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 Group Special 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 criteria:
 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.
In 1989 the responsibility 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 inter-working between the different elements of the GSM system. In 1990, the phase I of the GSM specifications was 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
1982 CEPT establishes a GSM group in order to develop the standards for a pan-
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European cellular mobile system
1985 Adoption of a list of recommendations to be generated by the group
1986
1987
Field tests were performed in order to test the different radio techniques proposed for the air interface
TDMA is chosen as access method (in fact, it will be used with FDMA)
Initial Memorandum of Understanding (MoU) signed by telecommunication operators (representing 12 countries)
1988 Validation of the GSM system
1989 The responsibility 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.
GSM networks are operational 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.
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>
Even though there are advantages in introducing wireless connectivity in Subscriber’s loop, we have to tackle certain issues Viz,
1.
Duplexing methodology.
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FDD, TDD
2.
Multiple Access methods.
FDMA, TDMA, CDMA, FDMA-CDMA
3.
cellular principle or reuse concept.
Even though multiple access techniques allowed multiple users to share the medium simultaneously, due to constraints in providing resources, an amount of blocking will exist.
Why Cellular?
Assuming 30mE traffic per subscriber, sub density of 30 per sq.km, and GOS 1%
Radius Area (KM2) Subs Total Traffic RF Channels
1
3
3.14
28.03
100
900
3.0E
27E
8
38
10 3.14 10000 300E 360
Providing 360 RF channels for 10,000 subscribers in an area of 314 sq.km on a single base station is not feasible and if still either the area of coverage or sub density increases, the system cannot function at all for want of bandwidth.
Hence the solution is dividing the service area into small units, called cell, with base stations radiating with low power, and limited number of carriers required as per traffic. The same carriers are again reused at a different cell, which is geographically separated. (Frequency
Reuse)
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In case of CDMA it appears that there is no limitation for simultaneous calls but practically there is a limit to CDMA capacity. And it is essentially the amount of interference a CDMA receiver can tolerate. As more and more units transmit, the amount of noise a receiver sees goes up, since all signals not using the receiver's specific PN code appear as noise. At some point there is so much noise that the receiver can no longer hear the transmitter. Boosting the transmitter power won't help overall, since it increases the noise for all the other receivers, who would in turn tell their transmitters to boost power, and the situation remains. In a nutshell, if a unit near a base station is transmitting with too much power, signals from units far from the base station will be lost in the noise.
Hence cellular concept is applicable even in the case of CDMA where code used for identification of cell/sector is reused.
Advantages of Cellular Principle
Base stations can transmit at low power compared to a single high power transmitter.
It requires less RF bandwidth to cover a given area. Frequency reuse gives good spectrum efficiency. (FDMA-TDMA)
Disadvantage of cellular principle
Reuse introduces interference.
Established calls should be handed over to next cell to avoid dropping of calls when the customer is in mobility.
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.
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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.
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.
The density of population in a country is so varied that different types of cells are used:
3.2.3.1
Macro cells
The macro cells are large cells for remote and sparsely populated areas
3.2.3.2
Micro cells
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.
3.2.3.3
Selective cells
It is not always useful to define a cell with a full coverage of 360 degrees. In some cases, cells with a particular shape and coverage are needed. These cells are called selective cells.
Typical examples of selective cells are the cells that may be located at the entrances of tunnels where coverage of 360 degrees is not needed. In this case, a selective cell with coverage of 120 degrees is used.
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3.2.3.4
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 micro cells. The power level inside an umbrella cell is increased comparing to the power levels used in the micro cells 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.
All mobile techniques incorporate some special features to overcome the hazards created by mobile environment. The following are a few to name:

Coding.( Speech coding, Convolution coding or Forward Error Correction coding, Interleaving)
No. Technology Bit rate per voice chl
Voice coding technique
1
2
3
GSM
CDMA IS95A
Cor-DECT
13Kbps
9.6Kbps/14.4
Kbps
32Kbps
RPE-LTP
QCELP/EVRC
ADPCM

Diversity techniques.
(Space Diversity, Polarisation Diversity, Frequency Diversity and Time
Diversity.)

Adaptive equalization( in case of GSM)

Rake Receiver (in case of CDMA)
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.
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The quality of the service can be considerably improved using a digital technology rather than an analog one.
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1 MSC=16 BSC
1 BSC=1024 TRU
HLR
B
T
S
B
T
S
C
MSC
VLR

B
T
S
MSC
VLR
BSC:BASE STATION CONTROLLER, BTS: BASE TRANSRECEIVER STATION, OSS: OPERATION AND SUPPORT
SUBSYSTEM.ss
Network Entities in GSM Architecture & Their Functions
The MS includes radio equipment and the man machine interface (MMI) that a subscribe needs in order to access the services provided by the GSM PLMN. MS can be installed in Vehicles or can be portable or handheld stations. The MS may include provisions for data communication as well as voice. A mobile transmits and receives message to and from the GSM system over the air interface to establish and continue connections through the system .
Different type of MSs can provide different type of data interfaces. To provide a common model for describing these different MS configuration, ”reference configuration” for MS, similar to those defined for ISDN land stations, has been defined.
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Each MS is identified by an IMEI that is permanently stored in the mobile unit.
Upon request, the MS sends this number over the signaling channel to the MSC.
The IMEI can be used to identify mobile units that are reported stolen or operating incorrectly.
Just as the IMEI identities the mobile equipment, other numbers are used to identity the mobile subscriber. Different subscriber identities are used in different phases of call setup. The Mobile Subscriber ISDN Number (MSISDN) is the number that the calling party dials in order to reach the subscriber. It is used by the land network to route calls toward an appropriate MSC. The international mobile subscribe identity (IMSI) is the primary function of the subscriber within the mobile network and is permanently assigned to him. The GSM system can also assign a
Temporary Mobile Subscriber Identity (TMSI) to identity a mobile. This number can be periodically changed by the system and protect the subscriber from being identified by those attempting to monitor the radio channel.
Functions of MS
The primary functions of MS are to transmit and receive voice and data over the air interface of the GSM system. MS performs the signal processing function of digitizing, encoding, error protecting, encrypting, and modulating the transmitted signals. It also performs the inverse functions on the received signals from the BS.
In order to transmit voice and data signals, the mobile must be in synchronization with the system so that the messages are the transmitted and received by the mobile at the correct instant. To achieve this, the MS automatically tunes and synchronizes to the frequency and TDMA timeslot specified by the BSC. This message is received over a dedicated timeslot several times within a multiframe period of 51 frames. We shall discuss the details of this in the next chapter. The exact synchronization will also include adjusting the timing advance to compensate for varying distance of the mobile from the BTS.
The MS monitors the power level and signal quality, determined by the BER for known receiver bit sequences (synchronization sequence), from both its current BTS and up to six surrounding BTSs. This data is received on the downlink broadcast control channel. The MS determines and send to the current
BTS a list of the six best-received BTS signals. The measurement results from
MS on downlink quality and surrounding BTS signal levels are sent to BSC and processed within the BSC. The system then uses this list for best cell handover decisions.
MS keeps the GSM network informed of its location during both national and international roaming, even when it is inactive. This enables the System to page in its present LA.
The MS includes an equalizer that compensates for multi-path distortion on the received signal. This reduces inter-symbol interface that would otherwise degrade the BER.
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Finally, the MS can store and display short received alphanumeric messages on the liquid crystal display (LCD) that is used to show call dialing and status information. These messages are limited to 160 characters in length.
Power Levels
These are five different categories of mobile telephone units specified by the
European GSM system: 20W, 8W, 5W, 2W, and 0.8W. These correspond to
43-dBm, 39-dBm, 37-dBm, 33-dBm, and 29-dBm power levels. The 20-W and
8-W units (peak power) are either for vehicle-mounted or portable station use.
The MS power is adjustable in 2-dB steps from its nominal value down to 20mW (13 dBm). This is done automatically under remote control from the
BTS, which monitors the received power and adjusts the MS transmitter to the minimum power setting necessary for reliable transmission.
SIM Card

As described in the first chapter, GSM subscribers are provided with a SIM card with its unique identification at the very beginning of the service. By divorcing the subscriber ID from the equipment ID, the subscriber may never own the GSM mobile equipment set. The subscriber is identified in the system when he inserts the SIM card in the mobile equipment. This provides an enormous amount of flexibility to the subscribers since they can now use any GSM-specified mobile equipment. Thus with a SIM card the idea of “Personalize” the equipment currently in use and the respective information used by the network (location information) needs to be updated. The smart card
SIM is portable between Mobile Equipment (ME) units. The user only needs to take his smart card on a trip. He can then rent a ME unit at the destination, even in another country, and insert his own SIM. Any calls he makes will be charged to his home GSM account. Also, the
GSM system will be able to reach him at the ME unit he is currently using.
The SIM is a removable SC, the size of a credit card, and contains an integrated circuit chip with a microprocessor, random access memory (RAM), and read only memory (ROM). It is inserted in the MS unit by the subscriber when he or she wants to use the MS to make or receive a call. As stated, a SIM also comes in a modular from that can be mounted in the subscriber’s equipment.
When a mobile subscriber wants to use the system, he or she mounts their
SIM card and provide their Personal Identification Number(PIN), which is compared with a PIN stored within the SIM. If the user enters three incorrect PIN codes, the SIM is disabled. The PIN can also be permanently bypassed by the service provider if requested by the subscriber. Disabling the PIN code simplifies
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the call setup but reduces the protection of the user’s account in the event of a stolen SIM.
International Mobile Subscriber Identity.
An IMSI is assigned to each authorized GSM user. It consists of a mobile country code (MSC), mobile network code (MNC), and a PLMN unique mobile subscriber identification number (MSIN). The IMSI is not hardware-specific. Instead, it is maintained on a SC by an authorized subscriber and is the only absolute identity that a subscriber has within the GSM system. The IMSI consists of the MCC followed by the NMSI and shall not exceed 15 digits.
3.5 TEMPORARY MOBILE SUBSCRIBER IDENTITY
A TMSI is a MSC-VLR specific alias that is designed to maintain user confidentiality.
It is assigned only after successful subscriber authentication. The correlation of a
TMSI to an IMSI only occurs during a mobile subscriber’s initial transaction with an
MSC (for example, location updating). Under certain condition (such as traffic system disruption and malfunctioning of the system), the MSC can direct individual
TMSIs to provide the MSC with their IMSI.
3.6 MOBILE STATION ISDN NUMBER
The MS international number must be dialed after the international prefix in order to obtain a mobile subscriber in another country. The MSISDN numbers is composed of the country code (CC) followed by the National Significant Number (N(S)N), which shall not exceed 15 digits.
The Mobile Station Roaming Number (MSRN)
The MSRN is allocated on temporary basis when the MS roams into another numbering area. The MSRN number is used by the HLR for rerouting calls to the
MS. It is assigned upon demand by the HLR on a per-call basis. The MSRN for
PSTN/ISDN routing shall have the same structure as international ISDN numbers in the area in which the MSRN is allocated. The HLR knows in what MSC/VLR service area the subscriber is located. At the reception of the MSRN, HLR sends it to the
GMSC, which can now route the call to the MSC/VLR exchange where the called subscriber is currently registered.
3.7 INTERNATIONAL MOBILE EQUIPMENT IDENTITY
The IMEI is the unique identity of the equipment used by a subscriber by each
PLMN and is used to determine authorized (white), unauthorized (black), and malfunctioning (gray) GSM hardware. In conjunction with the IMSI, it is used to
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ensure that only authorized usera are granted access to the system. An IMEI is never sent in cipher mode by MS.
The BSS is a set of BS equipment (such as transceivers and controllers) that is in view by the MSC through a single A interface as being the entity responsible for communicating with MSs in a certain area. The radio equipment of a BSS may be composed of one or more cells. A BSS may consist of one or more BS. The interface between BSC and BTS is designed as an A-bis interface. The BSS includes two types of machines: the BTS in contact with the MSs through the radio interface and the BSC, the latter being in contact with the MSC. The function split is basically between transmission equipment, the BTS, and managing equipment at the BSC.
A BTS compares radio transmission and reception devices, up to and including the antennas, and also all the signal processing specific to the radio interface. A single transceiver within BTS supports eight basic radio channels of the same TDM frame.
A BSC is a network component in the PLMN that function for control of one or more
BTS. It is a functional entity that handles common control functions within a BTS.
A BTS is a network component that serves one cell and is controlled by a
BSC. BTS is typically able to handle three to five radio carries, carrying between 24 and 40 simultaneous communication. Reducing the BTS volume is important to keeping down the cost of the cell sites.
An important component of the BSS that is considered in the GSM architecture as a part of the BTS is the Transcoder/Rate Adapter Unit (TRAU). The
TRAU is the equipment in which coding and decoding is carried out as well as rate adoption in case of data. Although the specifications consider the TRAU as a subpart of the BTS, it can be sited away from the BTS (at MSC), and even between the BSC and the MSC.
The interface between the MSC and the BSS is a standardized SS7 interface
(A-interface) that, as stated before, is fully defined in the GSM recommendations.
This allows the system operator to purchase switching equipment from one supplier and radio equipment and the controller from another. The interface between the
BSC and a remote BTS likewise is a standard the A-bis. In splitting the BSS functions between BTS and BSC, the main principle was that only such functions that had to reside close to the radio transmitters/receivers should be placed in BTS.
This will also help reduce the complexity of the BTS.
Functions of BTS

As stated, the primary responsibility of the BTS is to transmit and receive radio signals from a mobile unit over an air interface. To perform this function completely, the signals are encoded, encrypted, multiplexed, modulated, and
12

then fed to the antenna system at the cell site. Trans-coding to bring 13-kbps speech to a standard data rate of 16 kbps and then combining four of these signals to 64 kbps is essentially a part of BTS, though, it can be done at BSC or at MSC. The voice communication can be either at a full or half rate over logical speech channel. In order to keep the mobile synchronized, BTS transmits frequency and time synchronization signals over frequency correction channel
(FCCH and BCCH logical channels. The received signal from the mobile is decoded, decrypted, and equalized for channel impairments.
Random access detection is made by BTS, which then sends the message to
BSC. The channel subsequent assignment is made by BSC. Timing advance is determined by BTS. BTS signals the mobile for proper timing adjustment. Uplink radio channel measurement corresponding to the downlink measurements made by MS has to be made by BTS.
3.8 BTS-BSC CONFIGURATIONS
There are several BTS-BSC configurations: single site; single cell; single site; multicell; and multisite, multicell. These configurations are chosen based on the rular or urban application. These configurations make the GSM system economical since the operation has options to adapt the best layout based on the traffic requirement.
Thus, in some sense, system optimization is possible by the proper choice of the configuration. These include omni directional rural configuration where the BSC and
BTS are on the same site; chain and multidrop loop configuration in which several
BTSs are controlled by a single remote BSC with a chain or ring connection topology; rural star configuration in which several BTSs are connected by individual lines to the same BSC; and sectorized urban configuration in which three BTSs share the same site amd are controlled by either a collocated or remote BSC.
In rural areas, most BSs are installed to provide maximum coverage rather then maximum capacity.
Transcoder
Depending on the relative costs of a transmission plant for a particular cellular operator, there may be some benefit, for larger cells and certain network topologies, in having the transcoder either at the BTS, BSC or MSC location. If the trascoder is located at MSC, they are still considered functionally a part of the BSS. This approach allows for the maximum of flexibility and innovation in optimizing the transmission between MSC and BTS.
The transcoder is the device that takes 13-Kbps speech or 3.6/6/12-Kbps data multiplexes and four of them to convert into standard 64-Kbps data. First, the
13 Kbps or the data at 3.6/6/12 Kbps are brought up to the level of 16 Kpbs by inserting additional synchronizing data to make up the difference between a 13-Kbps speech or lower rate data, and then four of them are combined in the transcoder to provide 64 Kpbs channel within the BSS. Four traffic channel can then be multiplexed on one 64-Kpbs circuit. Thus, the TRAU output data rate is 64 Kpbs.
Then, up to 30 such 64-Kpbs channels are multiplexed onto a 2.048 Mpbs if a
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CEPT1 channel is provided on the A-bis interface. This channel can carry up to 120-
(16x 120) traffic and control signals. Since the data rate to the PSTN is normally at 2
Mbps, which is the result of combining 30-Kbps by 64-Kbph channels, or 120- Kbps by 16-Kpbs channels.
B) BSC
The BSC, as discussed, is connected to the MSC on one side and to the BTS on the other. The BSC performs the Radio Resource (RR) management for the cells under its control. It assigns and release frequencies and timeslots for all MSs in its own area. The BSC performs the intercell handover for MSs moving between BTS in its control. It also reallocates frequencies to the BTSs in its area to meet locally heavy demands during peak hours or on special events. The BSC controls the power transmission of both BSSs and MSs in its area. The minimum power level for a mobile unit is broadcast over the BCCH. The BSC provides the time and frequency synchronization reference signals broadcast by its BTSs. The BSC also measures the time delay of received MS signals relative to the BTS clock. If the received MS signal is not centered in its assigned timeslot at the BTS, The BSC can direct the
BTS to notify the MS to advance the timing such that proper synchronization takes place. The functions of BSC are as follows.
The BSC may also perform traffic concentration to reduce the number of transmission lines from the BSC to its BTSs, as discussed in the last section.
The network and the switching subsystem together include the main switching functions of GSM as well as the databases needed for subscriber data and mobility management (VLR). The main role of the MSC is to manage the communications between the GSM users and other telecommunication network users. The basic switching function of performed by the MSC, whose main function is to coordinate setting up calls to and from GSM users. The MSC has interface with the BSS on one side (through which MSC VLR is in contact with GSM users) and the external networks on the other (ISDN/PSTN/PSPDN). The main difference between a MSC and an exchange in a fixed network is that the MSC has to take into account the impact of the allocation of RRs and the mobile nature of the subscribers and has to perform, in addition, at least, activities required for the location registration and handover.
The MSC is a telephony switch that performs all the switching functions for MSs located in a geographical area as the MSC area. The MSC must also handle different types of numbers and identities related to the same MS and contained in different registers: IMSI, TMSI,ISDN number, and MSRN. In general identities are used in the interface between the MSC and the MS, while numbers are used in the fixed part of the network, such as, for routing.
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FUNCTIONS OF MSC
As stated, the main function of the MSC is to coordinate the set up of calls between GSM mobile and PSTN users. Specifically, it performs functions such as paging, resource allocation, location registration, and encryption.
Specifically, the call-handling function of paging is controlled by MSC. MSC coordinates the set up of call to and from all GSM subscribers operating in its areas.
The dynamics allocation of access resources is done in coordination with the BSS.
More specifically, the MSC decides when and which types of channels should be assigned to which MS. The channel identity and related radio parameters are the responsibility of the BSS, The MSC provides the control of interworking with different networks. It is transparent for the subscriber authentication procedure. The MSC supervises the connection transfer between different BSSs for MSs, with an active call, moving from one call to another. This is ensured if the two BSSs are connected to the same MSC but also when they are not . In this latter case the procedure is more complex, since more then one MSC in involved. The MSC performs billing on calls for all subscribers based in its areas. When the subscriber is roaming elsewhere, the MSC obtains data for the call billing from the visited MSC. Encryption parameters transfers from VLR to BSS to facilitate ciphering on the radio interface are done by MSC. The exchange of signaling information on the various interface toward the other network elements and the management of the interface themselves are all controlled by the MSC. Finally, the MSC serves as a SMS gateway to forward
SMS messages from Short Message Service Centers (SMSC) to the subscribers and from the subscribers to the SMSCs. It thus acts as a message mailbox and delivery system.
The VLR is collocated with an MSC. A MS roaming in an MSC area is controlled by the VLR responsible for that area. When a MS appears in a LA, it starts a registration procedure. The MSC for that area notices this registration and transfers to the VLR the identify of the LA where the MS is situated. A VLR may be in charge of one or several MSC LA’s. The VLR constitutes the databases that support the
MSC in the storage and retrieval of the data of subscribers present in its area. When an MS enters the MSC area borders, it signals its arrival to the MSC that stores its identify in the VLR. The information necessary to manage the MS is contained in the
HLR and is transferred to the VLR so that they can be easily retrieved if so required.
DATA STORED IN VLR

The data contained in the VLR and in the HLR are more or less the same.
Nevertheless the data are present in the VLR only as long as the MS is registered in the area related to that VLR. Data associated with the movement of mobile are IMSI, MSISDN, MSRN, and TMSI. The terms permanent and temporary, in this case, are meaningful only during that time interval. Some data are mandatory, others are optional.
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5) HOME LOCATION REGISTER
The HLR is a database that permanently stores data related to a given set of subscribers. The HLR is the reference database for subscriber parameters. Various identification numbers and addresses as well as authentication parameters, services subscribed, and special routing information are stored. Current subscriber status including a subscriber’s temporary roaming number and associated VLR if the mobile is roaming, are maintained.
The HLR provides data needed to route calls to all MS-SIMs home based in its MSC area, even when they are roaming out of area or in other GSM networks.
The HLR provides the current location data needed to support searching for and paging the MS-SIM for incoming calls, wherever the MS-SIM may be. The HLR is responsible for storage and provision of SIM authentication and encryption parameters needed by the MSC where the MS-SIM is operating. It obtains these parameters from the AUC.
The HLR maintains record of which supplementary service each user has subscribed to and provides permission control in granting services. The HLR stores the identification of SMS gateways that have messages for the subscriber under the
SMS until they can be transmitted to the subscriber and receipt is knowledge.
Some data are mandatory, other data are optional. Both the HLR and the
VLR can be implemented in the same equipment in an MSC (collocated). A PLMN may contain one or several HLRs.
The AUC stores information that is necessary to protect communication through the air interface against intrusions, to which the mobile is vulnerable. The legitimacy of the subscriber is established through authentication and ciphering, which protects the user information against unwanted disclosure. Authentication information and ciphering keys are stored in a database within the AUC, which protects the user information against unwanted disclosure and access.
In the authentication procedure, the key Ki is never transmitted to the mobile over the air path, only a random number is sent. In order to gain access to the system, the mobile must provide the correct Signed Response (SRES) in answer to a random number (RAND) generated by AUC.
Also, Ki and the cipher key Kc are never transmitted across the air interface between the BTS and the MS. Only the random challenge and the calculated response are transmitted. Thus, the value of Ki and Kc are kept secure. The cipher key, on the other hand, is transmitted on the SS7 link between the home HLR/AUC and the visited MSC, which is a point of potential vulnerability. On the other hand, the random number and cipher key is supposed to change with each phone call, so finding them on one call will not benefit using them on the next call.
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The HLR is also responsib le for the “authentication” of the subscriber each time he makes or receives a call. The AUC, which actually performs this function, is a separate GSM entity that will often be physically included with the HLR. Being separate, it will use separate processing equipment for the AUC database functions.
EIR is a database that stores the IMEI numbers for all registered ME units. The IMEI uniquely identifies all registered ME. There is generally one EIR per PLMN. It interfaces to the various HLR in the PLMN. The EIR keeps track of all ME units in the PLMN. It maintains various lists of message. The database stores the ME identification and has nothing do with subscriber who is receiving or originating call.
There are three classes of ME that are stored in the database, and each group has different characteristics.

White List: contains those IMEIs that are known to have been assigned to valid MS’s. This is the category of genuine equipment.

Black List: contains IMEIs of mobiles that have been reported stolen.

Gray List: contains IMEIs of mobiles that have problems (for example, faulty software, wrong make of the equipment). This list contains all MEs with faults not important enough for barring.

GSM provided a wide range of data services to its subscribers. The GSM system interface with the various forms of public and private data networks currently available. It is the job of the IWF to provide this interfacing capability.
The IWF, which in essence is a part of MSC, provides the subscriber with access to data rate and protocol conversion facilities so that data can be transmitted between
GSM Data Terminal Equipment (DTE) and a land-line DTE.
EC is used on the PSTN side of the MSC for all voice circuits. The EC is required at the MSC PSTN interface to reduce the effect of GSM delay when the mobile is connected to the PSTN circuit. The total round-trip delay introduced by the GSM system, which is the result of speech encoding, decoding and signal processing, is of the order of 180 ms. Normally this delay would not be an annoying factor to the mobile, except when communicating to PSTN as it requires a two-wire to four-wire hybrid transformer in the circuit. This hybrid is required at the local switching office because the standard local loop is a two-wire circuit. Due to the presence of this hybrid, some of the energy at its four-wire receive side from the mobile is coupled to the four-wire transmit side and thus retransmitted to the mobile. This causes the
17

echo, which does not effect the land subscriber but is an annoying factor to the mobile. The standard EC cancels about 70 ms of delay.
During a normal PSTN (land-to-land call), no echo is apparent because the delay is too short and the land user is unable to distinguish between the echo and the normal telephone “side tones” However, with the GSM round-trip delay added and without the EC, the effect would be irritating to the MS subscriber.
The OMC provides alarm-handling functions to report and log alarms generated by the other network entities. The maintenance personnel at the OMC can define that criticality of the alarm. Maintenance cover both technical and administrative actions to maintain and correct the system operation, or to restore normal operations after a breakdown, in the shortest possible time.
The fault management functions of the OMC allow network devices to be manually or automatically removed from or restored to service. The status of network devices can be checked, and tests and diagnostics on various devices can be invoked. For example, diagnostics may be initiated remotely by the OMC. A mobile call trace facility can also be invoked. The performance management functions included collecting traffic statistics from the GSM network entities and archiving them in disk files or displaying them for analysis. Because a potential to collect large amounts of data exists, maintenance personal can select which of the detailed statistics to be collected based on personal interests and past experience.
As a result of performance analysis, if necessary, an alarm can be set remotely.
The OMC provides system change control for the software revisions and configuration data bases in the network entities or uploaded to the OMC. The OMC also keeps track of the different software versions running on different subsystem of the GSM.
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•
•
•
•
GSM SERVICE AREA: TOTAL AREA SERVED BY THE COMBINATION OF
ALL MEMBER COUNTRIES WHERE A MOBILE CAN BE SERVED.
PLMN SERVICE AREA: IT IS ONE N/W AREA.
•
MSC SERVICE AREA: THERE CAN MANY MSC/VLR IN ONE PLMN AREA.IT
IS ONE MOBILE EXCH. AREA.
•
GMSC: ALL I/C CALLS FOR PLMN N/W WILL BE ROUTED THROUGH
GMSC. IN A GSM/PLMN N/W ALL MOBILE TERMINATED CALLS WILL BE
ROUTED TO A GATEWAY MSC. CALL CONNECTIONS BETWEEN PLMNS ,
OR TO FIXED N/WS MUST BE ROUTED TO A GMSC.THE GMSC CONTAINS
THE INTER WORKING FUNCTIONS TO MAKE THESE CONNECTIONS.
LOCATION AREA
CELLS
In this paragraph, the description of the GSM network is focused on the different functions to fulfill by the network and not on its physical components. In GSM, five main functions can be defined:
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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 transmission 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.
2.
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 responsibilities are:

Channel assignment, change and release.

Handover.

Frequency hopping.

Power-level control.

Discontinuous transmission and reception.

Timing advance.
3.
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. a.
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.
20

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 cancels 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. b.
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.
4.Communication Management (CM)
The CM function is responsible for: o Call control. o Supplementary Services management. o Short Message Services management. a.
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 dials the Mobile Subscriber ISDN (MSISDN) number, which includes:
 a country code
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
 a national destination code identifying the subscriber's operator a code corresponding to the subscriber's HLR
The call is then passed 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. b.
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. c.
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).
5.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 analyzing 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.
1)Trunked Radio System
2) Access Method – FDMA/TDMA
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3)FREQUENCY BANDS
Mobile to BTS

GSM-900 system:- 890Mhz to 915 Mhz

GSM-1800 system:- 1710 Mhz to 1785 Mhz
BTS To Mobile

GSM-900 system:- 935Mhz to 960 Mhz

GSM-1800 system:- 1805 Mhz to 1880 Mhz
4)Chanel/ carrier bandwidth :- 200Khz
5)Number of channels dedfined

GSM-900 system:- 124

GSM-1800 system:-374
6) TDMA Frame:- 8 time slot/ burst Period ( * voice channels , staggered in time are
transmitted over single RF carrier.
7) Voice coding:- RPE-LTP ( regular pulse excited long term prediction )
Bit rate per voice channel:- 13 Kbps.
Net bit rate of 1 carrier :- 270.833 Kbps
22.8Kbps------ Speech -----13kbps bit stream------ FEC
Additional Synchronisation & Signalling Data & guart band
33.9Kbps
3.9 *8 T.S.=270.833
8)Digital Modulation Technique:- Gaussian Minimum Shift Keying
Speech
Codin g
Channel
Coding
Inter
Speech
Decodin g
Channel
23 g
De-Inter

MS to BTS--------Um
BTS to BSC------A-Bis
BSC to BSC------A
Traffic
Channels
Logical
Channels
Control
Channels
Full Rate
Haft Rate
Broadcast
Channels
Common
Control
Channels
Dedicated
Control
Channels
Down
Link
Down
Link
Up
Link
Slow
Fast
TCH/F
TCH/H
BCCH
FCCH
SCH
CBCH
PCH
ACCH
RACH
SACC
H
FACC
H
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It is important to note that all the GSM services were not introduced since the appearance of
GSM but they have been introduced in a regular way. The GSM Memorandum of
Understanding (MoU) defined four 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.
- Telephony (E1® Eh).
- Facsimile group 3 (E1).
- Emergency calls (E1® Eh).
- Teletex.
Short Message Services (E1, E2, A) Using these services, a message of a maximum of 160 alphanumeric characters can be sent to or from a mobile station. If the mobile is powered off, the message is stored. With the SMS Cell Broadcast (SMS-CB), a message of a maximum of
93 characters can 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.
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).
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

Asynchronous PAD (packet-switched, packet assembler/dissembler) access, 300-
9600 bps (E1).
Synchronous dedicated packet data access, 2400-9600 bps (E2).
- Call Forwarding (E1). The subscriber can forward incoming calls to another number if the called mobile is busy (CFB), unreachable (CFNRc) or if there is no reply (CFNRy). Call forwarding can also be applied 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 holds (E2) puts an active call on hold.
- Call Waiting, CW (E2) informs the user, during a conversation, about another incoming call. The user can answer, reject or ignore this incoming call.
- Advice of Charge, AoC (E2) provides the user with 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 the calling 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 directory number he gets if his call is forwarded.
- Connected Line identification Restriction, CoLR (A). It enables the called user to restrict the presentation.
- Operator determined barring (A).Restriction of different services and call types by the operator.
26









 IT SUPPORT QOS
ADDITIONAL NODES USED ARE
1)SGSN : SERVING GPRS SUPPORT NODE (GPRS EQUIVALENT OF MSC)
2)GGSN : GATEWAY GPRS SUPPORT NODE (PROVIDES INTERWORKING
WITH OTHER PACKET SWITCHED N/W)
SGSN & GGSN ARE INTERCONNECTED VIA IP BASED GPRS BACKBONE N/W
27

1
28

.
29

GPRS SERVICES
1. COMMUNICATION : E-MAIL, FAX, INET,
INTRANET ACCESS
2. VAS : INFO SERVICES, GAMES,
E-COMMERCE
3. LOCATION BASED APPLICATION :
TRAFFIC CONDITION, AIR/RAIL
SCHEDULE
4. VERTICAL APPLICATION : FREIGHT DELIVERY, FLEET
MANAGEMENT ,
SALE-FORCE MANAGEMENT.
6. LOCATION SENSITIVE ADVERTISING :
FOR SUB NEAR CINEMA HALL/RESTAURANT RECEIVES FLASHES OF
ADVERTISEMENT
E
D
G
E
•
Next step towards 3G for GSM/GPRS Networks
•
Increased data rated up to 384 Kbps by bundling up to 8 channels of 48 Kbps/channel
• GPRS is based on modulation technique known as GMSK
•
EDGE is based on a new modulation scheme that allows a much higher bit rate across the air-interface called 8PSK modulation.
•
Since 8PSK will be used for UMTS, network operators will be required to introduce this at some stage before migration to 3G.
• Provide 3G services with existing licenses
• New modulation optimized for wireless data services
• Link adaptation: Take highest possible rate
• Covered by existing GSM licenses
• Same channel structure, network infrastructure, frequency planning and protocol as today’s GSM
•
Streaming Applications
• Very high speed downloads
• Corporate Intranet connections
•
Quicker MMS
30

• Video Phone
•
Vertical corporate applications
– Video
Conference, Remote presentations
. FUTURE STANDARD IN WHICH A SINGLE INEXPENSIVE MOBILE TERMINAL CAN
TRUELY PROVIDE COMMUNICATIONS ANY TIME AND ANY WHERE
• PROVISIONING OF THESE SERVICES OVER WIDE RANGE OF USER DENSITIES AND
COVERAGE AREAS.
(In-building , Urban , Sub-urban, Global)
• EFFICIENT USE OF RADIO SPECTRUM CONSISTENT WITH PROVIDING SERVICE AT
ACCEPTABLE COSY.
• IMT-2000 SHALL COVER APPLICATION AREAS PRESENTLY PROVIDED BY SEPERATELY
SYSTEMS I.E CELLULAR, CORDLESS AND PAGING etc
• A HIGH DEGREE OF COMMONALITY OF DESIGN WORLDWIDE
• A MODULAR STRUCTURE WHICH WILL ALLOW THE SYSTEM TO GROW IN SIZE AND
COMPLEXITY
• ITU’s VISION FOR THIRD GENERATION MOBILE SYSTEM
•
SINGLE UNIFIED STANDARD (Data & Multimedia Services)
• ANYWHERE, ANYTIME COMMUNICATION
ACROSS NETWORKS, ACROSS TECHNOLOGIES, SEAMLESS OPERATION USING A
SMALL POCKET TERMINAL WORLDWIDE.
•
HIGH SPEED ACCESS 144KB/S, 384 KB/S & 2MB/S FAST WIRELESS ACCESS TO INTERNET
• FULL MOTION VIDEOPHONE
• TERRESTRIAL & SATELLITE COMPONETS
IMT-2000 will provide..
•Enhanced voice quality, ubiquitous coverage and enable operators to provide service at reasonable cost
• WARC -92 IDENTIFIED 230 MHz GLOBAL SPECTRUM
• 1885 - 2025 MHz & 2110 - 2200 MHz for FPLMTS
• SATELLITE COMPONENTS OF IMT -2000 IDENTIFIED AS: 1980-2010 MHz & 2170- 2200
MHz
••
WARC -92 RESOLVED THAT ADMINISTRATIONS IMPLEMENTING IMT-2000 SHOULD
MAKE SPECTRUM AVAILABLE IN THIS IDENTIFIED BAND.
• WITH THE INTRODUCTION OF MULTIMEDIA SERVICES INCREASED SPECTRUM
REQUIREMENT IS TO BE TAKEN UP WITH WRC-2000
31

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