Javier Gozálvez Semperehome.scarlet.be/~pc030062/extra_info/GSM.pdf · 5.3.3 Interleaving 5.3.3.1...

An overview of the GSMsystem

Javier Gozálvez Sempere

PhD Student in Mobile Communications

Communications Division

Department of Electronic&Electrical Engineering

University of Strathclyde

Glasgow, Scotland

[email protected]

Table of Contents1 History of the cellular mobile radio and GSM2 Cellular systems

2.1 The cellular structure2.2 Cluster2.3 Types of cells

2.3.1 Macrocells2.3.2 Microcells2.3.3 Selective cells2.3.4 Umbrella cells

3 The transition from analog to digital technology

3.1 The capacity of the system

An overview of the GSM system by Javier Gozalvez Sempere

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3.2 Compatibility with other systems such as ISDN3.3 Aspects of quality

4 The GSM network

4.1 Architecture of the GSM network

4.1.1 Mobile Station

4.1.1.1 The Terminal4.1.1.2 The SIM

4.1.2 The Base Station Subsystem

4.1.2.1 The Base Transceiver Station4.1.2.2 The Base Station Controller

4.1.3 The Network and Switching Subsystem

4.1.3.1 The Mobile services Switching Center (MSC)4.1.3.2 The Gateway Mobile services Switching Center4.1.3.3 Home Location Register (HLR)4.1.3.4 Visitor Location Register (VLR)4.1.3.5 The Authentication Center (AuC)4.1.3.6 The Equipment Identity Register (EIR)4.1.3.7 The GSM Interworking Unit (GIWU)

4.1.4 The Operation and Support Subsystem (OSS)

4.2 The geographical areas of the GSM network4.3 The GSM functions

4.3.1 Transmission4.3.2 Radio Resources management (RR)

4.3.2.1 Handover

4.3.3 Mobility Management

4.3.3.1 Location management4.3.3.2 Authentication and security

4.3.4 Communication Management (CM)

4.3.4.1 Call Control (CC)4.3.4.2 Supplementary Services management4.3.4.3 Short Message Services management

4.3.5 Operation, Administration and Maintenance (OAM)

5 The GSM radio interface

5.1 Frequency allocation5.2 Multiple access scheme



5.2.1 FDMA and TDMA5.2.2 Channel structure

5.2.2.1 Traffic channels (TCH)5.2.2.2 Control channels

5.2.2.2.1 Broadcast channels5.2.2.2.2 Common Control Channels5.2.2.2.3 Dedicated Control Channels5.2.2.2.4 Associated Control Channels

5.2.3 Burst structure5.2.4 Frequency hopping

5.3 From source information to radio waves

5.3.1 Speech coding5.3.2 Channel coding

5.3.2.1 Channel coding for the GSM data TCH channels5.3.2.2 Channel coding for the GSM speech channels5.3.2.3 Channel coding for the GSM control channels

5.3.3 Interleaving

5.3.3.1 Interleaving for the GSM control channels5.3.3.2 Interleaving for the GSM speech channels5.3.3.3 Interleaving for the GSM data TCH channels

5.3.4 Burst assembling5.3.5 Ciphering5.3.6 Modulation

5.4 Discontinuous transmission (DTX)5.5 Timing advance5.6 Power control5.7 Discontinuous reception5.8 Multipath and equalisation

6 GSM services

6.1 Teleservices6.2 Bearer services6.3 Supplementary Services

7 Conclusion

Bibliography

Acronyms



Other GSM sites

The Global System for Mobile communications is a digital cellular communications system. It wasdeveloped in order to create a common European mobile telephone standard but it has been rapidlyaccepted worldwide. GSM was designed to be compatible with ISDN services.

1 History of the cellular mobile radio and GSMThe idea of cell-based mobile radio systems appeared at Bell Laboratories (in USA) in the early 1970s.However, mobile cellular systems were not introduced for commercial use until the 1980s. During theearly 1980s, analog cellular telephone systems experienced a very rapid growth in Europe, particularly inScandinavia and the United Kingdom. Today cellular systems still represent one of the fastest growingtelecommunications systems.

But in the beginnings of cellular systems, each country developed its own system, which was anundesirable situation for the following reasons:

The equipment was limited to operate only within the boundaries of each country.●

The market for each mobile equipment was limited.●

In order to overcome these problems, the Conference of European Posts and Telecommunications(CEPT) formed, in 1982, the Groupe Spécial Mobile (GSM) in order to develop a pan-European mobilecellular radio system (the GSM acronym became later the acronym for Global System for Mobilecommunications). The standardized system had to meet certain criterias:

Spectrum efficiency●

International roaming●

Low mobile and base stations costs●

Good subjective voice quality●

Compatibility with other systems such as ISDN (Integrated Services Digital Network)●

Ability to support new services●

Unlike the existing cellular systems, which were developed using an analog technology, the GSM systemwas developed using a digital technology. The reasons for this choice are explained in section 3.

In 1989 the responsability for the GSM specifications passed from the CEPT to the EuropeanTelecommunications Standards Institute (ETSI). The aim of the GSM specifications is to describe thefunctionality and the interface for each component of the system, and to provide guidance on the designof the system. These specifications will then standardize the system in order to guarantee the properinterworking between the different elements of the GSM system. In 1990, the phase I of the GSMspecifications were published but the commercial use of GSM did not start until mid-1991.

The most important events in the development of the GSM system are presented in the table 1.



Year Events

1982CEPT establishes a GSM group in order to develop the standards for a pan-Europeancellular mobile system

1985 Adoption of a list of recommendations to be generated by the group

1986Field tests were performed in order to test the different radio techniques proposed for the airinterface

1987TDMA is chosen as access method (in fact, it will be used with FDMA) InitialMemorandum of Understanding (MoU) signed by telecommunication operators(representing 12 countries)

1988 Validation of the GSM system

1989 The responsability of the GSM specifications is passed to the ETSI

1990 Appearance of the phase 1 of the GSM specifications

1991 Commercial launch of the GSM service

1992 Enlargement of the countries that signed the GSM- MoU> Coverage of larger cities/airports

1993 Coverage of main roads GSM services start outside Europe

1995 Phase 2 of the GSM specifications Coverage of rural areas

Table 1: Events in the development of GSM

From the evolution of GSM, it is clear that GSM is not anymore only a European standard. GSMnetworks are operationnal or planned in over 80 countries around the world. The rapid and increasingacceptance of the GSM system is illustrated with the following figures:

1.3 million GSM subscribers worldwide in the beginning of 1994.●

Over 5 million GSM subscribers worldwide in the beginning of 1995.●

Over 10 million GSM subscribers only in Europe by December 1995.●

Since the appearance of GSM, other digital mobile systems have been developed. The table 2 charts thedifferent mobile cellular systems developed since the commercial launch of cellular systems.

Year Mobile Cellular System

1981 Nordic Mobile Telephony (NMT), 450>

1983 American Mobile Phone System (AMPS)

1985 Total Access Communication System (TACS) Radiocom 2000 C-Netz

1986 Nordic Mobile Telephony (NMT), 900>

1991 Global System for Mobile communications> North American Digital Cellular (NADC)

1992 Digital Cellular System (DCS) 1800

1994 Personal Digital Cellular (PDC) or Japanese Digital Cellular (JDC)



1995 Personal Communications Systems (PCS) 1900- Canada>

1996 PCS-United States of America>

Table 2: Mobile cellular systems

2 Cellular systems

2.1 The cellular structure

In a cellular system, the covering area of an operator is divided into cells. A cell corresponds to thecovering area of one transmitter or a small collection of transmitters. The size of a cell is determined bythe transmitter's power.

The concept of cellular systems is the use of low power transmitters in order to enable the efficient reuseof the frequencies. In fact, if the transmitters used are very powerful, the frequencies can not be reusedfor hundred of kilometers as they are limited to the covering area of the transmitter.

The frequency band allocated to a cellular mobile radio system is distributed over a group of cells andthis distribution is repeated in all the covering area of an operator. The whole number of radio channelsavailable can then be used in each group of cells that form the covering area of an operator. Frequenciesused in a cell will be reused several cells away. The distance between the cells using the same frequencymust be sufficient to avoid interference. The frequency reuse will increase considerably the capacity innumber of users.

In order to work properly, a cellular system must verify the following two main conditions:

The power level of a transmitter within a single cell must be limited in order to reduce theinterference with the transmitters of neighboring cells. The interference will not produce anydamage to the system if a distance of about 2.5 to 3 times the diameter of a cell is reservedbetween transmitters. The receiver filters must also be very performant.

●

Neighboring cells can not share the same channels. In order to reduce the interference, thefrequencies must be reused only within a certain pattern.

●

In order to exchange the information needed to maintain the communication links within the cellularnetwork, several radio channels are reserved for the signaling information.

2.2 Cluster

The cells are grouped into clusters. The number of cells in a cluster must be determined so that thecluster can be repeated continuously within the covering area of an operator. The typical clusters contain4, 7, 12 or 21 cells. The number of cells in each cluster is very important. The smaller the number of cells



per cluster is, the bigger the number of channels per cell will be. The capacity of each cell will betherefore increased. However a balance must be found in order to avoid the interference that could occurbetween neighboring clusters. This interference is produced by the small size of the clusters (the size ofthe cluster is defined by the number of cells per cluster). The total number of channels per cell dependson the number of available channels and the type of cluster used.

2.3 Types of cells

The density of population in a country is so varied that different types of cells are used:

Macrocells●

Microcells●

Selective cells●

Umbrella cells●

2.3.1 Macrocells

The macrocells are large cells for remote and sparsely populated areas.

2.3.2 Microcells

These cells are used for densely populated areas. By splitting the existing areas into smaller cells, thenumber of channels available is increased as well as the capacity of the cells. The power level of thetransmitters used in these cells is then decreased, reducing the possibility of interference betweenneighboring cells.

2.3.3 Selective cells

It is not always useful to define a cell with a full coverage of 360 degrees. In some cases, cells with aparticular shape and coverage are needed. These cells are called selective cells. A typical example ofselective cells are the cells that may be located at the entrances of tunnels where a coverage of 360degrees is not needed. In this case, a selective cell with a coverage of 120 degrees is used.

2.3.4 Umbrella cells

A freeway crossing very small cells produces an important number of handovers among the differentsmall neighboring cells. In order to solve this problem, the concept of umbrella cells is introduced. Anumbrella cell covers several microcells. The power level inside an umbrella cell is increased comparingto the power levels used in the microcells that form the umbrella cell. When the speed of the mobile istoo high, the mobile is handed off to the umbrella cell. The mobile will then stay longer in the same cell(in this case the umbrella cell). This will reduce the number of handovers and the work of the network.

A too important number of handover demands and the propagation characteristics of a mobile can help todetect its high speed.



3 The transition from analog to digital technologyIn the 1980s most mobile cellular systems were based on analog systems. The GSM system can beconsidered as the first digital cellular system. The different reasons that explain this transition fromanalog to digital technology are presented in this section.

3.1 The capacity of the system

As it is explained in section 1, cellular systems have experienced a very important growth. Analogsystems were not able to cope with this increasing demand. In order to overcome this problem, newfrequency bands and new technologies were proposed. But the possibility of using new frequency bandswas rejected by a big number of countries because of the restricted spectrum (even if later on, otherfrequency bands have been allocated for the development of mobile cellular radio). The new analogtechnologies proposed were able to overcome the problem to a certain degree but the costs were tooimportant.

The digital radio was, therefore, the best option (but not the perfect one) to handle the capacity needs in acost-efficiency way.

3.2 Compatibility with other systems such as ISDN

The decision of adopting a digital technology for GSM was made in the course of developing thestandard. During the development of GSM, the telecommunications industry converted to digitalmethods. The ISDN network is an example of this evolution. In order to make GSM compatible with theservices offered by ISDN, it was decide that the digital technology was the best option.

Additionally, a digital system allows, easily than an analog one, the implementation of futureimprovements and the change of its own characteristics.

3.3 Aspects of quality

The quality of the service can be considerably improved using a digital technology rather than an analogone. In fact, analog systems pass the physical disturbances in radio transmission (such as fades, multipathreception, spurious signals or interferences) to the receiver. These disturbances decrease the quality ofthe communication because they produce effects such as fadeouts, crosstalks, hisses, etc. On the otherhand, digital systems avoid these effects transforming the signal into bits. This transformation combinedwith other techniques, such as digital coding, improve the quality of the transmission. The improvementof digital systems comparing to analog systems is more noticeable under difficult reception conditionsthan under good reception conditions.



4 The GSM network

4.1 Architecture of the GSM network

The GSM technical specifications define the different entities that form the GSM network by definingtheir functions and interface requirements.

The GSM network can be divided into four main parts:

The Mobile Station (MS).●

The Base Station Subsystem (BSS).●

The Network and Switching Subsystem (NSS).●

The Operation and Support Subsystem (OSS).●

The architecture of the GSM network is presented in figure 1.

figure 1: Architecture of the GSM network

4.1.1 Mobile Station

A Mobile Station consists of two main elements:

The mobile equipment or terminal.●

The Subscriber Identity Module (SIM).●



4.1.1.1 The Terminal

There are different types of terminals distinguished principally by their power and application:

The `fixed' terminals are the ones installed in cars. Their maximum allowed output power is 20 W.●

The GSM portable terminals can also be installed in vehicles. Their maximum allowed outputpower is 8W.

●

The handhels terminals have experienced the biggest success thanks to thei weight and volume,which are continuously decreasing. These terminals can emit up to 2 W. The evolution oftechnologies allows to decrease the maximum allowed power to 0.8 W.

●

4.1.1.2 The SIM

The SIM is a smart card that identifies the terminal. By inserting the SIM card into the terminal, the usercan have access to all the subscribed services. Without the SIM card, the terminal is not operational.

The SIM card is protected by a four-digit Personal Identification Number (PIN). In order to identify thesubscriber to the system, the SIM card contains some parameters of the user such as its InternationalMobile Subscriber Identity (IMSI).

Another advantage of the SIM card is the mobility of the users. In fact, the only element that personalizesa terminal is the SIM card. Therefore, the user can have access to its subscribed services in any terminalusing its SIM card.

4.1.2 The Base Station Subsystem

The BSS connects the Mobile Station and the NSS. It is in charge of the transmission and reception. TheBSS can be divided into two parts:

The Base Transceiver Station (BTS) or Base Station.●

The Base Station Controller (BSC).●

4.1.2.1 The Base Transceiver Station

The BTS corresponds to the transceivers and antennas used in each cell of the network. A BTS is usuallyplaced in the center of a cell. Its transmitting power defines the size of a cell. Each BTS has between oneand sixteen transceivers depending on the density of users in the cell.

4.1.2.2 The Base Station Controller

The BSC controls a group of BTS and manages their radio ressources. A BSC is principally in charge ofhandovers, frequency hopping, exchange functions and control of the radio frequency power levels of theBTSs.

4.1.3 The Network and Switching Subsystem

Its main role is to manage the communications between the mobile users and other users, such as mobile



users, ISDN users, fixed telephony users, etc. It also includes data bases needed in order to storeinformation about the subscribers and to manage their mobility. The different components of the NSS aredescribed below.

4.1.3.1 The Mobile services Switching Center (MSC)

It is the central component of the NSS. The MSC performs the switching functions of the network. It alsoprovides connection to other networks.

4.1.3.2 The Gateway Mobile services Switching Center (GMSC)

A gateway is a node interconnecting two networks. The GMSC is the interface between the mobilecellular network and the PSTN. It is in charge of routing calls from the fixed network towards a GSMuser. The GMSC is often implemented in the same machines as the MSC.

4.1.3.3 Home Location Register (HLR)

The HLR is considered as a very important database that stores information of the suscribers belongingto the covering area of a MSC. It also stores the current location of these subscribers and the services towhich they have access. The location of the subscriber corresponds to the SS7 address of the VisitorLocation Register (VLR) associated to the terminal.

4.1.3.4 Visitor Location Register (VLR)

The VLR contains information from a subscriber's HLR necessary in order to provide the subscribedservices to visiting users. When a subscriber enters the covering area of a new MSC, the VLR associatedto this MSC will request information about the new subscriber to its corresponding HLR. The VLR willthen have enough information in order to assure the subscribed services without needing to ask the HLReach time a communication is established.

The VLR is always implemented together with a MSC; so the area under control of the MSC is also thearea under control of the VLR.

4.1.3.5 The Authentication Center (AuC)

The AuC register is used for security purposes. It provides the parameters needed for authentication andencryption functions. These parameters help to verify the user's identity.

4.1.3.6 The Equipment Identity Register (EIR)

The EIR is also used for security purposes. It is a register containing information about the mobileequipments. More particularly, it contains a list of all valid terminals. A terminal is identified by itsInternational Mobile Equipment Identity (IMEI). The EIR allows then to forbid calls from stolen orunauthorized terminals (e.g, a terminal which does not respect the specifications concerning the output



RF power).

4.1.3.7 The GSM Interworking Unit (GIWU)

The GIWU corresponds to an interface to various networks for data communications. During thesecommunications, the transmission of speech and data can be alternated.

4.1.4 The Operation and Support Subsystem (OSS)

The OSS is connected to the different components of the NSS and to the BSC, in order to control andmonitor the GSM system. It is also in charge of controlling the traffic load of the BSS.

However, the increasing number of base stations, due to the development of cellular radio networks, hasprovoked that some of the maintenance tasks are transfered to the BTS. This transfer decreasesconsiderably the costs of the maintenance of the system.

4.2 The geographical areas of the GSM network

The figure 2 presents the different areas that form a GSM network.

figure 2: GSM network areas

As it has already been explained a cell, identified by its Cell Global Identity number (CGI), correspondsto the radio coverage of a base transceiver station. A Location Area (LA), identified by its Location AreaIdentity (LAI) number, is a group of cells served by a single MSC/VLR. A group of location areas underthe control of the same MSC/VLR defines the MSC/VLR area. A Public Land Mobile Network (PLMN)is the area served by one network operator.



4.3 The GSM functions

In this paragraph, the description of the GSM network is focused on the differents functions to fulfil bythe network and not on its physical components. In GSM, five main functions can be defined:

Transmission.●

Radio Resources management (RR).●

Mobility Management (MM).●

Communication Management (CM).●

Operation, Administration and Maintenance (OAM).●

4.3.1 Transmission

The transmission function includes two sub-functions:

The first one is related to the means needed for the transmission of user information.●

The second one is related to the means needed for the trasnmission of signaling information.●

Not all the components of the GSM network are strongly related with the transmission functions. TheMS, the BTS and the BSC, among others, are deeply concerned with transmission. But othercomponents, such as the registers HLR, VLR or EIR, are only concerned with the transmission for theirsignaling needs with other components of the GSM network. Some of the most important aspects of thetransmission are described in section 5.

4.3.2 Radio Resources management (RR)

The role of the RR function is to establish, maintain and release communication links between mobilestations and the MSC. The elements that are mainly concerned with the RR function are the mobilestation and the base station. However, as the RR function is also in charge of maintaining a connectioneven if the user moves from one cell to another, the MSC, in charge of handovers, is also concerned withthe RR functions.

The RR is also responsible for the management of the frequency spectrum and the reaction of thenetwork to changing radio environment conditions. Some of the main RR procedures that assure itsresponsabilities are:

Channel assignment, change and release.●

Handover.●

Frequency hopping.●

Power-level control.●

Discontinuous transmission and reception.●

Timing advance.●

Some of these procedures are described in section 5. In this paragraph only the handover, whichrepresents one of the most important responsabilities of the RR, is described.



4.3.2.1 Handover

The user movements can produce the need to change the channel or cell, specially when the quality ofthe communication is decreasing. This procedure of changing the resources is called handover. Fourdifferent types of handovers can be distinguished:

Handover of channels in the same cell.●

Handover of cells controlled by the same BSC.●

Handover of cells belonging to the same MSC but controlled by different BSCs.●

Handover of cells controlled by different MSCs.●

Handovers are mainly controlled by the MSC. However in order to avoid unnecessary signallinginformation, the first two types of handovers are managed by the concerned BSC (in this case, the MSCis only notified of the handover).

The mobile station is the active participant in this procedure. In order to perform the handover, themobile station controls continuously its own signal strengh and the signal strengh of the neighboringcells. The list of cells that must be monitored by the mobile station is given by the base station. Thepower measurements allow to decide which is the best cell in order to maintain the quality of thecommunication link. Two basic algorithms are used for the handover:

The `minimum acceptable performance' algorithm. When the quality of the transmission decreases(i.e the signal is deteriorated), the power level of the mbbile is increased. This is done until theincrease of the power level has no effect on the quality of the signal. When this happens, ahandover is performed.

●

The `power budget' algorithm. This algorithm performs a handover, instead of continuouslyincreasing the power level, in order to obtain a good communication quality.

●

4.3.3 Mobility Management

The MM function is in charge of all the aspects related with the mobility of the user, specially thelocation management and the authentication and security.

4.3.3.1 Location management

When a mobile station is powered on, it performs a location update procedure by indicating its IMSI tothe network. The first location update procedure is called the IMSI attach procedure.

The mobile station also performs location updating, in order to indicate its current location, when itmoves to a new Location Area or a different PLMN. This location updating message is sent to the newMSC/VLR, which gives the location information to the subscriber's HLR. If the mobile station isauthorized in the new MSC/VLR, the subscriber's HLR cancells the registration of the mobile stationwith the old MSC/VLR.

A location updating is also performed periodically. If after the updating time period, the mobile stationhas not registered, it is then deregistered.

When a mobile station is powered off, it performs an IMSI detach procedure in order to tell the networkthat it is no longer connected.



4.3.3.2 Authentication and security

The authentication procedure involves the SIM card and the Authentication Center. A secret key, storedin the SIM card and the AuC, and a ciphering algorithm called A3 are used in order to verify theauthenticity of the user. The mobile station and the AuC compute a SRES using the secret key, thealgorithm A3 and a random number generated by the AuC. If the two computed SRES are the same, thesubscriber is authenticated. The different services to which the subscriber has access are also checked.

Another security procedure is to check the equipment identity. If the IMEI number of the mobile isauthorized in the EIR, the mobile station is allowed to connect the network.

In order to assure user confidentiality, the user is registered with a Temporary Mobile Subscriber Identity(TMSI) after its first location update procedure.

Enciphering is another option to guarantee a very strong security but this procedure is going to bedescribed in section 5.

4.3.4 Communication Management (CM)

The CM function is responsible for:

Call control.●

Supplementary Services management.●

Short Message Services management.●

4.3.4.1 Call Control (CC)

The CC is responsible for call establishing, maintaining and releasing as well as for selecting the type ofservice. One of the most important functions of the CC is the call routing. In order to reach a mobilesubscriber, a user diales the Mobile Subscriber ISDN (MSISDN) number which includes:

a country code●

a national destination code identifying the subscriber's operator●

a code corresponding to the subscriber's HLR●

The call is then passsed to the GMSC (if the call is originated from a fixed network) which knows theHLR corresponding to a certain MISDN number. The GMSC asks the HLR for information helping tothe call routing. The HLR requests this information from the subscriber's current VLR. This VLRallocates temporarily a Mobile Station Roaming Number (MSRN) for the call. The MSRN number is theinformation returned by the HLR to the GMSC. Thanks to the MSRN number, the call is routed tosubscriber's current MSC/VLR. In the subscriber's current LA, the mobile is paged.

4.3.4.2 Supplementary Services management

The mobile station and the HLR are the only components of the GSM network involved with thisfunction. The different Supplementary Services (SS) to which the users have access are presented in



section 6.3.

4.3.4.3 Short Message Services management

In order to support these services, a GSM network is in contact with a Short Message Service Centerthrough the two following interfaces:

The SMS-GMSC for Mobile Terminating Short Messages (SMS-MT/PP). It has the same role asthe GMSC.

●

The SMS-IWMSC for Mobile Originating Short Messages (SMS-MO/PP).●

4.3.5 Operation, Administration and Maintenance (OAM)

The OAM function allows the operator to monitor and control the system as well as to modify theconfiguration of the elements of the system. Not only the OSS is part of the OAM, also the BSS and NSSparticipate in its functions as it is shown in the following examples:

The components of the BSS and NSS provide the operator with all the information it needs. Thisinformation is then passed to the OSS which is in charge of analize it and control the network.

●

The self test tasks, usually incorporated in the components of the BSS and NSS, also contribute tothe OAM functions.

●

The BSC, in charge of controlling several BTSs, is another example of an OAM functionperformed outside the OSS.

●

5 The GSM radio interfaceThe radio interface is the interface between the mobile stations and the fixed infrastructure. It is one ofthe most important interfaces of the GSM system.

One of the main objectives of GSM is roaming. Therefore, in order to obtain a complete compatibilitybetween mobile stations and networks of different manufacturers and operators, the radio interface mustbe completely defined.

The spectrum eficiency depends on the radio interface and the transmission, more particularly in aspectssuch as the capacity of the system and the techniques used in order to decrease the interference and toimprove the frequency reuse scheme. The specification of the radio interface has then an importantinfluence on the spectrum efficiency.

5.1 Frequency allocation

Two frequency bands, of 25 Mhz each one, have been allocated for the GSM system:

The band 890-915 Mhz has been allocated for the uplink direction (transmitting from the mobile●



station to the base station).

The band 935-960 Mhz has been allocated for the downlink direction (transmitting from the basestation to the mobile station).

●

But not all the countries can use the whole GSM frequency bands. This is due principally to militaryreasons and to the existence of previous analog systems using part of the two 25 Mhz frequency bands.

5.2 Multiple access scheme

The multiple access scheme defines how different simultaneous communications, between differentmobile stations situated in different cells, share the GSM radio spectrum. A mix of Frequency DivisionMultiple Access (FDMA) and Time Division Multiple Access (TDMA), combined with frequencyhopping, has been adopted as the multiple access scheme for GSM.

5.2.1 FDMA and TDMA

Using FDMA, a frequency is assigned to a user. So the larger the number of users in a FDMA system,the larger the number of available frequencies must be. The limited available radio spectrum and the factthat a user will not free its assigned frequency until he does not need it anymore, explain why the numberof users in a FDMA system can be "quickly" limited.

On the other hand, TDMA allows several users to share the same channel. Each of the users, sharing thecommon channel, are assigned their own burst within a group of bursts called a frame. Usually TDMA isused with a FDMA structure.

In GSM, a 25 Mhz frequency band is divided, using a FDMA scheme, into 124 carrier frequenciesspaced one from each other by a 200 khz frequency band. Normally a 25 Mhz frequency band canprovide 125 carrier frequencies but the first carrier frequency is used as a guard band between GSM andother services working on lower frequencies. Each carrier frequency is then divided in time using aTDMA scheme. This scheme splits the radio channel, with a width of 200 khz, into 8 bursts. A burst isthe unit of time in a TDMA system, and it lasts approximately 0.577 ms. A TDMA frame is formed with8 bursts and lasts, consequently, 4.615 ms. Each of the eight bursts, that form a TDMA frame, are thenassigned to a single user.

5.2.2 Channel structure

A channel corresponds to the recurrence of one burst every frame. It is defined by its frequency and theposition of its corresponding burst within a TDMA frame. In GSM there are two types of channels:

The traffic channels used to transport speech and data information.●

The control channels used for network management messages and some channel maintenancetasks.

●

5.2.2.1 Traffic channels (TCH)



Full-rate traffic channels (TCH/F) are defined using a group of 26 TDMA frames called a 26-Multiframe.The 26-Multiframe lasts consequently 120 ms. In this 26-Multiframe structure, the traffic channels forthe downlink and uplink are separated by 3 bursts. As a consequence, the mobiles will not need totransmit and receive at the same time which simplifies considerably the electronics of the system.

The frames that form the 26-Multiframe structure have different functions:

24 frames are reserved to traffic.●

1 frame is used for the Slow Associated Control Channel (SACCH).●

The last frame is unused. This idle frame allows the mobile station to perform other functions,such as measuring the signal strength of neighboring cells.

●

Half-rate traffic channels (TCH/H), which double the capacity of the system, are also grouped in a26-Multiframe but the internal structure is different.

5.2.2.2 Control channels

According to their functions, four different classes of control channels are defined:

Broadcast channels.●

Common control channels.●

Dedicated control channels.●

Associated control channels.●

5.2.2.2.1 Broadcast channels (BCH)

The BCH channels are used, by the base station, to provide the mobile station with the sufficientinformation it needs to synchronize with the network. Three different types of BCHs can bedistinguished:

The Broadcast Control Channel (BCCH), which gives to the mobile station the parameters neededin order to identify and access the network

●

The Synchronization Channel (SCH), which gives to the mobile station the training sequenceneeded in order to demodulate the information transmitted by the base station

●

The Frequency-Correction Channel (FCCH), which supplies the mobile station with the frequencyreference of the system in order to synchronize it with the network

●

5.2.2.2.2 Common Control Channels (CCCH)

The CCCH channels help to establish the calls from the mobile station or the network. Three differenttypes of CCCH can be defined:

The Paging Channel (PCH). It is used to alert the mobile station of an incoming cal●

The Random Access Channel (RACH), which is used by the mobile station to request access to thenetwork

●



The Access Grant Channel (AGCH). It is used, by the base station, to inform the mobile stationabout which channel it should use. This channel is the answer of a base station to a RACH fromthe mobile station

●

5.2.2.2.3 Dedicated Control Channels (DCCH)

The DCCH channels are used for message exchange between several mobiles or a mobile and thenetwork. Two different types of DCCH can be defined:

The Standalone Dedicated Control Channel (SDCCH), which is used in order to exchangesignaling information in the downlink and uplink directions.

●

The Slow Associated Control Channel (SACCH). It is used for channel maintenance and channelcontrol.

●

5.2.2.2.4 Associated Control Channels

The Fast Associated Control Channels (FACCH) replace all or part of a traffic channel when urgentsignaling information must be transmitted. The FACCH channels carry the same information as theSDCCH channels.

5.2.3 Burst structure

As it has been stated before, the burst is the unit in time of a TDMA system. Four different types ofbursts can be distinguished in GSM:

The frequency-correction burst is used on the FCCH. It has the same length as the normal burst buta different structure.

●

The synchronization burst is used on the SCH. It has the same length as the normal burst but adifferent structure.

●

The random access burst is used on the RACH and is shorter than the normal burst.●

The normal burst is used to carry speech or data information. It lasts approximately 0.577 ms andhas a length of 156.25 bits. Its structure is presented in figure 3.

●



figure 3*: Structure of the 26-Multiframe, the TDMA frame and the normal burst

*This figure has been taken, with the corresponding authorization, from "An Overview of GSM" by JohnScourias (see Other GSM sites)

The tail bits (T) are a group of three bits set to zero and placed at the beginning and the end of a burst.They are used to cover the periods of ramping up and down of the mobile's power.

The coded data bits corresponds to two groups, of 57 bits each, containing signaling or user data.

The stealing flags (S) indicate, to the receiver, whether the information carried by a burst corresponds totraffic or signaling data.

The training sequence has a length of 26 bits. It is used to synchronize the receiver with the incominginformation, avoiding then the negative effects produced by a multipath propagation.

The guard period (GP), with a length of 8.25 bits, is used to avoid a possible overlap of two mobilesduring the ramping time.

5.2.4 Frequency hopping

The propagation conditions and therefore the multipath fading depend on the radio frequency. In order toavoid important differences in the quality of the channels, the slow frequency hopping is introduced. Theslow frequency hopping changes the frequency with every TDMA frame. A fast frequency hoppingchanges the frequency many times per frame but it is not used in GSM. The frequency hopping alsoreduces the effects of co-channel interference.



There are different types of frequency hopping algorithms. The algorithm selected is sent through theBroadcast Control Channels.

Even if frequency hopping can be very useful for the system, a base station does not have to support itnecessarily On the other hand, a mobile station has to accept frequency hopping when a base stationdecides to use it.

5.3 From source information to radio waves

The figure 4 presents the different operations that have to be performed in order to pass from thespeech source to radio waves and vice versa.

figure 4: From speech source to radio waves



If the source of information is data and not speech, the speech coding will not be performed.

5.3.1 Speech coding

The transmission of speech is, at the moment, the most important service of a mobile cellular system.The GSM speech codec, which will transform the analog signal (voice) into a digital representation, hasto meet the following criterias:

A good speech quality, at least as good as the one obtained with previous cellular systems.●

To reduce the redundancy in the sounds of the voice. This reduction is essential due to the limitedcapacity of transmission of a radio channel.

●

The speech codec must not be very complex because complexity is equivalent to high costs.●

The final choice for the GSM speech codec is a codec named RPE-LTP (Regular Pulse ExcitationLong-Term Prediction). This codec uses the information from previous samples (this information doesnot change very quickly) in order to predict the current sample. The speech signal is divided into blocksof 20 ms. These blocks are then passed to the speech codec, which has a rate of 13 kbps, in order toobtain blocks of 260 bits.

5.3.2 Channel coding

Channel coding adds redundancy bits to the original information in order to detect and correct, ifpossible, errors ocurred during the transmission.

5.3.2.1 Channel coding for the GSM data TCH channels

The channel coding is performed using two codes: a block code and a convolutional code.

The block code corresponds to the block code defined in the GSM Recommendations 05.03. The blockcode receives an input block of 240 bits and adds four zero tail bits at the end of the input block. Theoutput of the block code is consequently a block of 244 bits.

A convolutional code adds redundancy bits in order to protect the information. A convolutional encodercontains memory. This property differentiates a convolutional code from a block code. A convolutionalcode can be defined by three variables : n, k and K. The value n corresponds to the number of bits at theoutput of the encoder, k to the number of bits at the input of the block and K to the memory of theencoder. The ratio, R, of the code is defined as follows : R = k/n. Let's consider a convolutional codewith the following values: k is equal to 1, n to 2 and K to 5. This convolutional code uses then a rate of R= 1/2 and a delay of K = 5, which means that it will add a redundant bit for each input bit. Theconvolutional code uses 5 consecutive bits in order to compute the redundancy bit. As the convolutionalcode is a 1/2 rate convolutional code, a block of 488 bits is generated. These 488 bits are punctured inorder to produce a block of 456 bits. Thirty two bits, obtained as follows, are not transmitted :

C (11 + 15 j) for j = 0, 1, ..., 31



The block of 456 bits produced by the convolutional code is then passed to the interleaver.

5.3.2.2 Channel coding for the GSM speech channels

Before applying the channel coding, the 260 bits of a GSM speech frame are divided in three differentclasses according to their function and importance. The most important class is the class Ia containing 50bits. Next in importance is the class Ib, which contains 132 bits. The least important is the class II, whichcontains the remaining 78 bits. The different classes are coded differently. First of all, the class Ia bits areblock-coded. Three parity bits, used for error detection, are added to the 50 class Ia bits. The resultant 53bits are added to the class Ib bits. Four zero bits are added to this block of 185 bits (50+3+132). Aconvolutional code, with r = 1/2 and K = 5, is then applied, obtaining an output block of 378 bits. Theclass II bits are added, without any protection, to the output block of the convolutional coder. An outputblock of 456 bits is finally obtained.

5.3.2.3 Channel coding for the GSM control channels

In GSM the signalling information is just contained in 184 bits. Forty parity bits, obtained using a firecode, and four zero bits are added to the 184 bits before applying the convolutional code (r = 1/2 and K =5). The output of the convolutional code is then a block of 456 bits, which does not need to be punctured.

5.3.3 Interleaving

An interleaving rearranges a group of bits in a particular way. It is used in combination with FEC codesin order to improve the performance of the error correction mechanisms. The interleaving decreases thepossibility of losing whole bursts during the transmission, by dispersing the errors. Being the errors lessconcentrated, it is then easier to correct them.

5.3.3.1 Interleaving for the GSM control channels

A burst in GSM transmits two blocks of 57 data bits each. Therefore the 456 bits corresponding to theoutput of the channel coder fit into four bursts (4*114 = 456). The 456 bits are divided into eight blocksof 57 bits. The first block of 57 bits contains the bit numbers (0, 8, 16, .....448), the second one the bitnumbers (1, 9, 17, .....449), etc. The last block of 57 bits will then contain the bit numbers (7, 15,.....455). The first four blocks of 57 bits are placed in the even-numbered bits of four bursts. The otherfour blocks of 57 bits are placed in the odd-numbered bits of the same four bursts. Therefore theinterleaving depth of the GSM interleaving for control channels is four and a new data block starts everyfour bursts. The interleaver for control channels is called a block rectangular interleaver.

5.3.3.2 Interleaving for the GSM speech channels

The block of 456 bits, obtained after the channel coding, is then divided in eight blocks of 57 bits in thesame way as it is explained in the previous paragraph. But these eight blocks of 57 bits are distributed



differently. The first four blocks of 57 bits are placed in the even-numbered bits of four consecutivebursts. The other four blocks of 57 bits are placed in the odd-numbered bits of the next four bursts. Theinterleaving depth of the GSM interleaving for speech channels is then eight. A new data block also startsevery four bursts. The interleaver for speech channels is called a block diagonal interleaver.

5.3.3.3 Interleaving for the GSM data TCH channels

A particular interleaving scheme, with an interleaving depth equal to 22, is applied to the block of 456bits obtained after the channel coding. The block is divided into 16 blocks of 24 bits each, 2 blocks of 18bits each, 2 blocks of 12 bits each and 2 blocks of 6 bits each. It is spread over 22 bursts in the followingway :

the first and the twenty-second bursts carry one block of 6 bits each●

the second and the twenty-first bursts carry one block of 12 bits each●

the third and the twentieth bursts carry one block of 18 bits each●

from the fourth to the nineteenth burst, a block of 24 bits is placed in each burst●

A burst will then carry information from five or six consecutive data blocks. The data blocks are said tobe interleaved diagonally. A new data block starts every four bursts.

5.3.4 Burst assembling

The busrt assembling procedure is in charge of grouping the bits into bursts. Section 5.2.3 presents thedifferent bursts structures and describes in detail the structure of the normal burst.

5.3.5 Ciphering Ciphering is used to protect signaling and user data. First of all, a ciphering key is computed using thealgorithm A8 stored on the SIM card, the subscriber key and a random number delivered by the network(this random number is the same as the one used for the authentication procedure). Secondly, a 114 bitsequence is produced using the ciphering key, an algorithm called A5 and the burst numbers. This bitsequence is then XORed with the two 57 bit blocks of data included in a normal burst.

In order to decipher correctly, the receiver has to use the same algorithm A5 for the decipheringprocedure.

5.3.6 Modulation

The modulation chosen for the GSM system is the Gaussian Modulation Shift Keying (GMSK).

The aim of this section is not to describe precisely the GMSK modulation as it is too long and it impliesthe presentation of too many mathematical concepts. Therefore, only brief aspects of the GMSK



modulation are presented in this section.

The GMSK modulation has been chosen as a compromise between spectrum efficiency, complexity andlow spurious radiations (that reduce the possibilities of adjacent channel interference). The GMSKmodulation has a rate of 270 5/6 kbauds and a BT product equal to 0.3. Figure 5 presents the principle ofa GMSK modulator.

figure 5: GMSK modulator

5.4 Discontinuous transmission (DTX)

This is another aspect of GSM that could have been included as one of the requirements of the GSMspeech codec. The function of the DTX is to suspend the radio transmission during the silence periods.This can become quite interesting if we take into consideration the fact that a person speaks less than 40or 50 percent during a conversation. The DTX helps then to reduce interference between different cellsand to increase the capacity of the system. It also extends the life of a mobile's battery. The DTXfunction is performed thanks to two main features:

The Voice Activity Detection (VAD), which has to determine whether the sound represents speechor noise, even if the background noise is very important. If the voice signal is considered as noise,the transmitter is turned off producing then, an unpleasant effect called clipping.

●

The comfort noise. An inconvenient of the DTX function is that when the signal is considered asnoise, the transmitter is turned off and therefore, a total silence is heard at the receiver. This can bevery annoying to the user at the reception because it seems that the connection is dead. In order toovercome this problem, the receiver creates a minimum of background noise called comfort noise.The comfort noise eliminates the impression that the connection is dead.

●

5.5 Timing advance

The timing of the bursts transmissions is very important. Mobiles are at different distances from the basestations. Their delay depends, consequently, on their distance. The aim of the timing advance is that thesignals coming from the different mobile stations arrive to the base station at the right time. The base



station measures the timing delay of the mobile stations. If the bursts corresponding to a mobile stationarrive too late and overlap with other bursts, the base station tells, this mobile, to advance thetransmission of its bursts.

5.6 Power control

At the same time the base stations perform the timing measurements, they also perform measurements onthe power level of the different mobile stations. These power levels are adjusted so that the power isnearly the same for each burst.

A base station also controls its power level. The mobile station measures the strength and the quality ofthe signal between itself and the base station. If the mobile station does not receive correctly the signal,the base station changes its power level.

5.7 Discontinuous reception

It is a method used to conserve the mobile station's power. The paging channel is divided intosubchannels corresponding to single mobile stations. Each mobile station will then only 'listen' to itssubchannel and will stay in the sleep mode during the other subchannels of the paging channel.

5.8 Multipath and equalisation

At the GSM frequency bands, radio waves reflect from buildings, cars, hills, etc. So not only the 'right'signal (the output signal of the emitter) is received by an antenna, but also many reflected signals, whichcorrupt the information, with different phases.

An equaliser is in charge of extracting the 'right' signal from the received signal. It estimates the channelimpulse response of the GSM system and then constructs an inverse filter. The receiver knows whichtraining sequence it must wait for. The equaliser will then , comparing the received training sequencewith the training sequence it was expecting, compute the coefficients of the channel impulse response. Inorder to extract the 'right' signal, the received signal is passed through the inverse filter.

6 GSM servicesIt is important to note that all the GSM services were not introduced since the appearance of GSM butthey have been introduced in a regular way. The GSM Memorandum of Understanding (MoU) definedfour classes for the introduction of the different GSM services:

E1: introduced at the start of the service.●

E2: introduced at the end of 1991.●



Eh: introduced on availability of half-rate channels.●

A: these services are optional.●

Three categories of services can be distinguished:

Teleservices.●

Bearer services.●

Supplementary Services.●

6.1 Teleservices

- Telephony (E1® Eh).

- Facsmile group 3 (E1).

- Emergency calls (E1® Eh).

- Teletex.

- Short Message Services (E1, E2, A). Using these services, a message of a maximum of 160alphanumeric characters can be sent to or from a mobile station. If the mobile is powered off, themessage is stored. With the SMS Cell Broadcast (SMS-CB), a message of a maximum of 93 characterscan be broadcast to all mobiles in a certain geographical area.

- Fax mail. Thanks to this service, the subscriber can receive fax messages at any fax machine.

- Voice mail. This service corresponds to an answering machine.

6.2 Bearer services

A bearer service is used for transporting user data. Some of the bearer services are listed below:

Asynchronous and synchronous data, 300-9600 bps (E1).●

Alternate speech and data, 300-9600 bps (E1).●

Asynchronous PAD (packet-switched, packet assembler/disassembler) access, 300-9600 bps (E1).●

Synchronous dedicated packet data access, 2400-9600 bps (E2).●

6.3 Supplementary Services

- Call Forwarding (E1). The subscriber can forward incoming calls to another number if the calledmobile is busy (CFB), unreachable (CFNRc) or if there is no reply (CFNRy). Call forwarding can also beapplied unconditionally (CFU).



- Call Barring. There are different types of `call barring' services:

Barring of All Outgoing Calls, BAOC (E1).●

Barring of Outgoing International Calls, BOIC (E1).●

Barring of Outgoing International Calls except those directed toward the Home PLMN Country,BOIC-exHC (E1).

●

Barring of All Incoming Calls, BAIC (E1)●

Barring of incoming calls when roaming (A).●

- Call hold (E2). Puts an active call on hold.

- Call Waiting, CW (E2). Informs the user, during a conversation, about another incoming call. The usercan answer, reject or ignore this incoming call.

- Advice of Charge, AoC (E2). Provides the user with an online charge information.

- Multiparty service (E2). Possibility of establishing a multiparty conversation.

- Closed User Group, CUG (A). It corresponds to a group of users with limited possibilities of calling(only the people of the group and certain numbers).

- Calling Line Identification Presentation, CLIP (A). It supplies the called user with the ISDN of thecalling user.

- Calling Line Identification Restriction, CLIR (A). It enables the calling user to restrict the presentation.

- Connected Line identification Presentation, CoLP (A). It supplies the calling user with the directorynumber he gets if his call is forwarded.

- Connected Line identification Restriction, CoLR (A). It enables the called user to restrict thepresentation.

- Operator determined barring (A). Restriction of different services and call types by the operator.

7 ConclusionThe aim of this paper was to give an overview of the GSM system and not to provide a complete andexhaustive guide.

As it is shown in this chapter, GSM is a very complex standard. It can be considered as the first seriousattempt to fulfil the requirements for a universal personal communication system. GSM is then used as abasis for the development of the Universal Mobile Telecommunication System (UMTS).

Bibliography



`An introduction to GSM' by Redl, Weber and Oliphant. Published by Artech House. ISBN0-89006-785-6.

'The GSM System for Mobile communications' by Mouly and Pautet. Published by Cell & Sys. ISBN2-9507190-0-7.

`Telecommunications Engineering' by J.Dunlop and D.G. Smith. Published by Chapman & Hall. ISBN0-412-56270-7.

`Modern Personal Radio Systems'. Edited by R.C.V. Macario. The Institution of Electrical Engineers.ISBN 0-85296-861-2.

`Mobile Radio Communications' by Raymond Steele. Pentech Press publishers and IEEE Press. ISBN0-7803-1102-7.

'Overview of the Global System for Mobile communications' by John Scourias (University of Waterloo).Web document found in: http://ccnga.uwaterloo.ca/~jscouria/GSM/index.html

'A brief overview of the GSM radio interface' by Thierry Turletti (Laboratory for Computer Science,Massachussets Institute of Technology).

'An introduction to GSM' from the book 'Cellular Radio Systems', edited by Balston and Macario.Published by Artech House.

'The GSM tutorial'. Web document found in: http:/www.iec.org

AcronymsA3 Authentication algorithm A5 Ciphering algorithm A8 Ciphering key computation AGCH Access Grant CHannel AMPS Advanced Mobile Phone Service AoC Advice of Charge ARQ Automatic Repeat reQuest mechanism AUC Authentication Center BAIC Barring of All Incoming Calls BAOC Barring of All Outgoing Calls BOIC Barring of Outgoing International Calls

BOIC-exHC Barring of Outgoing International Calls except those directed toward the HomePLMN Country

BCCH Broadcast Control CHannel BCH Broadcast CHannel BER Bit Error Rate



bps bits per second BSC Base Station Controller BSS Base Station Subsystem BTS Base Transceiver Station CC Call Control CCCH Common Control CHannel CDMA Code Division Multiple Access CEPT Conference of European Posts and Telecommunications CFB Call Forwarding on mobile subscriber Busy CFNRc Call Forwarding on mobile subscriber Not Reachable CFNRy Call Forwarding on No Reply CFU Call Forwarding Unconditional CGI Cell Global Identity C/I Carrier-to-Interference ratio C/I Carrier-to-Interference ratio CLIP Calling Line Identification Presentation CLIR Calling Line Identification Restriction CM Communication Management CoLP Connected Line identification Presentation CoLR Connected Line identification Restriction CUG Closed User Group CW Call Waiting DCS Digital Cellular System DCCH Dedicated Control CHannel DTX Discontinuous transmission EIR Equipment Identity Register ETSI European Telecommunications Standards Institute FACCH Fast Associated Control CHannel FCCH Frequency-Correction CHannel FDMA Frequency Division Multiple Access FEC Forward Error Correction code FER Frame Erasure Rate GIWU GSM Interworking Unit GMSC GSM Mobile services Switching Center GMSK Gaussian Minimum Shift Keying GP Guard Period GSM Global System for Mobile communications HLR Home Location Register



IMEI International Mobile Equipment Identity IMSI International Mobile Subscriber Identity ISDN Integrated Services Digital Network JDC Japanese Digital Cellular LA Location Area LAI Location Area Identity LOS Line-Of-Sight MM Mobility Management MoU Memorandum of Understanding MS Mobile Station MSC Mobile services Switching Center MSISDN Mobile Station ISDN number MSRN Mobile Station Roaming Number NADC North American Digital Cellular NMT Nordic Mobile Telephone NSS Network and Switching Subsystem OAM Operation, Administration and Maintenance OSS Operation and Support Subsystem PAD Packet Assembler Disassembler PCH Paging CHannel PCS Personal Communications Services PDC Personal Digital Cellular PIN Personal Identification Number PLMN Public Land Mobile Network PSPDN Packet Switched Public Data Network PSTN Public Switched Telephone Network RACH Random Access CHannel RF Radio Frequency RPE-LTP Regular Pulse Excitation Long-Term Prediction RR Radio Resources management S Stealing flags SACCH Slow Associated Control CHannel SCH Synchronisation CHannel SDCCH Standalone Dedicated Control CHannel SDCCH Standalone Dedicated Control CHannel SIM Subscriber Identity Module SMS Short Message Services SMS-CB Short Message Services Cell Broadcast



SMS-MO/PP Short Message Services Mobile Originating/Point-to-Point SMS-MT/PP Short Message Services Mobile Terminating/Point-to-Point SNR Signal to Noise Ratio SRES Signed RESult SS Supplementary Services T Tail bits TACS Total Access Communication System TCH Traffic CHannel TCH/F Traffic CHannel/Full rate TCH/H Traffic CHannel/Half rate TDMA Time Division Multiple Access TMSI Temporary Mobile Subscriber Identity UMTS Universal Mobile Telecommunications System VAD Voice Activity Detection VLR Visitor Location Register

Other GSM sites

● The Telecoms Virtual Library about mobile communications. You can find information about GSMbut also about other mobile commmunications systems. http://www.analysys.co.uk/vlib/mobile.htm

● An overview of the Global System for Mobile Communications by John Scouriashttp://ccnga.uwaterloo.ca/~jscouria/GSM/gsmreport.html

● GSM in Belgiumhttp://www.luc.ac.be/~hbaerten/gsm/

● GSM World, the world wide web site of the GSM MoU Association http://www.gsmworld.com/

● The magazine GSMag Internationalhttp://www.gsmag.com/

● A list of GSM operators and network codes by countryhttp://kbs.cs.tu-berlin.de/~jutta/gsm/gsm-list.html

● Send messages to GSM Mobile phones



http://www.analysys.co.uk/vlib/mobile.htm

http://www.luc.ac.be/~hbaerten/gsm/

http://www.gsmworld.com/

http://www.gsmag.com/

http://kbs.cs.tu-berlin.de/~jutta/gsm/gsm-list.html

http://www.mtn.co.za/regulars/sms/

● Mobile Worldhttp://www.mobileworld.org/

● ITU Selected Sites-Telecom-Wirelesshttp://www.itu.int/Sites/wwwfiles/tel_wireless.html

● GSM information networkhttp://www.gin.nl/

● Radiophonehttp://radiophone.dhp.com/

● SMS referencehttp://www.virtua.co.uk/sms/sms/index.html

● Ben Wood's GSM reference sitehttp://ds.dial.pipex.com/benw/

● A complete french web page about GSM (includes an overview of GSM, GSM services, usefulinformation for GSM users, etc...). http://massena.univ-mlv.fr/~turloy/Gsm/frmGSM.htm

● Some of the most important manufacturers of cellular phones: Motorola, Ericsson, Nokia and Alcatel

Go back to my home page

Javier Gozalvez Sempere web's page - April 1998



http://www.mtn.co.za/regulars/sms/

http://www.mobileworld.org/

http://www.itu.int/Sites/wwwfiles/tel_wireless.html

http://www.gin.nl/

http://radiophone.dhp.com/

http://www.virtua.co.uk/sms/sms/index.html

http://ds.dial.pipex.com/benw/

http://massena.univ-mlv.fr/~turloy/Gsm/frmGSM.html

http://www.mot.com/

http://www.ericsson.com/

http://www.nokia.com/

http://www.alcatel.com/

http://www.comms.eee.strath.ac.uk/~gozalvez/

Overview of the Global System for MobileCommunications

John [email protected]

Table of Contents

1. History of GSM2. Services provided by GSM3. Architecture of the GSM network

3.1. Mobile Station3.2. Base Station Subsystem3.3. Network Subsystem

4. Radio link aspects

4.1. Multiple access and channel structure

4.1.1. Traffic channels4.1.2. Control channels4.1.3. Burst structure

4.2. Speech coding4.3. Channel coding and modulation4.4. Multipath equalization4.5. Frequency hopping4.6. Discontinuous transmission4.7. Discontinuous reception4.8. Power control

5. Network aspects

5.1. Radio resources management

5.1.1. Handover

5.2. Mobility management

5.2.1. Location updating

Overview of the Global System for Mobile Communications

http://ccnga.uwaterloo.ca/~jscouria/GSM/gsmreport.html (1 of 18) [24/09/1999 13:09:05]

5.2.2. Authentication and security

5.3. Communication management

5.3.1. Call routing

6. Conclusion and comments

History of GSMDuring the early 1980s, analog cellular telephone systems were experiencing rapid growth in Europe,particularly in Scandinavia and the United Kingdom, but also in France and Germany. Each countrydeveloped its own system, which was incompatible with everyone else's in equipment and operation.This was an undesirable situation, because not only was the mobile equipment limited to operation withinnational boundaries, which in a unified Europe were increasingly unimportant, but there was also a verylimited market for each type of equipment, so economies of scale and the subsequent savings could notbe realized.

The Europeans realized this early on, and in 1982 the Conference of European Posts and Telegraphs(CEPT) formed a study group called the Groupe Spécial Mobile (GSM) to study and develop apan-European public land 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 European Telecommunication Standards Institute(ETSI), and phase I of the GSM specifications were published in 1990. Commercial service was startedin mid-1991, and by 1993 there were 36 GSM networks in 22 countries [6]. Although standardized inEurope, GSM is not only a European standard. Over 200 GSM networks (including DCS1800 andPCS1900) are operational in 110 countries around the world. In the beginning of 1994, there were 1.3million subscribers worldwide [18], which had grown to more than 55 million by October 1997. WithNorth America making a delayed entry into the GSM field with a derivative of GSM called PCS1900,GSM systems exist on every continent, and the acronym GSM now aptly stands for Global System forMobile communications.

The developers of GSM chose an unproven (at the time) digital system, as opposed to the then-standardanalog cellular systems like AMPS in the United States and TACS in the United Kingdom. They hadfaith that advancements in compression algorithms and digital signal processors would allow thefulfillment of the original criteria and the continual improvement of the system in terms of quality and



cost. The over 8000 pages of GSM recommendations try to allow flexibility and competitive innovationamong suppliers, but provide enough standardization to guarantee proper interworking between thecomponents of the system. This is done by providing functional and interface descriptions for each of thefunctional entities defined in the system.

Services provided by GSMFrom the beginning, the planners of GSM wanted ISDN compatibility in terms of the services offeredand the control signalling used. However, radio transmission limitations, in terms of bandwidth and cost,do not allow the standard ISDN B-channel bit rate of 64 kbps to be practically achieved.

Using the ITU-T definitions, telecommunication services can be divided into bearer services,teleservices, and supplementary services. The most basic teleservice supported by GSM is telephony. Aswith all other communications, speech is digitally encoded and transmitted through the GSM network asa digital stream. There is also an emergency service, where the nearest emergency-service provider isnotified by dialing three digits (similar to 911).

A variety of data services is offered. GSM users can send and receive data, at rates up to 9600 bps, tousers on POTS (Plain Old Telephone Service), ISDN, Packet Switched Public Data Networks, andCircuit Switched Public Data Networks using a variety of access methods and protocols, such as X.25 orX.32. Since GSM is a digital network, a modem is not required between the user and GSM network,although an audio modem is required inside the GSM network to interwork with POTS.

Other data services include Group 3 facsimile, as described in ITU-T recommendation T.30, which issupported by use of an appropriate fax adaptor. A unique feature of GSM, not found in older analogsystems, is the Short Message Service (SMS). SMS is a bidirectional service for short alphanumeric (upto 160 bytes) messages. Messages are transported in a store-and-forward fashion. For point-to-pointSMS, a message can be sent to another subscriber to the service, and an acknowledgement of receipt isprovided to the sender. SMS can also be used in a cell-broadcast mode, for sending messages such astraffic updates or news updates. Messages can also be stored in the SIM card for later retrieval [2].

Supplementary services are provided on top of teleservices or bearer services. In the current (Phase I)specifications, they include several forms of call forward (such as call forwarding when the mobilesubscriber is unreachable by the network), and call barring of outgoing or incoming calls, for examplewhen roaming in another country. Many additional supplementary services will be provided in the Phase2 specifications, such as caller identification, call waiting, multi-party conversations.

Architecture of the GSM networkA GSM network is composed of several functional entities, whose functions and interfaces are specified.Figure 1 shows the layout of a generic GSM network. The GSM network can be divided into three broadparts. The Mobile Station is carried by the subscriber. The Base Station Subsystem controls the radio linkwith the Mobile Station. The Network Subsystem, the main part of which is the Mobile servicesSwitching Center (MSC), performs the switching of calls between the mobile users, and between mobileand fixed network users. The MSC also handles the mobility management operations. Not shown is theOperations and Maintenance Center, which oversees the proper operation and setup of the network. The



Mobile Station and the Base Station Subsystem communicate across the Um interface, also known as theair interface or radio link. The Base Station Subsystem communicates with the Mobile servicesSwitching Center across the A interface.

Figure 1. General architecture of a GSM network

Mobile Station

The mobile station (MS) consists of the mobile equipment (the terminal) and a smart card called theSubscriber Identity Module (SIM). The SIM provides personal mobility, so that the user can have accessto subscribed services irrespective of a specific terminal. By inserting the SIM card into another GSMterminal, the user is able to receive calls at that terminal, make calls from that terminal, and receive othersubscribed services.

The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI). TheSIM card contains the International Mobile Subscriber Identity (IMSI) used to identify the subscriber tothe system, a secret key for authentication, and other information. The IMEI and the IMSI areindependent, thereby allowing personal mobility. The SIM card may be protected against unauthorizeduse by a password or personal identity number.

Base Station Subsystem

The Base Station Subsystem is composed of two parts, the Base Transceiver Station (BTS) and the BaseStation Controller (BSC). These communicate across the standardized Abis interface, allowing (as in therest of the system) operation between components made by different suppliers.

The Base Transceiver Station houses the radio tranceivers that define a cell and handles the radio-link



protocols with the Mobile Station. In a large urban area, there will potentially be a large number of BTSsdeployed, thus the requirements for a BTS are ruggedness, reliability, portability, and minimum cost.

The Base Station Controller manages the radio resources for one or more BTSs. It handles radio-channelsetup, frequency hopping, and handovers, as described below. The BSC is the connection between themobile station and the Mobile service Switching Center (MSC).

Network Subsystem

The central component of the Network Subsystem is the Mobile services Switching Center (MSC). It actslike a normal switching node of the PSTN or ISDN, and additionally provides all the functionalityneeded to handle a mobile subscriber, such as registration, authentication, location updating, handovers,and call routing to a roaming subscriber. These services are provided in conjuction with severalfunctional entities, which together form the Network Subsystem. The MSC provides the connection tothe fixed networks (such as the PSTN or ISDN). Signalling between functional entities in the NetworkSubsystem uses Signalling System Number 7 (SS7), used for trunk signalling in ISDN and widely usedin current public networks.

The Home Location Register (HLR) and Visitor Location Register (VLR), together with the MSC,provide the call-routing and roaming capabilities of GSM. The HLR contains all the administrativeinformation of each subscriber registered in the corresponding GSM network, along with the currentlocation of the mobile. The location of the mobile is typically in the form of the signalling address of theVLR associated with the mobile station. The actual routing procedure will be described later. There islogically one HLR per GSM network, although it may be implemented as a distributed database.

The Visitor Location Register (VLR) contains selected administrative information from the HLR,necessary for call control and provision of the subscribed services, for each mobile currently located inthe geographical area controlled by the VLR. Although each functional entity can be implemented as anindependent unit, all manufacturers of switching equipment to date implement the VLR together with theMSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR,thus simplifying the signalling required. Note that the MSC contains no information about particularmobile stations --- this information is stored in the location registers.

The other two registers are used for authentication and security purposes. The Equipment IdentityRegister (EIR) is a database that contains a list of all valid mobile equipment on the network, where eachmobile station is identified by its International Mobile Equipment Identity (IMEI). An IMEI is marked asinvalid if it has been reported stolen or is not type approved. The Authentication Center (AuC) is aprotected database that stores a copy of the secret key stored in each subscriber's SIM card, which is usedfor authentication and encryption over the radio channel.

Radio link aspectsThe International Telecommunication Union (ITU), which manages the international allocation of radiospectrum (among many other functions), allocated the bands 890-915 MHz for the uplink (mobile stationto base station) and 935-960 MHz for the downlink (base station to mobile station) for mobile networksin Europe. Since this range was already being used in the early 1980s by the analog systems of the day,



the CEPT had the foresight to reserve the top 10 MHz of each band for the GSM network that was stillbeing developed. Eventually, GSM will be allocated the entire 2x25 MHz bandwidth.

Multiple access and channel structure

Since radio spectrum is a limited resource shared by all users, a method must be devised to divide up thebandwidth among as many users as possible. The method chosen by GSM is a combination of Time- andFrequency-Division Multiple Access (TDMA/FDMA). The FDMA part involves the division byfrequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart. Oneor more carrier frequencies are assigned to each base station. Each of these carrier frequencies is thendivided in time, using a TDMA scheme. The fundamental unit of time in this TDMA scheme is called aburst period and it lasts 15/26 ms (or approx. 0.577 ms). Eight burst periods are grouped into a TDMAframe (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 thesedefinitions are cyclic, and the entire pattern repeats approximately every 3 hours. Channels can bedivided into dedicated channels, which are allocated to a mobile station, and common channels, whichare used by mobile stations in idle mode.

Traffic channels

A traffic channel (TCH) is used to carry speech and data traffic. Traffic channels are defined using a26-frame multiframe, or group of 26 TDMA frames. The length of a 26-frame multiframe is 120 ms,which is how the length of a burst period is defined (120 ms divided by 26 frames divided by 8 burstperiods per frame). Out of the 26 frames, 24 are used for traffic, 1 is used for the Slow AssociatedControl Channel (SACCH) and 1 is currently unused (see Figure 2). TCHs for the uplink and downlinkare separated in time by 3 burst periods, so that the mobile station does not have to transmit and receivesimultaneously, thus simplifying the electronics.

In addition to these full-rate TCHs, there are also half-rate TCHs defined, although they are not yetimplemented. Half-rate TCHs will effectively double the capacity of a system once half-rate speechcoders are specified (i.e., speech coding at around 7 kbps, instead of 13 kbps). Eighth-rate TCHs are alsospecified, and are used for signalling. In the recommendations, they are called Stand-alone DedicatedControl Channels (SDCCH).



Figure 2. Organization of bursts, TDMA frames, and multiframes for speech and data

Control channels

Common channels can be accessed both by idle mode and dedicated mode mobiles. The commonchannels are used by idle mode mobiles to exchange the signalling information required to change todedicated mode. Mobiles already in dedicated mode monitor the surrounding base stations for handoverand other information. The common channels are defined within a 51-frame multiframe, so thatdedicated mobiles using the 26-frame multiframe TCH structure can still monitor control channels. Thecommon channels include:

Broadcast Control Channel (BCCH)

Continually broadcasts, on the downlink, information including base station identity, frequencyallocations, and frequency-hopping sequences.

Frequency Correction Channel (FCCH) and Synchronisation Channel (SCH)

Used to synchronise the mobile to the time slot structure of a cell by defining the boundaries ofburst periods, and the time slot numbering. 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 the network.

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 signalling (in order to obtain a dedicated channel),following a request on the RACH.



Burst structure

There are four different types of bursts used for transmission in GSM [16]. The normal burst is used tocarry data and most signalling. It has a total length of 156.25 bits, made up of two 57 bit information bits,a 26 bit training sequence used for equalization, 1 stealing bit for each information block (used forFACCH), 3 tail bits at each end, and an 8.25 bit guard sequence, as shown in Figure 2. The 156.25 bitsare transmitted in 0.577 ms, giving a gross bit rate of 270.833 kbps.

The F burst, used on the FCCH, and the S burst, used on the SCH, have the same length as a normalburst, but a different internal structure, which differentiates them from normal bursts (thus allowingsynchronization). The access burst is shorter than the normal burst, and is used only on the RACH.

Speech coding

GSM is a digital system, so speech which is inherently analog, has to be digitized. The method employedby ISDN, and by current telephone systems for multiplexing voice lines over high speed trunks andoptical fiber lines, is Pulse Coded Modulation (PCM). The output stream from PCM is 64 kbps, too higha rate to be feasible over a radio link. The 64 kbps signal, although simple to implement, contains muchredundancy. The GSM group studied several speech coding algorithms on the basis of subjective speechquality and complexity (which is related to cost, processing delay, and power consumption onceimplemented) before arriving at the choice of a Regular Pulse Excited -- Linear Predictive Coder(RPE--LPC) with a Long Term Predictor loop. Basically, information from previous samples, which doesnot change very quickly, is used to predict the current sample. The coefficients of the linear combinationof the previous samples, plus an encoded form of the residual, the difference between the predicted andactual sample, represent the signal. Speech is divided into 20 millisecond samples, each of which isencoded as 260 bits, giving a total bit rate of 13 kbps. This is the so-called Full-Rate speech coding.Recently, an Enhanced Full-Rate (EFR) speech coding algorithm has been implemented by some NorthAmerican GSM1900 operators. This is said to provide improved speech quality using the existing 13kbps bit rate.

Channel coding and modulation

Because of natural and man-made electromagnetic interference, the encoded speech or data signaltransmitted over the radio interface must be protected from errors. GSM uses convolutional encoding andblock interleaving to achieve this protection. The exact algorithms used differ for speech and for differentdata rates. The method used for speech blocks will be described below.

Recall that the speech codec produces a 260 bit block for every 20 ms speech sample. From subjectivetesting, it was found that some bits of this block were more important for perceived speech quality thanothers. The bits are thus divided 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, theframe is judged too damaged to be comprehensible and it is discarded. It is replaced by a slightly



attenuated version of the previous correctly received frame. These 53 bits, together with the 132 Class Ibbits and a 4 bit tail sequence (a total of 189 bits), are input into a 1/2 rate convolutional encoder ofconstraint length 4. Each input bit is encoded as two output bits, based on a combination of the previous4 input bits. The convolutional encoder thus outputs 378 bits, to which are added the 78 remaining ClassII bits, which are unprotected. Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rateof 22.8 kbps.

To further protect against the burst errors common to the radio interface, each sample is interleaved. The456 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 burst can carry two 57 bit blocks,each burst carries traffic from two different speech samples.

Recall that each time-slot burst is transmitted at a gross bit rate of 270.833 kbps. This digital signal ismodulated onto the analog carrier frequency using Gaussian-filtered Minimum Shift Keying (GMSK).GMSK was selected over other modulation schemes as a compromise between spectral efficiency,complexity of the transmitter, and limited spurious emissions. The complexity of the transmitter isrelated to power consumption, which should be minimized for the mobile station. The spurious radioemissions, outside of the allotted bandwidth, must be strictly controlled so as to limit adjacent channelinterference, and allow for the co-existence of GSM and the older analog systems (at least for the timebeing).

Multipath equalization

At the 900 MHz range, radio waves bounce off everything - buildings, hills, cars, airplanes, etc. Thusmany reflected signals, each with a different phase, can reach an antenna. Equalization is used to extractthe desired signal from the unwanted reflections. It works by finding out how a known transmitted signalis modified by multipath fading, and constructing an inverse filter to extract the rest of the desired signal.This known signal is the 26-bit training sequence transmitted in the middle of every time-slot burst. Theactual implementation of the equalizer is not specified in the GSM specifications.

Frequency hopping

The mobile station already has to be frequency agile, meaning it can move between a transmit, receive,and monitor time slot within one TDMA frame, which normally are on different frequencies. GSMmakes use of this inherent frequency agility to implement slow frequency hopping, where the mobile andBTS transmit each TDMA frame on a different carrier frequency. The frequency hopping algorithm isbroadcast on the Broadcast Control Channel. Since multipath fading is dependent on carrier frequency,slow frequency hopping helps alleviate the problem. In addition, co-channel interference is in effectrandomized.

Discontinuous transmission

Minimizing co-channel interference is a goal in any cellular system, since it allows better service for agiven 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 [22], by turning the transmitter off during silence



periods. An added benefit of DTX is that power is conserved at the mobile unit.

The most important component of DTX is, of course, Voice Activity Detection. It must distinguishbetween voice and noise inputs, a task that is not as trivial as it appears, considering background noise. Ifa voice signal is misinterpreted as noise, the transmitter is turned off and a very annoying effect calledclipping is heard at the receiving end. If, on the other hand, noise is misinterpreted as a voice signal toooften, the efficiency of DTX is dramatically 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 connection is not dead, comfort noise is created at the receiving end bytrying to match the characteristics of the transmitting end's background noise.

Discontinuous reception

Another method used to conserve power at the mobile station is discontinuous reception. The pagingchannel, used by the base station to signal an incoming call, is structured into sub-channels. Each mobilestation needs to listen only to its own sub-channel. In the time between successive paging sub-channels,the mobile can go into sleep mode, when almost no power is used.

Power control

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

The mobile station measures the signal strength or signal quality (based on the Bit Error Ratio), andpasses the information to the Base Station Controller, which ultimately decides if and when the powerlevel should be changed. Power control should be handled carefully, since there is the possibility ofinstability. This arises from having mobiles in co-channel cells alternatingly increase their power inresponse to increased co-channel interference caused by the other mobile increasing its power. This inunlikely to occur in practice but it is (or was as of 1991) under study.

Network aspectsEnsuring the transmission of voice or data of a given quality over the radio link is only part of thefunction of a cellular mobile network. A GSM mobile can seamlessly roam nationally andinternationally, which requires that registration, authentication, call routing and location updatingfunctions exist and are standardized in GSM networks. In addition, the fact that the geographical areacovered by the network is divided into cells necessitates the implementation of a handover mechanism.These functions are performed by the Network Subsystem, mainly using the Mobile Application Part(MAP) built on top of the Signalling System No. 7 protocol.



Figure 3. Signalling protocol structure in GSM

The signalling protocol in GSM is structured into three general layers [1], [19], depending on theinterface, as shown in Figure 3. Layer 1 is the physical layer, which uses the channel structures discussedabove over the air interface. Layer 2 is the data link layer. Across the Um interface, the data link layer isa modified version of the LAPD protocol used in ISDN, called LAPDm. Across the A interface, theMessage Transfer Part layer 2 of Signalling System Number 7 is used. Layer 3 of the GSM signallingprotocol is itself divided into 3 sublayers.

Radio Resources Management

Controls the setup, maintenance, and termination of radio and fixed channels, including handovers.

Mobility Management

Manages the location updating and registration procedures, as well as security and authentication.

Connection Management

Handles general call control, similar to CCITT Recommendation Q.931, and managesSupplementary Services and the Short Message Service.

Signalling between the different entities in the fixed part of the network, such as between the HLR andVLR, is accomplished throught the Mobile Application Part (MAP). MAP is built on top of theTransaction Capabilities Application Part (TCAP, the top layer of Signalling System Number 7. Thespecification of the MAP is quite complex, and at over 500 pages, it is one of the longest documents inthe GSM recommendations [16].

Radio resources management

The radio resources management (RR) layer oversees the establishment of a link, both radio and fixed,between the mobile station and the MSC. The main functional components involved are the mobilestation, and the Base Station Subsystem, as well as the MSC. The RR layer is concerned with themanagement of an RR-session [16], which is the time that a mobile is in dedicated mode, as well as theconfiguration of radio channels including the allocation of dedicated channels.



An RR-session is always initiated by a mobile station through the access procedure, either for anoutgoing call, or in response to a paging message. The details of the access and paging procedures, suchas when a dedicated channel is actually assigned to the mobile, and the paging sub-channel structure, arehandled in the RR layer. In addition, it handles the management of radio features such as power control,discontinuous transmission and reception, and timing advance.

Handover

In a cellular network, the radio and fixed links required are not permanently allocated for the duration ofa call. Handover, or handoff as it is called in North America, is the switching of an on-going call to adifferent channel or cell. The execution and measurements required for handover form one of basicfunctions of the RR layer.

There are four different types of handover in the GSM system, which involve 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 SwitchingCenter (MSC), and

●

Cells under the control of different MSCs.●

The first two types of handover, called internal handovers, involve only one Base Station Controller(BSC). To save signalling bandwidth, they are managed by the BSC without involving the Mobileservices Switching Center (MSC), except to notify it at the completion of the handover. The last twotypes of handover, called external handovers, are handled by the MSCs involved. An important aspect ofGSM is that the original MSC, the anchor MSC, remains responsible for most call-related functions, withthe exception of subsequent inter-BSC handovers under the control of the new MSC, called the relayMSC.

Handovers can be initiated by either the mobile or the MSC (as a means of traffic load balancing).During its idle time slots, the mobile scans the Broadcast Control Channel of up to 16 neighboring cells,and forms a list of the six best candidates for possible handover, based on the received signal strength.This information is passed to the BSC and MSC, at least once per second, and is used by the handoveralgorithm.

The algorithm for when a handover decision should be taken is not specified in the GSMrecommendations. There are two basic algorithms used, both closely tied in with power control. This isbecause the BSC usually does not know whether the poor signal quality is due to multipath fading or tothe mobile having moved to another cell. This is especially true in small urban cells.

The 'minimum acceptable performance' algorithm [3] gives precedence to power control over handover,so that when the signal degrades beyond a certain point, the power level of the mobile is increased. Iffurther power increases do not improve the signal, then a handover is considered. This is the simpler andmore common method, but it creates 'smeared' cell boundaries when a mobile transmitting at peak powergoes some distance beyond its original cell boundaries into another cell.

The 'power budget' method [3] uses handover to try to maintain or improve a certain level of signalquality at the same or lower power level. It thus gives precedence to handover over power control. It



avoids the 'smeared' cell boundary problem and reduces co-channel interference, but it is quitecomplicated.

Mobility management

The Mobility Management layer (MM) is built on top of the RR layer, and handles the functions thatarise from the mobility of the subscriber, as well as the authentication and security aspects. Locationmanagement is concerned with the procedures that enable the system to know the current location of apowered-on mobile station so that incoming call routing can be completed.

Location updating

A powered-on mobile is informed of an incoming call by a paging message sent over the PAGCHchannel of a cell. One extreme would be to page every cell in the network for each call, which isobviously a waste of radio bandwidth. The other extreme would be for the mobile to notify the system,via location updating messages, of its current location at the individual cell level. This would requirepaging messages to be sent to exactly one cell, but would be very wasteful due to the large number oflocation updating messages. A compromise solution used in GSM is to group cells into location areas.Updating messages are required when moving between location areas, and mobile stations are paged inthe cells of their current location area.

The location updating procedures, and subsequent call routing, use the MSC and two location registers:the Home Location Register (HLR) and the Visitor Location Register (VLR). When a mobile station isswitched on in a new location area, or it moves to a new location area or different operator's PLMN, itmust register with the network to indicate its current location. In the normal case, a location updatemessage is sent to the new MSC/VLR, which records the location area information, and then sends thelocation information to the subscriber's HLR. The information sent to the HLR is normally the SS7address of the new VLR, although it may be a routing number. The reason a routing number is notnormally assigned, even though it would reduce signalling, is that there is only a limited number ofrouting numbers available in the new MSC/VLR and they are allocated on demand for incoming calls. Ifthe subscriber is entitled to service, the HLR sends a subset of the subscriber information, needed for callcontrol, to the new MSC/VLR, and sends a message to the old MSC/VLR to cancel the old registration.

For reliability reasons, GSM also has a periodic location updating procedure. If an HLR or MSC/VLRfails, to have each mobile register simultaneously to bring the database up to date would causeoverloading. Therefore, the database is updated as location updating events occur. The enabling ofperiodic updating, and the time period between periodic updates, is controlled by the operator, and is atrade-off between signalling traffic and speed of recovery. If a mobile does not register after the updatingtime period, it is deregistered.

A procedure related to location updating is the IMSI attach and detach. A detach lets the network knowthat the mobile station is unreachable, and avoids having to needlessly allocate channels and send pagingmessages. An attach is similar to a location update, and informs the system that the mobile is reachableagain. The activation of IMSI attach/detach is up to the operator on an individual cell basis.

Authentication and security



Since the radio medium can be accessed by anyone, authentication of users to prove that they are whothey claim to be, is a very important element of a mobile network. Authentication involves twofunctional entities, the SIM card 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 other in the AuC. Duringauthentication, the AuC generates a random number that it sends to the mobile. Both the mobile and theAuC then use the random number, in conjuction with the subscriber's secret key and a cipheringalgorithm called A3, to generate a signed response (SRES) that is sent back to the AuC. If the numbersent by the mobile is the same as the one calculated by the AuC, the subscriber is authenticated [16].

The same initial random number and subscriber key are also used to compute the ciphering key using analgorithm called A8. This ciphering key, together with the TDMA frame number, use the A5 algorithmto create a 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 is already coded, interleaved, andtransmitted in a TDMA manner, thus providing protection from all but the most persistent and dedicatedeavesdroppers.

Another level of security is performed on the mobile equipment itself, as opposed to the mobilesubscriber. As mentioned earlier, each GSM terminal is identified by a unique International MobileEquipment 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 the EIR is one of the following:

White-listed

The terminal is allowed to connect to the network.

Grey-listed

The terminal is under observation from the network for possible problems.

Black-listed

The terminal has either been reported stolen, or is not type approved (the correct type of terminalfor a GSM network). The terminal is not allowed to connect to the network.

Communication management

The Communication Management layer (CM) is responsible for Call Control (CC), supplementaryservice management, and short message service management. Each of these may be considered as aseparate sublayer within the CM layer. Call control attempts to follow the ISDN procedures specified inQ.931, although routing to a roaming mobile subscriber is obviously unique to GSM. Other functions ofthe CC sublayer include call establishment, selection of the type of service (including alternatingbetween services during a call), and call release.

Call routing

Unlike routing in the fixed network, where a terminal is semi-permanently wired to a central office, aGSM user can roam nationally and even internationally. The directory number dialed to reach a mobilesubscriber is called the Mobile Subscriber ISDN (MSISDN), which is defined by the E.164 numberingplan. This number includes a country code and a National Destination Code which identifies thesubscriber'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 isbasically a switch which is able to interrogate the subscriber's HLR to obtain routing information, andthus contains a table linking MSISDNs to their corresponding HLR. A simplification is to have a GSMChandle one specific PLMN. It should be noted that the GMSC function is distinct from the MSCfunction, but is usually implemented in an MSC.

The routing information that is returned to the GMSC is the Mobile Station Roaming Number (MSRN),which is also defined by the E.164 numbering plan. MSRNs are related to the geographical numberingplan, and not assigned to subscribers, nor are they visible to subscribers.

The most general routing procedure begins with the GMSC querying the called subscriber's HLR for anMSRN. The HLR typically stores only the SS7 address of the subscriber's current VLR, and does nothave the MSRN (see the location updating section). The HLR must therefore query the subscriber'scurrent VLR, which will temporarily allocate an MSRN from its pool for the call. This MSRN is returnedto the HLR and back to the GMSC, which can then route the call to the new MSC. At the new MSC, theIMSI corresponding to the MSRN is looked up, and the mobile is paged in its current location area (seeFigure 4).

Figure 4. Call routing for a mobile terminating call

Conclusion and commentsIn this paper I have tried to give an overview of the GSM system. As with any overview, and especiallyone covering a standard 6000 pages long, there are many details missing. I believe, however, that I gavethe general flavor of GSM and the philosophy behind its design. It was a monumental task that theoriginal GSM committee undertook, and one that has proven a success, showing that internationalcooperation on such projects between academia, industry, and government can succeed. It is a standardthat ensures interoperability without stifling competition and innovation among suppliers, to the benefit



of the public both in terms of cost and service quality. For example, by using Very Large ScaleIntegration (VLSI) microprocessor technology, many functions of the mobile station can be built on onechipset, resulting in lighter, more compact, and more energy-efficient terminals.

Telecommunications are evolving towards personal communication networks, whose objective can bestated as the availability of all communication services anytime, anywhere, to anyone, by a singleidentity number and a pocketable communication terminal [25]. Having a multitude of incompatiblesystems throughout the world moves us farther away from this ideal. The economies of scale created by aunified system are enough to justify its implementation, not to mention the convenience to people ofcarrying just one communication terminal anywhere they go, regardless of national boundaries.

The GSM system, and its sibling systems operating at 1.8 GHz (called DCS1800) and 1.9 GHz (calledGSM1900 or PCS1900, and operating in North America), are a first approach at a true personalcommunication system. The SIM card is a novel approach that implements personal mobility in additionto terminal mobility. Together with international roaming, and support for a variety of services such astelephony, data transfer, fax, Short Message Service, and supplementary services, GSM comes close tofulfilling the requirements for a personal communication system: close enough that it is being used as abasis for the next generation of mobile communication technology in Europe, the Universal MobileTelecommunication System (UMTS).

Another point where GSM has shown its commitment to openness, standards and interoperability is thecompatibility with the Integrated Services Digital Network (ISDN) that is evolving in most industrializedcountries, and Europe in particular (the so-called Euro-ISDN). GSM is also the first system to makeextensive use of the Intelligent Networking concept, in in which services like 800 numbers areconcentrated and handled from a few centralized service centers, instead of being distributed over everyswitch in the country. This is the concept behind the use of the various registers such as the HLR. Inaddition, the signalling between these functional entities uses Signalling System Number 7, aninternational standard already deployed in many countries and specified as the backbone signallingnetwork for ISDN.

GSM is a very complex standard, but that is probably the price that must be paid to achieve the level ofintegrated service and quality offered while subject to the rather severe restrictions imposed by the radioenvironment.

References[1]

Jan A. Audestad. Network aspects of the GSM system. In EUROCON 88, June 1988.

[2]D. M. Balston. The pan-European system: GSM. In D. M. Balston and R.C.V. Macario, editors,Cellular Radio Systems. Artech House, Boston, 1993.

[3]David M. Balston. The pan-European cellular technology. In R.C.V. Macario, editor, Personal andMobile Radio Systems. Peter Peregrinus, London, 1991.

[4]



M. Bezler et al. GSM base station system. Electrical Communication, 2nd Quarter 1993.

[5]David Cheeseman. The pan-European cellular mobile radio system. In R.C.V. Macario, editor,Personal and Mobile Radio Systems. Peter Peregrinus, London, 1991.

[6]C. Déchaux and R. Scheller. What are GSM and DCS. Electrical Communication, 2nd Quarter1993.

[7]M. Feldmann and J. P. Rissen. GSM network systems and overall system integration. ElectricalCommunication, 2nd Quarter 1993.

[8]John M. Griffiths. ISDN Explained: Worldwide Network and Applications Technology. John Wiley&Sons, Chichester, 2nd edition, 1992.

[9]I. Harris. Data in the GSM cellular network. In D. M. Balston and R.C.V. Macario, editors,Cellular Radio Systems. Artech House, Boston, 1993.

[10]I. Harris. Facsimile over cellular radio. In D. M. Balston and R.C.V. Macario, editors, CellularRadio Systems. Artech House, Boston, 1993.

[11]Thomas Haug. Overview of the GSM project. In EUROCON 88, June 1988.

[12]Josef-Franz Huber. Advanced equipment for an advanced network. Telcom Report International,15(3-4), 1992.

[13]Hans Lobensommer and Helmut Mahner. GSM - a European mobile radio standard for the worldmarket. Telcom Report International, 15(3-4), 1992.

[14]Bernard J. T. Mallinder. Specification methodology applied to the GSM system. In EUROCON 88,June 1988.

[15]Seshadri Mohan and Ravi Jain. Two user location strategies for personal communication services.IEEE Personal Communications, 1(1), 1994.

[16]Michel Mouly and Marie-Bernadette Pautet. The GSM System for Mobile Communications.Published by the authors, 1992.

[17]Jon E. Natvig, Stein Hansen, and Jorge de Brito. Speech processing in the pan-European digitalmobile radio system (GSM) - system overview. In IEEE GLOBECOM 1989, November 1989.



[18]Torbjorn Nilsson. Toward a new era in mobile communications. http://193.78.100.33/ (EricssonWWW server).

[19]Moe Rahnema. Overview of the GSM system and protocol architecture. IEEE CommunicationsMagazine, April 1993.

[20]E. H. Schmid and M. Kähler. GSM operation and maintenance. Electrical Communication, 2ndQuarter 1993.

[21]Marko Silventoinen. Personal email, quoted from European Mobile Communications Business andTechnology Report, March 1995, and December 1995.

[22]C. B. Southcott et al. Voice control of the pan-European digital mobile radio system. In IEEEGLOBECOM 1989, November 1989.

[23]P. Vary et al. Speech codec for the European mobile radio system. In IEEE GLOBECOM 1989,November 1989.

[24]C. Watson. Radio equipment for GSM. In D. M. Balston and R.C.V. Macario, editors, CellularRadio Systems. Artech House, Boston, 1993.

[25]Robert G. Winch. Telecommunication Transmission Systems. McGraw-Hill, New York, 1993.

Copyright © John Scourias 1996, 1997Written by John ScouriasLast modified October 14, 1997.




How They Work: GSM Cellphones

Select an area inside this demo cellphone:

SIM ReaderRF UnitCODEC

DSPMemory

LCDKeypad

SpeakerKeypad

Mic

AntennaBattery

Connectors

See also, How A GSM Network Operates

Option International - Inside A GSM Cellphone

http://www.option.com/in-fone.htm (1 of 4) [24/09/1999 13:09:53]

Component Purpose

MicrophoneCaptures your voice for conversion from analogue to digitalmode

Speaker Allows monitoring of remote phone

LCD Display Shows Call, Phone, Signal & Network Info

Keypad Allows access to specific remote phones

Battery + Meter While battery housings on cellphones are standard inputdeigns, some cellphones also have some "batteryprocessing" intelligence built in. For example, they willcheck the charge level to start or stop the charge when thephone is connected to a desktop, car or quick charger andeven automatically discharge the battery for you whennecessary. This is usually linked to the LCD display and toan audible beep to warn you of the battery charge status.

LED Lights Status Information, usuallay Green, white & Red.

Digital Signal Processor The DSP chipset is a critical component. It co-ordinates thevoice, SMS and data/fax features of a cellphone. Itprocesses speech, handles voice activity detection, as wellas discontinuous GSM transmission and reception. Anothersection amplifies the input signal received from themicrophone, while another converts this microphone voicesignal from "analogue" to "digital". The digital conversion isnecessary because the GSM cellular standard is acompletely digital system.

CODEC This DSP's voice processing is done in tandem with highlysophisticated compression technique mediated by the"CODEC" (compressor/decompressor) portion of thecellphone. T



RF Unit The CODEC chipset instantly transfers this "compressed"information to the cellphone’s Radio Frequency (RF) unit.This RF unit, which is essentially the transmit and receivesection of the cellphone, then sends out the voice or datainformation via the cellphone antenna, over the air and onto the nearest cellular base station - and ultimately to yourcall destination.

The incoming voice also travels much the same route,although it is first uncompressed from it’s incoming digitalform into an audible analogue form which is then piped outas sound through the cellphone’s speaker. Thisanalogue-to-digital and digital-to-analogue voice conversionvia the CODEC is done at very high speeds, so that younever really experience any delay between talking and theother person hearing you (and visa versa).

SIM Card Reader When you switch on your phone with a "live" SIM cardinside, the subscriber information on the chip inside theSIM card is read by the SIM card reader and thentransmitted digitally to the network via the RF unit. Thesame route is followed when you hit the Call button (and it’svariants) on the cellphone: the number you’ve inputted isinstantly and digitally transferred to the network forprocessing.

External Connectors At the bottom of most cellphones there is an externalconnector system. You can usually plug in a data/faxadapter, or a battery charger, or a personal hands freedevice, or a car-kit with external antenna connections.You’ll also find many with separate "speaker" and LEDlights that are activated when the phone rings and/or whenthe battery is low. Many phones also have tiny LED lightsunder the keypad that light up when you press a key and/orwhen the phone rings.

On-Board Memory Many cellphones also have a certain amount of on-boardmemory chip capacity available for storing outgoingtelephone numbers, your own telephone number, as wellas incoming and outgoing SMS messages. Some allowcopying between the (limited) memory on the SIM card andthe phone’s own internal memory.



Antenna System Cellphone manufacturers are implementing many weirdand wonderful permutations of antenna system designs.While some are stubby, fixed types, the most predominantdesigns though are those with thin, pull-out steel rods all ofwhom usually fit snugly into a special antenna shaft. Theseantenna designs, be they the stubby or pull-out types, allconform to the same circa 900 MHz frequency transmit andreceive range required by the GSM specification.

See also, How A GSM Network Operates

Back to GSM Resource

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DisclaimerAll trademarks or product names mentioned herein are the property of their respective owners.

Specifications subject to change without notice.This page serves merely as an informational aid &, unless stated, does not imply any endorsement of or by the parties mentioned within.

The above is merely a very rough, schematic interpretation of what is inside a GSM cellphoneThe components will vary from cellphone model/brand to cellphone model/brand.

We make no guarantees for the accuracy of the information contained herein.Copyright © 1998 Option International



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How They Work: GSM Networks

See also, What's Inside a GSM cellphone

GSM 900/DCS 1800 networks use a sophisticated array of digitalequipment to provide you with a seamless, hiss-free connection.

Below are some of the critical components & procedures that allow themto do so:

Component/Procedure Purpose

Option International - How A GSM Cellular Network Operates

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Base Station GSM (Global System for Mobile Communications) uses aseries of radio transmitters called Base Stations (BS) toconnect you and your cellphone to your cellular network.

Each BS is also termed a cell, so named because it covers acertain range within a discrete area (cell).

Base Stations are all interconnected, which is why you canmove from one cell to another - a process called "hand-over"- without (hopefully) losing your connection.

Base Station Controller A set of Base Stations is connected to a particular BaseStation Controller.

MSC The combination of a cellphone and the SIM card creates aspecial digital "signature" - that includes your subscribernumber - which is sent from your cellphone to the nearest BSasking that you as a subscriber of a particular network beallowed to use the network. The request is passed on alongthe network of BSs to the multifaceted heart of a cellularnetwork - the Mobile Switching Center (MSC).

The MSC also routes all your incoming and outgoing calls toand from the fixed-line networks or other cellular networks.

HLR The MSC also contains a critical component called the HomeLocation Register (HLR) which provides the administrativeinformation required to authenticate, register and locate youas a that network's subscriber. Once it’s received your log-onrequest, the HLR immediately checks the special "signature"contained in the request against it’s special subscriberdatabase.

If your subscription is current, the MSC sends a messageback to the phone via the network of BSs that indicates thatyou’re allowed to access the network. The name or code ofthat network will appear on the LCD screen of the cellphone.

Once this network "name" message appears on your phone’sLCD screen, it means you’re connected to the network andable to make and receive calls. The entire log-on processusually takes only a couple of seconds.



Polling At the same time, the HLR also registers which BS yourcellphone is currently connected to, so that when thenetwork’s MSC needs to route an incoming call to yourcellphone number, it will first check the HLR to see whereyou are. Every now and gain, the cellphone will send amessage to the network indicating where it is, a processcalled Polling.

Multiplexing Each BS uses digital techniques to enable a number ofphones to be simultaneously connected to it, as well assimultaneously allowing a number of subscribers to makeand receive calls. This sophisticated digital call-jugglingability is called Multiplexing.

However, the combination of the tracking function and yourunique digital signature allows the MSC to route that call tothe precise BS your cellphone happens to be connected to,and then exclusively to your cellphone - even if a number ofother subscribers are simultaneously connected to that BS.

Hand Over When you "hand-over" to another cell whilst driving, the HLRis automatically updated, and continues to monitor whereexactly it should route your calls should you then move withinrange of to another Vodacom BS. This sophisticated routingprocedure means that out of hundreds of thousands ofsubscribers, only the correct cellphone will ring whennecessary.

VLR When you want to make an outgoing call, another section ofthe MSC called the Visitor Location Register (VLR) checkswhether you are actually allowed to make that call. Forexample, if you are barred for international dialing, amessage to that effect will be generated by the VLR, sentalong the network, and almost instantly back to yourcellphone.

MailBox If you’re unavailable for some reason and your Mailbox hasbeen activated, any incoming voice calls will be transferred tothe Mail system.



SMSC Some VoiceMail systems are linked to a network's SMSCenter (SMSC), a special facility that handles ShortMessages. The SMSC generates the special SMS messagethat notifies you that you have mail waiting in your Mailbox.

SMS messages can be received on your SMS-capablecellphone even while you’re on a voice call. That’s becausethey are sent on a different radio frequency - the GSM datachannel - than voice calls, so that the two never interfere.These sophisticated digital facilities are the reason why GSMis now considered the de facto global cellular standard.

See also, What's Inside a GSM cellphone

Back to GSM Resource

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DisclaimerAll trademarks or product names mentioned herein are the property of their respective owners.

Specifications subject to change without notice.This page serves merely as an informational aid &, unless stated, does not imply any endorsement of or by the parties mentioned within.

We make no guarantees for the accuracy of the information contained herein.Copyright © 1998 Option International



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MobileWorld's GSM Info Page, Information on the GSM system. Last Modified: 6 April 1999

GSM InformationIntroduction

Highlights

GSM Requirements

GSM Features

The GSM Network

Spectrum Efficency

Conclusion

Introduction

The development of GSM started in the early 1980s. It was seen then as the mainstay of the plans for Europe´s mobilecommunication infrastructure for the 1990s. Today, GSM and its DCS 1800 and PCS 1900 versions have spread far beyondWestern Europe with networks installed across all continents.

The story begins in 1982 when the European Conference of Posts and Telecommunications Administrations (CEPT), consistingthen of the telecommunication administrations of twenty six nations made two very significant decisions. The first was to establisha team with the title "Groupe Spéciale Mobile" (hence the term "GSM", which today stands for Global System for MobileCommunications) todevelop a set of common standards for a future pan-European cellular network. The second was to recommend that two blocks offrequencies in the 900 MHz band be set aside for the system.

The CEPT made these decisions in an attempt to solve the problems created by the uncoordinated development of individualnational mobile communication systems using incompatible standards. The impossibility of using the same terminal in differentcountries whilst travelling across Europe was one of these problems; another was the difficulty of establishing a Europe-widemobile communicationsindustry that would be competitive in world markets due to the lack of a sufficiently larger home market with common standards -with its attendant economies of scale.

By 1986 it was clear that some of these analogue cellular networks would run out of capacity by the early 1990s. As a result, adirective was issued for two blocks of frequencies in the 900 MHz band, albeit somewhat smaller than recommended by theCEPT, to be reserved absolutely for a pan-European service to be opened in 1991.

In the meantime the GSM members were making excellent progress with the development of agreed standards. One majordecision was to adopt a digital rather than an analogue system.

The digital system would offer improved spectrum efficiency, better quality transmission and new services with enhanced featuresincluding security. It would also permit the use of Very Large Scale Integration (VLSI) technology which would lead to smaller andcheaper mobiles, including hand held terminals. Finally, a digital approach would complement the development of the IntegratedServices Digital Network (ISDN) with which GSM would have to interface.

GSM initially stood for Group Spécial Mobile, the CEPT (Conference of European Posts & Telegraphs) formed the group todevelop a Pan-European cellular system to replace the many systems already in place in Europe that were all incompatible.

MobileWorld's GSM Information

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The main features of GSM were to be International Roaming ability, good sound quality, small cheap handsets and ability tohandle high volumes of users. GSM was taken over in 1989 by the ETSI (European Telecommunications Standards Institute) andthey finalised the GSM standard in 1990. GSM service started in 1991. It was also renamed this year to Global System for Mobilecommunications (GSM).

Today there are approx 105 countries with GSM networks or planned networks and many more are planned with around 32 millionsubscribers world wide on the 139 networks. This accounts for over 25% of the world's cellular market.

The MoU "Memorandum of Understanding" has over 210 members from 105 countries, this organisation meets ever three to fourmonths to look at new or better implementions to the GSM system.

The MoU has a website that goes into more details at http://www.gsmworld.com.

Highlights

1982 CEPT forms Groupe Spéciale Mobile (GSM) and recommends reservation of frequencies in 900 MHz band for futurepan-European cellular system.

1987 Memorandum of Understanding (MoU) signed in Copenhagen by operators from thirteen European countries.

1992 First commercial GSM networks start to come into service.

1992 First international roaming agreement signed between Telecom Finland´s and Vodafone (UK´s) GSM networks.

1992 Australian operators are first non-European operators to sign the MoU.

1993 Status report: thirty GSM networks in (end) service worldwide with more than one million customers. Seventy MoU membersfrom forty five countries.

1994 Status report: sixty GSM networks in service (end) worldwide with more than four million customers. Over one hundred MoUmembers from sixty countries.

1995 Status report: one hundred and twenty (end) GSM networks in service worldwide with more than twelve million customers.Over one hundred and fifty MoU members from ninety countries.

GSM Requirements

The quality of Voice in the GSM system must be better then that achieved by the 900MHz analogue systems over all theoperating conditions.

●

The system must offer encryption of user information●

The system must operate in the entire frequency band 890-915MHz and 935-960MHz.●

An international standardised signalling system must be used to allow the interconnection of mobile switching centres andlocation registers.

●

Minimise modifications to the existing fixed public networks.●

Design the system so handset costs are minimised●

Handsets must be able to be used in all participating countries●

Maximum flexibility for other services like ISDN●

System should maximise the functions and services available to cater for the special nature of mobile communications.●

GSM Features

Quality

With digital, sound quality is sharp and clear. Background sounds and static are vastly reduced and crossed-line conversationsare also eliminated. In comparison with analogue there are also far fewer dropouts, and overall the quality is more like that of afixed telephone.

Security

Unlike analogue, everything you say and send within the digital network is safe and secure. Some features are user authenticationthat prohibits unauthorised access, encryption key distribution that guarantees the privacy of the call and caller identificationrestrictions that can prevent the delivery of the calling users number to the receiver.

Convenience



http://www.gsmworld.com/

With digital, better technology means better battery life. You get up to twice as much talk time from each battery charge, comparedwith analogue. In addition the digital service allows more calls to be handled at any one time, therefore reducing congestion inareas of dense population and high usage.

Roaming

With digital, you are able to use your mobile phone, and number in other countries around the world who operate a GSM network.Click HERE to view the list of GSM operators around the world. Or you can just take your SIM card and use another GSM phone.Your home carrier must have a roaming agreement in place and must be notified before leaving so that you can be activated inthat country. All you need to do is switch on the phone at your destination and you will automatically log into the network.Dependent on the country you can still use your old SIM, but some countries will require you to get a loan SIM from your carrierbefore going there. This will give you a new number whilst in that country but you can easily set up a diversion to the new numberif need be.

GSM Phase 1 features

Call Forwarding●

All Calls●

No Answer●

Engaged●

Unreachable●

Call Barring●

Outgoing - Bar certain outgoing calls(e.g. ISD)●

Incoming - Bar certain incoming calls (Useful if in another country)●

Global roaming - Visit any other country with GSM and a roaming agreement and use your phone and existing number*(see section on roaming)

●

GSM Phase 2 features

SMS - Short Message Service - Allows you to send text messages too and from phones●

Multi Party Calling - Talk to five other parties as well as yourself at the same time●

Call Holding - Place a call on Hold●

Call Waiting - Notifies you of another call whilst on a call●

Mobile Data Services - Allows handsets to communicate with computers●

Mobile Fax Service - Allows handsets to send, retrieve and receive faxes●

Calling Line Identity Service - This facility allows you to see the telephone number of the incoming caller on our handsetbefore answering

●

Advice of Charge - Allows you to keep track of call costs●

Cell Broadcast - Allows you to subscribe to local news channels●

Mobile Terminating Fax - Another number you are issued with that receives faxes that you can then download to thenearest fax machine.

●

GSM Phase 2 + features

Available by 1998●

Upgrade and improvements to existing services●

Majority of the upgrade concerns data transmission, including bearer services and packet switched data at 64 kbit/s andabove

●

DECT access to GSM●

PMR/Public Access Mobile Radio (PAMR)-like capabilities●

GSM in the local loop●

Virtual Private Networks●

Packet Radio●

SIM enhancements●

Premium rate services (eg Stock prices sent to your phone)●

GSM 96 features

In fact, there is no such thing as GSM 96. In MoU SERG there is a document called SE.03. In SE.03 you find the date for●



implementation of services. The date is 'coded' E in case this is essential at the start of operation of a GSM network.

Services of that kind are: TS11 (basic speech), TS12 (emergency calls/112), SMS MT, Call forwarding/Call barring servicesand data/fax. Then there are E96 services, servrvices to be implemented for roamers before end 1996. The only service inthis section is ODB Phase 2. (ODB=Operator Determined Barring).

E97 is SMS MO (Short Message/Mobile Originated).

The list for E98 is longer. One reason is to put presure on suppliers. Services included are CAMEL (to support PNP as astart), SOR, USSD, HSCSD and GPRS.

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The GSM Network

The GSM Network comprises three parts, Mobile Station (MS) which is similar to a cordless phone with extra features, the BaseTransceiver Station (BTS) that controls the connection with the Mobile Station, the Base Station Controller (BSC) that controlsmultiply Base Transceiver Station's and then the rest of the network covered further below..

Mobile Station (MS)

A Digital Mobile Phone and a SIM card make up the Mobile Station. The SIM (Subscriber Identity Module) is a card that fits intoyour handset and is one of two sizes - either full size (same size as a credit card) or the smaller plug in version. The SIMmicroprocessor is based on a silicon chip which is designed to tolerate temperatures between -25 Degrees Celsius and +70Degrees Celsius, and will also withstand up to 85% humidity. However silicon is fragile and, therefore, if the card is tampered with,physically or electronically, the card will be rendered useless.The SIM contains all of your identification details, such as your IMSI (International Mobile Subscriber Identity. This is a numericstring, where the first 3 digits represent the country where the SIM is from, the next represent the operator in that specificcountry.The other digits represent the subscribers identity in his home-network), phone memories, billing information, SMS textmessages, pin numbers and international roaming information.

A IMEI (International Mobile Equipment Identity) card is the serial number of the GSM phone that is the equivalent of the ESNnumber in a Analogue Phone, this is fixed in the phone and cannot be changed. The SIM card contains a IMSI (InternationalMobile Subscriber Identity) number that identifies the user to the network along with other user and security information.

Base Transceiver Station (BTS)

The Base Transceiver Station consists of a radio transceiver with antenna that covers a single cell. It handles the communicationswith the MS via radio interface.

BTS are all connected together to allow you to move from one cell to another. The antenna can take on various forms, in the UKlamp-posts are being used, but normally it has three directional cells.

Base Station Controller (BSC)

The Base Station Controller manages multiple BTS's. It controls the allocation and release of radio channels and handoversbetween cells.

A series of BTS's are connected to each Base Station Controller, the BSC keeps a eye on each call and decides when to pass thecall off to another BTS and to which one.

The Rest of the Network

Several BSC's are controlled by the Mobile service Switching Centre (MSC), the MSC works with four databases (HLR, VLR, EIRand the AuC) and together they manage the communications between Mobile Station user and the other network types. Each ofthe databases has a separate job, these are as follows

Mobile Switching Centre (MSC)

The Mobile Switching Centre is the interface between the base station system and the switching subsystem of the mobile phonenetwork. Furthermore, the MSC is also the interface between the cellular network and the PSTN. The MSC generates all billingrecords and ensures that all usage is directed to the appropriate account.



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The MSC has a relatively complex task, as unlike a conventional telephone exchange, when GSM subscribers make calls theycould be anywhere within the network.

The MSC must ensure that calls are routed through to those subscribers, wherever they are and wherever they move tothroughout the duration of each cell. This situation becomes even more complex when two mobile subscribers wish to contacteach other from two distant locations.

In order to simplify the subscriber management function, a specific service area is allocated to each MSC. The MSC has to controlthe switching of tariff to and from the subscribers within it's service area which involves the co-ordination of all radio resources andthe inter cell hand-off activities.

Home Location Register (HLR)

The HLR is the central data base for all the subscribers which contains details on the identity of each subscriber, the services towhich they have access and the locations where the subscriber was last registered.

All subscriber administration procedures are communicated to the HLR where the data is stored until it is required by another partof the Public Land Mobile Network (PLMN).

The two key references used to route calls to each subscriber are the International Mobile Subscriber Identity (IMSI) and theMobile Subscriber Integrated Services Digital Network (MSISDN) number.

The IMSI is the unique number allocated to the subscriber which is stored in the Sim Card and is used by the network for internalcommunications. When the Sim Card is inserted into a Mobile Equipment it becomes a Mobile Station.

The MSISDN is the subscriber's mobile number which is linked to the IMSI in the HLR. Incoming calls to a subscriber aretranslated back to the IMSI at the HLR thus enabling them to be delivered to the Mobile Station.

Once the Mobile Station's MSISDN has been used to identify the IMSI, the HLR verifies the subscription records to ensure that thecall can be delivered to the last known location of the Mobile Station.

Visitor's Location Register (VLR)

The VLR is a database that is linked to an MSC and temporarily stares information about each Mobile Station within the areaserved by that MSC.

The information that is temporarily stored in the VLR is sufficient to allow any Mobile Station within that MSC area to make andreceive calls. This includes the Mobile Station's identity, the area in which it was last registered and data pertaining to thesubscriber and any supplementary services that have been selected by the subscriber.

The MSC refers to the VLR each time that a Mobile Station attempts to make a call in order to verify that the request can befulfilled. This process is to establish that no call restrictions or call barring instructions are in place.

Equipment Identity Register (EIR)

The EIR ensures that all Mobile Equipments are valid and authorised to function on the PLMN. Three categories exist on the EIR,a white list, a grey list and a black list.

The white list comprises the IMEI ranges of all the Mobile Equipments that have been approved by any one of the three European,GSM approval centres.

Any Mobile Equipment that appears on the grey list will be allowed to function but will trigger an alert to the network operator. Thisfacility allows the network operator to identify any subscriber that is using a lost or stolen Mobile Equipment.

Mobiles that are lost or stolen can be blacklisted which will prevent them from functioning on the home PLMN or on other PLMNsaround the world.

Central Equipment Identity Register (CEIR)

A central EIR is managed by the MoU Permanent Secretariat in Dublin, Ireland. Every MoU member is committed to linking theirnetwork's EIR to the CEIR by January 1995.

The advantage in having the CEIR concept is that it empowers each network operator to restrict or prevent the operation of anygiven MS throughout all PLMNs that are linked up to the CEIR.



Authentication Centre (AUC)

The authentication centre is used to validate the Sim Card being used by the Mobile Station. Secret information that is held in theAUC and which is also contained within the Sim Card is used to perform a complex mathematical calculation. Authenticationoccurs if the results of these two calculations agree.

SMSC (SMS Centre or Service Centre), the SMSC handled all the SMS messages that are sent. The messages are senton a data channel so you can receive them whilst on a call.

●

GMSC (Gateway MSC), is a gateway switch where the call is directed when setting up a call to a GSM user. The GMSClooks for the subscriber by interrogating the right HLR which then interrogates the VLR and routes the incoming calltowards the MSC where the subscriber can be reached.

●

Spectrum Efficiency

The frequency bands allocated are 890-915MHz and 935-960MHz. Half is used for transmitting and the other half is used forreceiving. To allow maximum number of users access, each band is subdivided into 124 carrier frequencies spaced 200KHz apart,using FDMA techniques. By applying TDMA techniques, each of these carrier frequencies is further subdivided into time slotswhich provide each user with the carrier frequency for approximately 0.577ms. This equates to approx 217 jumps per second, butamongst a very small frequency range so encryption is a must for proper security of calls. In fact it is not exactly that, it is hopping13 times every 60 ms, which gives 13/0.06 per second. 0.577ms = 13 frames/60 ms /8 time slots There is also an extension bandof 15 MHz in both directions. There is also DCS 1800 which is equivalent to GSM but at 1800 MHz and the USA will use the 1900MHz band for what they call the PCS (which is either CDMA or GSM like).

The application of speech coding, frequency hopping, channel coding, power control, discontinuous transmission and modulationscheme assists the high level of spectrum Efficiency.

Speech Coding - The algorithm used is RPELPC which converts the digitally converted analog speech.The LPC (long termprediction), is very important as this allows an efficient speech extrapolation when a speech block has not been receivedproperly, the system can accept up to 2% of missing speech block without audible effect.

●

Frequency Hopping - The transmit and receive carrier frequencies are dynamically assigned. This avoids collisionsbetween frequencies.What avoids the collision is the fact that all the mobiles of a cell, even when they use the 'random'frequency hopping, do it in such a way that they never use the same frequency at the same time. In fact, the Frequencyhopping gives both the frequency diversity (protection against the fading on one frequency, better effect with cyclic hopping)and the interfere diversity (protection against user mobiles using the same frequency, better effect with random hopping).

●

Channel Coding - Channel coding is used to achieve very reliable communication with error correction. A unique feature ofthe GSM is to provide a different protection of the speech bits, according to their importance, most important bits areprotected, less important bits are not protected at all. This is a difference with the Qualcom CDMA which offers the sameprotection of all the bits, it is simpler to realise but it is a waste of bandwidth. This does not apply to Data Calls which offersprotection to all the bits.

●

Power Control - Power output is controlled dependent upon signal strength so that is good signal areas the phone uses less●



power.

Discontinuous Transmission - During transmission silence which occurs 60% of the time during a conversation thetransmitter is turned off. This allows reducing the radiated power, hence reducing interferences and of course saves thebatteries.

●

Modulation Scheme - GMSK is used to modulate the digital signal to minimise co-channel interference. (GMSK hasprobably be chosen because it did not require linear amplifier (one of the most beautiful error of GSM, the amplifier *must*be linear), and because of its spectral efficiency (2.70 bit/Hertz))

●

PowerClass

GSM 900Maximum output power

DCS 1800Maximum output power

Tolerance (dB)normal

Tolerance (dB)extreme

1 - 1 (30 dBM) 2 2.52 8 (39 dBM) .25W (24 dBM) 2 2.53 5 (37 dBM) 4W (36 dBM) 2 2.54 2 (33 dBM) - 2 2.55 .8 (29 dBM) - 2 2.5

NOTE: The lowest power control level for all classes of GSM 900 MS is 19 (5 dBm) and for all classes of DCS 1800 MS is 15 (0dBm)

GSM900

PowerOutput


Tolerance (db)extreme

DCS1800

PowerOutput


Tolerance (db)extreme

0 - - - 29 36 2 2.51 - - - 30 34 3 42 39 2 2.5 31 32 3 43 37 3 4 0 30 3 44 35 3 4 1 28 3 45 33 3 4 2 26 3 46 31 3 4 3 24 3 47 29 3 4 4 22 3 48 27 3 4 5 20 3 49 25 3 4 6 18 3 410 23 3 4 7 16 3 411 21 3 4 8 14 3 412 19 3 4 9 12 4 513 17 3 4 10 10 4 514 15 3 4 11 8 4 515 13 3 4 12 6 4 516 11 5 6 13 4 4 517 9 5 6 14 2 5 618 7 5 6 15 0 5 619 5 5 6

Conclusion

The truly international standard of GSM has been crucial to it's success worldwide. This has created a large market for mobileequipment and promotes widespread competition amongst manufacturers and cheaper prices for phones and network equipment.

GSM also has all the features above in GSM Features and many more will be added as the networks develop more.

GSM has developed to the point where it will be used in all countries (including Japan and America), to facilitate easy roaming andthe ultimate in convient personal communications. Countries like America have networks now based on GSM but instead of in the900MHz are they operate at 1800MHz and 1900MHz.

Reference: Various Web Sites and M. Mouly (Michel Mouly) and M.B. Pauter (Marie Bernadette Pauter): The GSM system for Mobile Communications:published by them in1992 and a big thank you to Pierre Dupuy, Eric Tholome and Ron Peereboom.






Javier Gozálvez Semperehome.scarlet.be/~pc030062/extra_info/GSM.pdf · 5.3.3 Interleaving 5.3.3.1...

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Transcript of Javier Gozálvez Semperehome.scarlet.be/~pc030062/extra_info/GSM.pdf · 5.3.3 Interleaving 5.3.3.1...