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

History of the cellular mobile radio and GSM

The 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 the early 1980s, analog cellular telephone systems experienced a very rapid growth in Europe, particularly in Scandinavia and the United Kingdom. Today cellular systems still represent one of the fastest growing telecommunications systems.

But in the beginnings of cellular systems, each country developed its own system, which was an undesirable 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 mobile cellular radio system (the GSM acronym became later the acronym for Global System for Mobile communications). 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 system was 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 European Telecommunications Standards Institute (ETSI). The aim of the GSM specifications is to describe the functionality and the interface for each component of the system, and to provide guidance on the design of the system. These specifications will then standardize the system in order to guarantee the proper interworking between the different elements of the GSM system. In 1990, the phase I of the GSM specifications 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.

From the evolution of GSM, it is clear that GSM is not anymore only a European standard. GSM networks are operationnal or planned in over 80 countries around the world. The rapid and increasing acceptance 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 the different mobile cellular systems developed since the commercial launch of cellular systems.

Cellular systems
 

The cellular structure

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

The concept of cellular systems is the use of low power transmitters in order to enable the efficient reuse of the frequencies. In fact, if the transmitters used are very powerful, the frequencies can not be reused for 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 and this distribution is repeated in all the covering area of an operator. The whole number of radio channels available can then be used in each group of cells that form the covering area of an operator. Frequencies used in a cell will be reused several cells away. The distance between the cells using the same frequency must be sufficient to avoid interference. The frequency reuse will increase considerably the capacity in number 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 the interference with the transmitters of neighboring cells. The interference will not produce any damage to the system if a distance of about 2.5 to 3 times the diameter of a cell is reserved between transmitters. The receiver filters must also be very performant.
• Neighboring cells can not share the same channels. In order to reduce the interference, the frequencies must be reused only within a certain pattern.

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

Cluster

The cells are grouped into clusters. The number of cells in a cluster must be determined so that the cluster can be repeated continuously within the covering area of an operator. The typical clusters contain 4, 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 be therefore increased. However a balance must be found in order to avoid the interference that could occur between neighboring clusters. This interference is produced by the small size of the clusters (the size of the cluster is defined by the number of cells per cluster). The total number of channels per cell depends on the number of available channels and the type of cluster used.

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

   Macrocells

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

   Microcells

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

 
   Selective cells

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

   Umbrella cells

A freeway crossing very small cells produces an important number of handovers among the different small neighboring cells. In order to solve this problem, the concept of umbrella cells is introduced. An umbrella cell covers several microcells. The power level inside an umbrella cell is increased comparing to the power levels used in the microcells that form the umbrella cell. When the speed of the mobile is too 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 to detect its high speed.

The transition from analog to digital technology

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

 
The capacity of the system

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

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

 
Compatibility with other systems such as ISDN

The decision of adopting a digital technology for GSM was made in the course of developing the standard. During the development of GSM, the telecommunications industry converted to digital methods. The ISDN network is an example of this evolution. In order to make GSM compatible with the services 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 future improvements and the change of its own characteristics.
 

Aspects of quality

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

The GSM network  
 

Architecture of the GSM network

The GSM technical specifications define the different entities that form the GSM network by defining their 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.
 

 

   Mobile Station

A Mobile Station consists of two main elements:

• The mobile equipment or terminal.
• The Subscriber Identity Module (SIM).

       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 output power 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 of technologies allows to decrease the maximum allowed power to 0.8 W.

 
       The SIM

The SIM is a smart card that identifies the terminal. By inserting the SIM card into the terminal, the user can 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 the subscriber to the system, the SIM card contains some parameters of the user such as its International Mobile Subscriber Identity (IMSI).

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

 
   The Base Station Subsystem

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

• The Base Transceiver Station (BTS) or Base Station.
• The Base Station Controller (BSC).

     The Base Transceiver Station

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

 
       The Base Station Controller

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

 
   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 store information about the subscribers and to manage their mobility. The different components of the NSS are described below.

 
        The Mobile services Switching Center (MSC)

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

 
       The Gateway Mobile services Switching Center (GMSC)

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

 
        Home Location Register (HLR)

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

       Visitor Location Register (VLR)

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

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

 
       The Authentication Center (AuC)

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

 
       The Equipment Identity Register (EIR)

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

 
       The GSM Interworking Unit (GIWU)

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

 
   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 and monitor 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, has provoked that some of the maintenance tasks are transfered to the BTS. This transfer decreases considerably the costs of the maintenance of the system.

The geographical areas of the GSM network

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

 

 
As it has already been explained a cell, identified by its Cell Global Identity number (CGI), corresponds to the radio coverage of a base transceiver station. A Location Area (LA), identified by its Location Area Identity (LAI) number, is a group of cells served by a single MSC/VLR. A group of location areas under the 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.
 
 

The GSM functions

In this paragraph, the description of the GSM network is focused on the differents functions to fulfil by the 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).

   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. The MS, the BTS and the BSC, among others, are deeply concerned with transmission. But other components, such as the registers HLR, VLR or EIR, are only concerned with the transmission for their signaling needs with other components of the GSM network. Some of the most important aspects of the transmission are described in section 5.

 
    Radio Resources management (RR)

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

The RR is also responsible for the management of the frequency spectrum and the reaction of the network to changing radio environment conditions. Some of the main RR procedures that assure its responsabilities 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, which represents one of the most important responsabilities of the RR, is described.

 
      Handover

The user movements can produce the need to change the channel or cell, specially when the quality of the communication is decreasing. This procedure of changing the resources is called handover. Four different 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 signalling information, the first two types of handovers are managed by the concerned BSC (in this case, the MSC is only notified of the handover).

The mobile station is the active participant in this procedure. In order to perform the handover, the mobile station controls continuously its own signal strengh and the signal strengh of the neighboring cells. The list of cells that must be monitored by the mobile station is given by the base station. The power measurements allow to decide which is the best cell in order to maintain the quality of the communication 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 the increase of the power level has no effect on the quality of the signal. When this happens, a handover is performed.
• The `power budget’ algorithm. This algorithm performs a handover, instead of continuously increasing the power level, in order to obtain a good communication quality.

 
    Mobility Management

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

 
       Location management

When a mobile station is powered on, it performs a location update procedure by indicating its IMSI to the 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 it moves to a new Location Area or a different PLMN. This location updating message is sent to the new MSC/VLR, which gives the location information to the subscriber’s HLR. If the mobile station is authorized in the new MSC/VLR, the subscriber’s HLR cancells the registration of the mobile station with the old MSC/VLR.

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

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

 
       Authentication and security

The authentication procedure involves the SIM card and the Authentication Center. A secret key, stored in the SIM card and the AuC, and a ciphering algorithm called A3 are used in order to verify the authenticity of the user. The mobile station and the AuC compute a SRES using the secret key, the algorithm A3 and a random number generated by the AuC. If the two computed SRES are the same, the subscriber 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 is authorized 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 be described in section 5.

 
   Communication Management (CM)

The CM function is responsible for:

• Call control.
• Supplementary Services management.
• Short Message Services management.

        Call Control (CC)

The CC is responsible for call establishing, maintaining and releasing as well as for selecting the type of service. One of the most important functions of the CC is the call routing. In order to reach a mobile subscriber, 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 the HLR corresponding to a certain MISDN number. The GMSC asks the HLR for information helping to the call routing. The HLR requests this information from the subscriber’s current VLR. This VLR allocates temporarily a Mobile Station Roaming Number (MSRN) for the call. The MSRN number is the information returned by the HLR to the GMSC. Thanks to the MSRN number, the call is routed to subscriber’s current MSC/VLR. In the subscriber’s current LA, the mobile is paged.

 
       Supplementary Services management

The mobile station and the HLR are the only components of the GSM network involved with this function. The different Supplementary Services (SS) to which the users have access are presented in section 6.3.

 
       Short Message Services management

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

• The SMS-GMSC for Mobile Terminating Short Messages (SMS-MT/PP). It has the same role as the GMSC.
• The SMS-IWMSC for Mobile Originating Short Messages (SMS-MO/PP).

    Operation, Administration and Maintenance (OAM)

The OAM function allows the operator to monitor and control the system as well as to modify the configuration of the elements of the system. Not only the OSS is part of the OAM, also the BSS and NSS participate 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. This information 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 to the OAM functions.
• The BSC, in charge of controlling several BTSs, is another example of an OAM function performed outside the OSS.

 
The GSM radio interface

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

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

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

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 base station to the mobile station).

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

Multiple access scheme

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

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