457-1236 2

415-387-8884 1

5. Mobile Communications

The use of mobile radio-telephones has seen an enormous boost in the

1980s and 1990s. Previous to this time, citizen band (CB) radio had serveda limited market. However, the bandwidth assignation for CB radio wasvery limited and rapidly saturated. Even in the U.S., a total of only 40 10KHz channels were available around 27MHz. The use of digital mobiletelephones has a number of advantages over CB radio:

Access to national and international telephone system. Privacy of communication. Data independent transmission. An infinitely extendable number of channels.

Mobile communications are usually allocated bands in the 50MHz to 1GHzband. At these frequencies the effects of scattering and shadowing aresignificant. Lower frequencies would improve this performance, but HF

bandwidth is not available for this purpose. The primary problemsassociated with mobile communication at these frequencies are:

Maintaining transmission in the fading circumstances of mobilecommunication.

The extensive investigation of propagation characteristics required

prior to installation.

Mobile communication work by limiting transmitter powers. This restrictsthe range of communication to a small region. Outside this region, othertransmitters can operate independently. Each region is termed a cell.These cells are often represented in diagrams as hexagons.


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Figure: Use of cells to provide geographical coverage for mobile phone

service

Figure: Frequency re-use in cellsWithin each cell, the user communicates with a transmitter within the cell.As the mobile approaches a cell boundary, the signal strength fades, and theuser is passed on to a transmitter from the new cell. Each cell is equipped

with cell-site(s) that transmit/receive to/from the mobile within the cell.Within a single cell, a number of channels are available. These channels are(usually) separated by frequency. Then a mobile initiates a call, it isassigned an idle channel within the current cell by the mobile-servicesswitching centre (MSC). He/she uses the channel within the cell untilhe/she reaches the boundary. He/she is then allocated a new idle channel

within the next cell.For example, the American advanced mobile phone service(AMPS) makes use of a 40MHz channel in the 800 - 900MHz band. This

band is split into a 20MHz transmit and 20MHz receive bandwidth. Thesebands are split into 666 two-way channels, each having a bandwidth of 30KHz. These channels are subdivided into 21 sets of channels, arranged in 7groups of 3. The nominally hexagonal pattern contains 7 cells, a central one


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and its 6 nearest neighbours. Each cell is assigned a different group in sucha way that at least two cells lie between it and the next block using that set.

With a total of 666 channels, it is possible to assign three sets of 31channels per cell.The great strength of this type of network is the ease with which morechannels may be introduced. As demand rises, one simply reduces the cellsize. Then the same number of channels is available in a smaller area,increasing the total number of channels per unit area. In a well plannedsystem, the density of cells would reflect the user density.

AMPS is a first generation mobile phone system. It uses analoguemodulation. It is one of six incompatible first generation systems that existaround the world. Currently, second generation systems are beingintroduced. These are digital in nature. One aim of the second generationmobile systems was to try and develop one global standard, allowing use of

the same mobile phone anywhere in the world. However, there arecurrently three digital standards in use, so this seems unlikely. The pan-European standard is known as GSM (Groupe Special Mobile), and isnow available in the UK. The services planned for the GSM are similar tothose for ISDN (e.g. call forwarding, charge advice, etc. ). Full GSM willhave 200KHz physical channels offering 270Kb/s. Currently, one physicalchannel is split between 8 users, each having use of 13Kb/s (the rest is usedfor channel overhead). The aims of the GSM system were:

Good speech quality Low terminal cost Low service cost International roaming Ability to support hand-held portables A range of new services and facilities (ISDN!)

The heart of the mobile telephone network is the MSC. Its task is toacknowledge the paging of the user, assign him/her a channel, broadcasthis/her dialed request, return the call. In addition it automatically monitorsthe signal strength of both transmitter and receiver, and allocates newchannels as required. This latter process, known ashand-off, is completelyhidden to the user, although is a major technical problem. In addition, theMSC is responsible for charging the call. The decision making ability of theMSC relies to a great extent on modern digital technology. It is the maturityof this technology that has permitted the rapid growth of mobilecommunications.


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Figure: Hand-off between cells

The principle problem with mobile communication is the variation in signalstrength as the communicating parties move. This variation is due to the

varying interference of scattered radiation -- fading. Fading causes rapidvariation in signal strength. The normal solution to fading, increasing thetransmitter power, is not available in mobile communication wheretransmitter power is limited.The installation of a mobile telephone system requires a large initial effortin determining the propagation behaviour in the area covered by thenetwork. Propagation planning, by a mixture of observation and computersimulation, is necessary if the system is to work properly. At UHF and VHFfrequencies, the effects of obstructions is significant. Some of the effectsthat need to be considered are:

Free space loss. This significantly increases in urban environments.

Studies have indicated that a relationship is more often

followed than a law. Effect of street orientation. Streets have a significant waveguide

effect. Variations of up to 20dB have been measured in urbanenvironments as a result of street direction.

Effects of foliage. Propagation in rural areas is significantlyeffected by the presence of leaves. Variations of18dB between

summer and winter have been observed in forested areas. Effect of tunnels. Tunnels can introduce signal attenuation of up to

30dB according to the tunnel length and frequency.

1.Radio link aspects


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The International Telecommunication Union (ITU), which manages theinternational allocation of radio spectrum (among many other functions),allocated the bands 890-915 MHz for the uplink (mobile station to basestation) and 935-960 MHz for the downlink (base station to mobile station)for mobile networks in Europe. Since this range was already being used inthe early 1980s by the analog systems of the day, the CEPT had theforesight to reserve the top 10 MHz of each band for the GSM network that

was still being developed. Eventually, GSM will be allocated the entire 2x25MHz bandwidth.

1. Multiple access and channel structureSince radio spectrum is a limited resource shared by all users, a methodmust be devised to divide up the bandwidth among as many users as

possible. The method chosen by GSM is a combination of Time- andFrequency-Division Multiple Access (TDMA/FDMA). The FDMA partinvolves the division by frequency of the (maximum) 25 MHz bandwidthinto 124 carrier frequencies spaced 200 kHz apart. One or more carrierfrequencies are assigned to each base station. Each of these carrierfrequencies is then divided in time, using a TDMA scheme. Thefundamental unit of time in this TDMA scheme is called a burst periodandit lasts 15/26 ms (or approx. 0.577 ms). Eight burst periods are groupedinto a TDMA frame (120/26 ms, or approx. 4.615 ms), which forms the

basic unit for the definition of logical channels. One physical channel is one

burst period per TDMA frame.Channels are defined by the number and position of their corresponding

burst periods. All these definitions are cyclic, and the entire pattern repeatsapproximately every 3 hours. Channels can be divided into dedicatedchannels, which are allocated to a mobile station, and common channels,

which are used by mobile stations in idle mode.

1. Traffic channelsA traffic channel (TCH) is used to carry speech and data traffic. Traffic

channels are defined using a 26-frame multiframe, or group of 26 TDMAframes. The length of a 26-frame multiframe is 120 ms, which is how thelength of a burst period is defined (120 ms divided by 26 frames divided by8 burst periods per frame). Out of the 26 frames, 24 are used for traffic, 1 isused for the Slow Associated Control Channel (SACCH) and 1 is currentlyunused (see Figure 2). TCHs for the uplink and downlink are separated in


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time by 3 burst periods, so that the mobile station does not have to transmitand receive simultaneously, thus simplifying the electronics.In addition to thesefull-rate TCHs, there are alsohalf-rate TCHs defined,although they are not yet implemented. Half-rate TCHs will effectivelydouble the capacity of a system once half-rate speech coders are specified(i.e., speech coding at around 7 kbps, instead of 13 kbps). Eighth-rate TCHsare also specified, and are used for signalling. In the recommendations,they are called Stand-alone Dedicated Control Channels (SDCCH).

2. Control channelsCommon channels can be accessed both by idle mode and dedicated modemobiles. The common channels are used by idle mode mobiles to exchangethe signaling information required to change to dedicated mode. Mobiles

already in dedicated mode monitor the surrounding base stations forhandover and other information. The common channels are defined withina 51-frame multiframe, so that dedicated mobiles using the 26-framemultiframe TCH structure can still monitor control channels. The commonchannels include:Broadcast Control Channel (BCCH)

Continually broadcasts, on the downlink, information including basestation identity, frequency allocations, and frequency-hoppingsequences.

Frequency Correction Channel (FCCH) and Synchronization Channel

(SCH)Used to synchronize the mobile to the time slot structure of a cell bydefining the boundaries of burst periods, and the time slotnumbering. Every cell in a GSM network broadcasts exactly oneFCCH and one SCH, which are by definition on time slot number 0(within a TDMA frame).

Random Access Channel (RACH)Slotted Aloha channel used by the mobile to request access to thenetwork.

Paging Channel (PCH)Used to alert the mobile station of an incoming call.

Access Grant Channel (AGCH)Used to allocate an SDCCH to a mobile for signaling (in order to obtain adedicated channel), following a request on the RACH.

3. Burst structure


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There are four different types of bursts used for transmission in GSM. Thenormal burst is used to carry data and most signaling. It has a total lengthof 156.25 bits, made up of two 57 bit information bits, a 26 bit trainingsequence used for equalization, 1 stealing bit for each information block(used for FACCH), 3 tail bits at each end, and an 8.25 bit guard sequence.The 156.25 bits are transmitted in 0.577 ms, giving a gross bit rate of270.833 kbps.The F burst, used on the FCCH, and the S burst, used on the SCH, have thesame length as a normal burst, but a different internal structure, whichdifferentiates them from normal bursts (thus allowing synchronization).The access burst is shorter than the normal burst, and is used only on theRACH.

2. Speech codingGSM is a digital system, so speech which is inherently analog, has to bedigitized. The method employed by ISDN, and by current telephonesystems for multiplexing voice lines over high speed trunks and opticalfiber lines, is Pulse Coded Modulation (PCM). The output stream from PCMis 64 kbps, too high a rate to be feasible over a radio link. The 64 kbpssignal, although simple to implement, contains much redundancy. TheGSM group studied several speech coding algorithms on the basis ofsubjective speech quality and complexity (which is related to cost,processing delay, and power consumption once implemented) before

arriving at the choice of a Regular Pulse Excited -- Linear Predictive Coder(RPE--LPC) with a Long Term Predictor loop. Basically, information fromprevious samples, which does not change very quickly, is used to predict thecurrent sample. The coefficients of the linear combination of the previoussamples, plus an encoded form of the residual, the difference between thepredicted and actual sample, represent the signal. Speech is divided into 20millisecond samples, each of which is encoded as 260 bits, giving a total bitrate of 13 kbps. This is the so-called Full-Rate speech coding. Recently, anEnhanced Full-Rate (EFR) speech coding algorithm has been implemented

by some North American GSM1900 operators. This is said to provideimproved speech quality using the existing 13 kbps bit rate.

3. Channel coding and modulationBecause of natural and man-made electromagnetic interference, theencoded speech or data signal transmitted over the radio interface must beprotected from errors. GSM uses convolutional encoding and block


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interleaving to achieve this protection. The exact algorithms used differ forspeech and for different data rates. The method used for speech blocks will

be described below.From subjective testing, it was found that some bits of this block were moreimportant for perceived speech quality than others. The bits are thusdivided into three classes:

Class Ia 50 bits - most sensitive to bit errors Class Ib 132 bits - moderately sensitive to bit errors Class II 78 bits - least sensitive to bit errors

Class Ia bits have a 3 bit Cyclic Redundancy Code added for error detection.If an error is detected, the frame is judged too damaged to becomprehensible and it is discarded. It is replaced by a slightly attenuated

version of the previous correctly received frame. These 53 bits, togetherwith the 132 Class Ib bits and a 4 bit tail sequence (a total of 189 bits), areinput into a 1/2 rate convolutional encoder of constraint length 4. Eachinput bit is encoded as two output bits, based on a combination of theprevious 4 input bits. The convolutional encoder thus outputs 378 bits, to

which are added the 78 remaining Class II bits, which are unprotected.Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rate of22.8 kbps.To further protect against the burst errors common to the radio interface,each sample is interleaved. The 456 bits output by the convolutional

encoder are divided into 8 blocks of 57 bits, and these blocks aretransmitted in eight consecutive time-slot bursts. Since each time-slot burstcan carry two 57 bit blocks, each burst carries traffic from two differentspeech samples.Recall that each time-slot burst is transmitted at a gross bit rate of 270.833kbps. This digital signal is modulated onto the analog carrier frequencyusing Gaussian-filtered Minimum Shift Keying (GMSK). GMSK wasselected over other modulation schemes as a compromise between spectralefficiency, complexity of the transmitter, and limited spurious emissions.The complexity of the transmitter is related to power consumption, whichshould be minimized for the mobile station. The spurious radio emissions,outside of the allotted bandwidth, must be strictly controlled so as to limitadjacent channel interference, and allow for the co-existence of GSM andthe older analog systems (at least for the time being).

4. Multipath equalization


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At the 900 MHz range, radio waves bounce off everything - buildings, hills,cars, airplanes, etc. Thus many reflected signals, each with a differentphase, can reach an antenna. Equalization is used to extract the desiredsignal from the unwanted reflections. It works by finding out how a knowntransmitted signal is modified by multipath fading, and constructing aninverse filter to extract the rest of the desired signal. This known signal isthe 26-bit training sequence transmitted in the middle of every time-slot

burst. The actual implementation of the equalizer is not specified in theGSM specifications.

6.5 Discontinuous transmission

Minimizing co-channel interference is a goal in any cellular system, since itallows better service for a given cell size, or the use of smaller cells, thus

increasing the overall capacity of the system. Discontinuous transmission(DTX) is a method that takes advantage of the fact that a person speaks lessthat 40 percent of the time in normal conversation by turning thetransmitter off during silence periods. An added benefit of DTX is thatpower is conserved at the mobile unit.The most important component of DTX is, of course, Voice ActivityDetection. It must distinguish between voice and noise inputs, a task that isnot as trivial as it appears, considering background noise. If a voice signal ismisinterpreted as noise, the transmitter is turned off and a very annoyingeffect called clipping is heard at the receiving end. If, on the other hand,

noise is misinterpreted as a voice signal too often, the efficiency of DTX isdramatically decreased. Another factor to consider is that when thetransmitter is turned off, there is total silence heard at the receiving end,due to the digital nature of GSM. To assure the receiver that the connectionis not dead,comfort noise is created at the receiving end by trying to matchthe characteristics of the transmitting end's background noise.

1. Discontinuous receptionAnother method used to conserve power at the mobile station is

discontinuous reception. The paging channel, used by the base station tosignal an incoming call, is structured into sub-channels. Each mobilestation needs to listen only to its own sub-channel. In the time betweensuccessive paging sub-channels, the mobile can go into sleep mode, whenalmost no power is used.

2. Power control


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There are five classes of mobile stations defined, according to their peaktransmitter power, rated at 20, 8, 5, 2, and 0.8 watts. To minimize co-channel interference and to conserve power, both the mobiles and the BaseTransceiver Stations operate at the lowest power level that will maintain anacceptable signal quality. Power levels can be stepped up or down in stepsof 2 dB from the peak power for the class down to a minimum of 13 dBm(20 milliwatts).

2.Network aspectsEnsuring the transmission of voice or data of a given quality over the radiolink is only part of the function of a cellular mobile network. A GSM mobilecan seamlessly roam nationally and internationally, which requires thatregistration, authentication, call routing and location updating functions

exist and are standardized in GSM networks. In addition, the fact that thegeographical area covered by the network is divided into cells necessitatesthe implementation of a handover mechanism. These functions areperformed by the Network Subsystem, mainly using the Mobile ApplicationPart (MAP) built on top of the Signalling System No. 7 protocol.

Figure 3. Signaling protocol structure in GSM

The signaling protocol in GSM is structured into three general layers

depending on the interface, as shown in Figure 3. Layer 1 is the physicallayer, which uses the channel structures discussed above over the airinterface. Layer 2 is the data link layer. Across the Um interface, the datalink layer is a modified version of the LAPD protocol used in ISDN, calledLAPDm. Across the A interface, the Message Transfer Part layer 2 ofSignaling System Number 7 is used. Layer 3 of the GSM signaling protocolis itself divided into 3 sub layers.


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Radio Resources ManagementControls the setup, maintenance, and termination of radio and fixedchannels, including handovers.

Mobility ManagementManages the location updating and registration procedures, as well assecurity and authentication.

Connection ManagementHandles general call control, similar to CCITT RecommendationQ.931, and manages Supplementary Services and the Short MessageService.

Signaling between the different entities in the fixed part of the network,such as between the HLR and VLR, is accomplished through the Mobile

Application Part (MAP). MAP is built on top of the Transaction CapabilitiesApplication Part (TCAP, the top layer of Signaling System Number 7. The

specification of the MAP is quite complex, and at over 500 pages, it is oneof the longest documents in the GSM recommendations .

1. Radio resources managementThe radio resources management (RR) layer oversees the establishment ofa link, both radio and fixed, between the mobile station and the MSC. Themain functional components involved are the mobile station, and the BaseStation Subsystem, as well as the MSC. The RR layer is concerned with themanagement of an RR-session which is the time that a mobile is in

dedicated mode, as well as the configuration of radio channels including theallocation of dedicated channels.

An RR-session is always initiated by a mobile station through the accessprocedure, either for an outgoing call, or in response to a paging message.The details of the access and paging procedures, such as when a dedicatedchannel is actually assigned to the mobile, and the paging sub-channelstructure, are handled in the RR layer. In addition, it handles themanagement of radio features such as power control, discontinuoustransmission and reception, and timing advance.

1. HandoverIn a cellular network, the radio and fixed links required are notpermanently allocated for the duration of a call. Handover, or handoff as itis called in North America, is the switching of an on-going call to a differentchannel or cell. The execution and measurements required for handoverform one of basic functions of the RR layer.


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There are four different types of handover in the GSM system, whichinvolve transferring a call between:

Channels (time slots) in the same cell Cells (Base Transceiver Stations) under the control of the same Base

Station Controller (BSC), Cells under the control of different BSCs, but belonging to the same

Mobile services Switching Center (MSC), and Cells under the control of different MSCs.

The first two types of handover, called internal handovers, involve only oneBase Station Controller (BSC). To save signaling bandwidth, they aremanaged by the BSC without involving the Mobile services SwitchingCenter (MSC), except to notify it at the completion of the handover. The last

two types of handover, called external handovers, are handled by the MSCsinvolved. An important aspect of GSM is that the original MSC, the anchorMSC, remains responsible for most call-related functions, with theexception of subsequent inter-BSC handovers under the control of the newMSC, called the relay MSC.Handovers can be initiated by either the mobile or the MSC (as a means oftraffic load balancing). During its idle time slots, the mobile scans theBroadcast Control Channel of up to 16 neighboring cells, and forms a list ofthe six best candidates for possible handover, based on the received signalstrength. This information is passed to the BSC and MSC, at least once per

second, and is used by the handover algorithm.The algorithm for when a handover decision should be taken is notspecified in the GSM recommendations. There are two basic algorithmsused, both closely tied in with power control. This is because the BSCusually does not know whether the poor signal quality is due to multipathfading or to the mobile having moved to another cell. This is especially truein small urban cells.The 'minimum acceptable performance' algorithm gives precedence topower control over handover, so that when the signal degrades beyond acertain point, the power level of the mobile is increased. If further powerincreases do not improve the signal, then a handover is considered. This isthe simpler and more common method, but it creates 'smeared' cell

boundaries when a mobile transmitting at peak power goes some distancebeyond its original cell boundaries into another cell.The 'power budget' method uses handover to try to maintain or improve acertain level of signal quality at the same or lower power level. It thus gives


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precedence to handover over power control. It avoids the 'smeared' cellboundary problem and reduces co-channel interference, but it is quitecomplicated.

2. Authentication and securitySince the radio medium can be accessed by anyone, authentication of usersto prove that they are who they claim to be is a very important element of amobile network. Authentication involves two functional entities, the SIMcard in the mobile, and the Authentication Center (AuC). Each subscriber isgiven a secret key, one copy of which is stored in the SIM card and the otherin the AuC. During authentication, the AuC generates a random numberthat it sends to the mobile. Both the mobile and the AuC then use therandom number, in conjunction with the subscriber's secret key and a

ciphering algorithm called A3, to generate a signed response (SRES) that issent back to the AuC. If the number sent by the mobile is the same as theone calculated by the AuC, the subscriber is authenticated.The same initial random number and subscriber key are also used tocompute the ciphering key using an algorithm called A8. This cipheringkey, together with the TDMA frame number, use the A5 algorithm to createa 114 bit sequence that is XORed with the 114 bits of a burst (the two 57 bit

blocks). Enciphering is an option for the fairly paranoid, since the signal isalready coded, interleaved, and transmitted in a TDMA manner, thusproviding protection from all but the most persistent and dedicated

eavesdroppers.Another level of security is performed on the mobile equipment itself, asopposed to the mobile subscriber. As mentioned earlier, each GSM terminalis identified by a unique International Mobile Equipment Identity (IMEI)number. A list of IMEIs in the network is stored in the Equipment IdentityRegister (EIR). The status returned in response to an IMEI query to theEIR is one of the following:

White-listedThe terminal is allowed to connect to the network.

Grey-listedThe terminal is under observation from the network for possibleproblems.

Black-listedThe terminal has either been reported stolen, or is not type approved(the correct type of terminal for a GSM network). The terminal is notallowed to connect to the network.


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3. Call routingUnlike routing in the fixed network, where a terminal is semi-permanently

wired to a central office, a GSM user can roam nationally and eveninternationally. The directory number dialed to reach a mobile subscriber iscalled the Mobile Subscriber ISDN (MSISDN), which is defined by theE.164 numbering plan.The MSISDN is the dialable number that callers use to reach a mobilesubscriber. Some phones can support multiple MSISDNs - for example, aU.S.-based MSISDN and a Canadian-based MSISDN. Callers dialing eithernumber will reach the subscriber. This number includes a country codeand a National Destination Code which identifies the subscriber's operator.The first few digits of the remaining subscriber number may identify thesubscriber's HLR within the home PLMN.

An incoming mobile terminating call is directed to the Gateway MSC(GMSC) function. The GMSC is basically a switch which is able tointerrogate the subscriber's HLR to obtain routing information, and thuscontains a table linking MSISDNs to their corresponding HLR. Asimplification is to have a GSMC handle one specific PLMN. It should benoted that the GMSC function is distinct from the MSC function.The routing information that is returned to the GMSC is the Mobile StationRoaming Number (MSRN), which is also defined by the E.164 numberingplan. MSRNs are related to the geographical numbering plan, and notassigned to subscribers, nor are they visible to subscribers.

The most general routing procedure begins with the GMSC querying thecalled subscriber's HLR for an MSRN. The HLR typically stores only theSS7 address of the subscriber's current VLR, and does not have the MSRN(see the location updating section). The HLR must therefore query thesubscriber's current VLR, which will temporarily allocate an MSRN from itspool for the call. This MSRN is returned to the HLR and back to the GMSC,

which can then route the call to the new MSC. At the new MSC, the IMSIcorresponding to the MSRN is looked up, and the mobile is paged in itscurrent location area (see Figure 4).


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The GSM system operates on a number of frequencies around 900 MHz(CDMA operates from 824-894MHz). The pie chart below shows a typical

example of the relationship of the GSM system with other broadcastersusing radio frequency transmission. Television and FM radio usefrequencies of about 100MHz and AM radio uses frequencies near 1MHz.The pie chart gives the relative amount of RFR emitted by various sourcesmeasured in Burwood a middle class suburb East of Melbourne and about25km from the television transmission antennas and 0.1km from thenearest base station. Measurements of power density levels (inmicro wattsper square centimeter- white text) are made at a position which maximizesthe exposure from the mobile phone base station. It can be seen thatexposure levels are less than those from FM radio stations (100 MHz) andsignificantly less than levels from AM radio stations (1 MHz).
http://www.google.com/url?q=http%3A%2F%2Fwww.arpansa.gov.au%2Fmph_sys.htm%23emf&sa=D&sntz=1&usg=AFQjCNGIE6CwZU5myYDH5tSGQRZLtBqnyghttp://www.google.com/url?q=http%3A%2F%2Fwww.arpansa.gov.au%2Fmph_sys.htm%23emf&sa=D&sntz=1&usg=AFQjCNGIE6CwZU5myYDH5tSGQRZLtBqnyghttp://www.google.com/url?q=http%3A%2F%2Fwww.arpansa.gov.au%2Fmph_sys.htm%23emf&sa=D&sntz=1&usg=AFQjCNGIE6CwZU5myYDH5tSGQRZLtBqnyghttp://www.google.com/url?q=http%3A%2F%2Fwww.arpansa.gov.au%2Fmph_sys.htm%23emf&sa=D&sntz=1&usg=AFQjCNGIE6CwZU5myYDH5tSGQRZLtBqnyghttp://www.google.com/url?q=http%3A%2F%2Fwww.arpansa.gov.au%2Fmph_sys.htm%23emf&sa=D&sntz=1&usg=AFQjCNGIE6CwZU5myYDH5tSGQRZLtBqnyghttp://www.google.com/url?q=http%3A%2F%2Fwww.arpansa.gov.au%2Fmph_sys.htm%23emf&sa=D&sntz=1&usg=AFQjCNGIE6CwZU5myYDH5tSGQRZLtBqnyg


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These levels are well below the former Australian Standard requirement of0.2mW/cm2. The average exposure from a base station antenna is similarto the exposure (albeit visible rather than RF radiation) from a 2 Watt torch

bulb where the light is used to illuminate an area of approximately 7 acres.

4.Conclusion and CommentsIn this paper we have tried to give an overview of the GSM system. It is astandard that ensures interoperability without stifling competition and

innovation among suppliers, to the benefit of the public both in terms ofcost and service quality. For example, by using Very Large Scale Integration(VLSI) microprocessor technology, many functions of the mobile stationcan be built on one chipset, resulting in lighter, more compact and moreenergy-efficient terminals.Telecommunications are evolving towards personal communicationnetworks, whose objective can be stated as the availability of allcommunication services anytime, anywhere, to anyone, by a single identitynumber and a pocketable communication terminal. Having a multitude ofincompatible systems throughout the world moves us farther away fromthis ideal. The economies of scale created by a unified system are enough to

justify its implementation, not to mention the convenience to people ofcarrying just one communication terminal anywhere they go, regardless ofnational boundaries.The GSM system, and its sibling systems operating at 1.8 GHz (calledDCS1800) and 1.9 GHz (called GSM1900 or PCS1900, and operating in


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North America), are a first approach at a true personal communicationsystem. The SIM card is a novel approach that implements personalmobility in addition to terminal mobility. Together with internationalroaming, and support for a variety of services such as telephony, datatransfer, fax, Short Message Service, and supplementary services, GSMcomes close to fulfilling the requirements for a personal communicationsystem: close enough that it is being used as a basis for the next generationof mobile communication technology in Europe, the Universal MobileTelecommunication System (UMTS).

Another point where GSM has shown its commitment to openness,standards and interoperability is the compatibility with the IntegratedServices Digital Network (ISDN) that is evolving in most industrializedcountries and Europe in particular (the so-called Euro-ISDN). GSM is alsothe first system to make extensive use of the Intelligent Networking

concept, in which services like 800 numbers are concentrated and handledfrom a few centralized service centers, instead of being distributed overevery switch in the country. This is the concept behind the use of the

various registers such as the HLR. In addition, the signaling between thesefunctional entities uses Signaling System Number 7, an internationalstandard already deployed in many countries and specified as the backbonesignaling network for ISDN.]

10. Bibliography

[1] Jan A. Audestad. Network aspects of the GSM system[2] D. M. Balston. The pan-European system: GSM. In D. M. Balston andR.C.V. Macario, editors.[3] David M. Balston. The pan-European cellular technology. In R.C.V.Macario, editor,Personal and Mobile Radio Systems.[4] David Cheeseman. The pan-European cellular mobile radio system. InR.C.V. Macario, editor,Personal and Mobile Radio Systems.[5] C. Dchaux and R. Scheller. What are GSM and DCS.

[6] M. Feldmann and J. P. Rissen. GSM network systems and overallsystem integration.[7] John M. Griffiths.ISDN Explained: Worldwide Network and

Applications Technology.[8] I. Harris. Data in the GSM cellular network.1

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