Global Systems for Mobile Communication (GSM)

Global Systems for Mobile Communication (GSM)

ABSTRACT:

GSM (Global System for Mobile Communication) is globally accepted standard for digital cellular communication.

Before GSM network there were public radio networks. They use analog technologies varying from country to country. These analog network did not comply with any uniform standard. Therefore GSM system which is based on digital communication are preferred.

GSM system architecture includes main blocks are MS. BTS, BSC, OMC, MSC, HLR, VLR, EIR, AUC having particular functions.

The GSM network is made up of geographic areas, including cells, location areas, Msc/VLR service areas and public land mobile network areas.

Frequency range of GSM is 1850 to 1990 MHz. Duplex distance is 80MHz, 200KHz channel separation.

Transmission rate of GSM is 270 Kbps. GSM utilizes the time division multiple access (TDMA). IT uses linear predictive coding.

GSM provides two basic subscriber services such as teleservices and data telephony services. With this it also supports some supplementary services that can complement and support both telephony and data services.

A GSM mobile can seamlessly roam nationally and internationally requiring standardized call routine and location updating functions. Within a GSM network different protocol are used. These are basically divided into three layers.

1. Physical layer
2. Data Link layer
3. Network layer.

GSM system have some features which are:

1. Roaming
2. Handover
3. Multiple equalization
4. Discontinuous transmission
5. Discontinuous reception
6. Short Message service
Security is major concern of any mobile system. GSM also supports the moderate level of security.

WIRELESS

            Wireless is an old fashioned term for a radio receiver, referring to its use as a wireless telegraph; now the term is used to describe modern wireless connections such as in cellular network and wireless broadband internet.

            In modern usage wireless is a method of communication that uses low powered radio waves to transmit data between devices. The term refers to communication without cables or cords, chiefly using radio frequency and infrared waves.

            Software and hardware developers are creating smaller computer networks ad-hoc wireless network with protocols such as WiFi and ZigBee.

            In cellular system there are two main competing network technologies, global system of Mobile Communication (GSM) and CDMA. One of the key feature of GSM is the subscriber Identity Module (SIM) commonly known as SIM card.

DEFINITION

            Global system for mobile communication (GSM) is a globally acceptable standard for digital cellular communication. GSM is the name of a standardization group established in 1982 to create a common European mobile system standard that would formulate specifications for a pan European mobile cellular radio system operating at 900 MHz. It is estimated that many countries outside of Europe will join the GSM partnership.

Topics :

1. Introduction: The evolution of mobile telephone
2. GGSM System?
3. The GSM Architecture
4. GSM network areas
5. GSM specifications
6. GSM subscriber services
7. GSM protocols/Interfacing
8. features.
9. Security.

1) Introduction : The evolution of mobile telephone system.        

            Cellular is one to the fastest growing and most demanding telecommunication application. Today it represents a continuously increasing percentage of all new telephone subscriptions around the world. Currently there are more tan 45 millions cellular subscribes worldwide.

            The concept of cellular service is the use of low power transmitters where frequencies can be reused within a geographic area. The idea of cell based mobile radio service was formulated in the United States at Bell labs in the early 1970. However the Nordic countries were the 1^st to introduce cellular services for commercial use with the introduction of the Nordic Mobile telephone (NMT) in 1981.

            Cellular Systems began in the United States with the release of the advanced mobile phone service (AMPS) system in 1983. The AMPS standard was adopted by Asia, Latin America and Oceanic countries, creating the largest potential market in the world for cellular.

            In the early 1980s most mobile telephone systems were analog rather than digital, like today’s newer systems. One challenge facing analog systems was the inability to handle the growing capacity needs in a cost efficient manner. As a result, digital technology was welcomed. The advantages of digital systems over analog systems include ease of signaling, lower switching, and increased ability to meet capacity.

2) GSM :

Throughout the evolution of cellular telecommunications various systems have been developed without the benefit of standardized specifications. This presented many problems directly related to compatibility, especially with the development of digital radio technology. The GSM standard is intended to address these problems.

GSM Overview

Before GSM networks there were public mobile radio networks (cellular). They normally used analog technologies, which varied from country to country and from manufacturer to another. These networks did not comply with any uniform standard. There was no way to use a single mobile phone form one country to another. The speech quality in most networks was not satisfactory.

GSM became popular very quickly because it provided improved speech quality and, through a uniform international standard, made it possible to use a single telephone number and mobile unit around the world. The European Telecommunications Standardization Institute (ETSI) adopted the GSM standard in 1991, and GSM is now used in 135 countries.

The benefits of GSM include :

·         Support for international roaming

·         Distinction between user and device identification

·         Excellent speech quality

·         Wide range of services

·         Interlocking (e.g. with ISDN, DECT)

·         Extensive security features

GSM also stands out form other technologies with its wide range of services :

·         Telephony

·         Asynchronous and synchronous data services ( 2.1/4.8/9.6 dkit/s)

·         Access to packet data network (S.25)

·         Telematic services (SMS, fax, videotext, etc.)

·         Many value-added features (call forwarding, caller ID, voice mailbox)

·         E-mail and Internet connections.

GSM System Architecture

The best way to create a manageable communications system is to divide it into various subgroups that are interconnected using standardized interfaces. A GSM network can be divided into three groups The mobile station (MS), the base station subsystem (BSS) and the network subsystem.

They are characterized as follows :

The Mobile Station (MS) :

            A mobile station may be referred to as a “handset ”,  a ‘mobile”, a “portable terminal” or “mobile equipment” (ME). It also includes a subscriber identity module (SIM) that is normally removable and comes in two sizes. Each SIM card has unique identification number called IMSI (international mobile subscriber identity) In addition, each MS is assigned a unique hardware identification called IMEI (International mobile equipment identity)

            In some of the newer application (data communication in particular) an MS can also be a terminal that acts as a GSM interface, e.g. for a laptop computer. In this new application the MS does not look like normal GSM telephone.

The seemingly low price of a mobile phone can five the (false) impression that the product is not of high quality. Besides providing a transceiver (TRS) for transmission and reception of voice and data, the mobile also performs a number of very demanding tasks such as authentication, handover, encoding and channel encoding.

The base station subsystem (BSS) :

            The base station subsystem (BSS) is made up of the base station controller (BSC) and the base transceiver station (BTS.)

The base transceiver station (BTS) :

GSM uses a series of radio transmitters called BTSs to connect the mobiles to cellular network. Their tasks include channel coding/decoding and encryption/decryption. A BTS is comprised of radio transmitters and receivers, antennas, the interface to the PCM facility, etc. The BTS may contain one or more transceivers to provide the require call handling capacity. A cell site may be omni directional or split into typically three directional cells.

The base station controller  (BSC) :

A group of BTSs are connected to a particular BSC which manages the radio resources for them. Today’s new and intelligent BTSs have taken over many tasks that were previously handled by the BSCs.

The primary function of the BSC is call maintenance. The mobile stations normally send a report of their received signal strength to the BSC every 480 ms. With this information the BSC decides to initiate handovers to other cells, change the BTS transmitter power, etc.

The mobile switching center (MSC) :

Acts like a standard exchange in a fixed network and additionally provides all the functionality needed to handle a mobile subscriber. The signaling between functional entities (registers) in the network subsystem uses signaling system 7 (SS7). If the MSC also has a gateway function for communicating with other networks, it is called Gateway (GMSC).

The home location register (HLR) :

A database used for management of mobile subscribers. It stores the international mobile subscriber identity (IMSI), mobiles station ISDN number (MSISDN) and current visitor location register (VLR) address. The main information stored there concerns the location of each mobile station in order to be able to route calls to the mobile subscribers managed by each HLR. The HLR also maintains the services associated with each MS. One HLR can serve several MSCs.

The visitor location register (VLR) :

Contains the current location of the MS and selected administrative information form the HLR, necessary for call control and provision of the subscribed services, for each mobile currently located in the geographical area controlled by the VLR. AVLR is connected to one MSC and is normally integrated into the MSC’s hardware.

The authentication center (AuC) :

A protected database that holds a copy of the secret key stored in each subscriber’s SIM card, which is used fro authentication and encryption over the radio channel. The AuC provides additional security against fraud. It is normally located close to each HLR within a GSM network.

The equipment identity register (EIR) :

The EIR is a database that contains a list of all valid mobile station equipment within the network, where each mobile station is identified by its international mobile equipment identity (IMEI). The EIR has three databases :

White list : for all known, good IMEIs

Black list : for bad or stolen handsets

Grey list : for handsets/IMEIs that are uncertain

Operation and Maintenance Center (OMC)

            The OMC is a management system that oversees the GSM functional blocks. The OMC assists the network operator in maintaining satisfactory operation of the GSM network. Hardware redundancy and intelligent error detection mechanisms help prevent network down-time. The OMC is responsible for controlling and maintaining the MSC. BSC and BTS. It can be in charge of an entire public land mobile network (PLMN) or just some parts of the PLMN.

GSM Network Areas

The GSM network is made up of geographic areas. As shown in figure. These areas include cells, location areas (Las), MSC/VLR service areas, and public land mobile network (PLMN) areas.

The cell is the area given radio coverage by one base transceiver station. The GSM network identifies each cell via the cell global identity (CGI) number assigned to each cell. The location are is a group of cells. It is the area in which the subscriber is paged. Each LA is served by one or more base station controllers yet only by a single MSC (See figure) Each LA is assigned a location area identity (LAI) number.

An MSC/VLR service area represents the part of the GSM network that is covered by one MSC and which is reachable, as it is registered in the VLR of the MSC (See figure)

The PLMN service area is an area served by one network operator (see figure)

GSM Specifications :

Before looking at the GSM specifications, it is important to understand the following basic terms :

Bandwidth : the range of channel’s limits; the broader the bandwidth, the faster data can

be sent.

Bits per second (bps) : a single on-off pulse of data; eight bits are equivalent to one byte.

Frequency : the number of cycles per units of time; frequency is measured in hertz (Hz)

Kilo (k) : kilo is the designation for 10002; the abbreviation kbps represents 1000 bits per

                 second.

Megahertz (MHz): 1000000 hertz (cycle per second)

Milliseconds (ms) : one thousand of a second

Watt (W) : a measure of power of a transmitter.

Specific for different personal communication services (PCS) systems vary among the different PCS networks. Listed below is a description of the specification and characteristics for GSM.

Frequency band :

The frequency range specified for GSM is 1,850 to 1,990 MHz (mobile station to base station)

Duplex distance :

The duplex distance is 80 Mhz. Duplex distance is the distance between the uplink and downwind frequencies. A channel has two frequencies, 80 MHz apart.

Channel separation :

The separation between adjacent carrier frequencies. In GSM, this is 200 KHz.

Modulation :

Modulation is the process of sending a signal by changing the characteristics of a carrier frequency. This is done in GXM via Gaussian minimum shift keying (GMSK)

Transmission rate : GSM is a digital system with an over the air bit rate of 270 kbps.

Access method :

GSM utilizes the time division multiple access (TDMA)  concept. TDMA is a technique in which several different calls may share the same carrier. Each call is assigned a particular time slot.

Speech coder :

GSM uses linear predictive coding (LPC). The purpose of LPC is to reduce the bit rate. The LPC provides parameters for a filter that mimics the vocal tract. The signal passes through this filter leaving behind a residual signal. Speech is encoded at 13 kbps.

GSM Subscriber Services

There are two basic types of services offered through GSM :

telephony also referred to as teleservices) and data (also referred to as bearer services)

Telephony services are mainly voice services that provide subscribers with the complete capability (including necessary terminal equipment) to communicate with other subscribers. Data services provide the capacity necessary to transmit appropriate data signals between two access points creating an interface to the network. In addition to normal telephony and emergency calling, the following subscriber services are supported by GSM:

Dual-tone multifriquency (DTMF) : DTMF is a tone signaling scheme often used for various control purposes via the telephone network, such as remote control of an answering machine. GSM supports full originating DTMF.

Facsimile group II :

GSM supports CCITT Group 3 facsimile. As standard fax machines are designed to be connected to a telephone using analog signals, a special fax converter connected to the exchange is used in the GSM system. This enables a GSM : connected fax to communicate with any analog fax in the network.

Short message services :

A convenient facility of the GSM network is the short message service. A message consisting of a maximum of 160 alphanumeric characters can be sent to or from a mobile station. This service can be viewed as an advanced form of alphanumeric paging with a number of advantages. If the subscriber’s mobile unit is offered back to the subscriber when the mobile is powered on or has tendered the coverage area of the network. This function ensures that the message will be received.

Cell broadcast :

A variation of the short message service is the cell broadcast facility. A message of a maximum of 93 characters can be broadcast to all mobile subscribers in a certain geographic area. Typical applications include traffic congestion warnings and reports on accidents.

Voice mail :

This service is actually an answering machine within the network, which is controlled by the subscriber. Calls can be forwarded to the subscriber’s voice-mail box and the subscriber checks for messages via a personal security code.

Fax mail :

With this service, the subscriber can receive fax messages at any fax machine. The messages are stored in a service center form which they can be retrieved by the subscriber via a personal security code to the desired fax number.

Supplementary Services

GSM supports a comprehensive set of supplementary services that can complement and support both telephony and data services. Supplementary services are defined by GSM and are characterized as revenue-generating features. A partial listing of

Supplementary services follows :

Call forwarding : This service gives the subscriber the ability to forward incoming calls to another number if the called mobile unit is not reachable, if it is busy, if there is no reply, or if call forwarding is allowed unconditionally.

Barring of outgoing calls : This service makes it possible for a mobile subscriber to prevent all outgoing.

Barring of incoming calls : This function allows the subscriber to prevent incoming calls. The following two conditions for incoming call barring exits : baring of all incoming calls and barring of incoming calls when roaming outside the home PLMN.

Advice of charge (AoC) : The AoC service provides the mobile subscriber with an estimate of the call charges. There are two types of AoC information : One that provides the subscriber with an estimate of the bill and one that can be used for immediate charging purposes. AoC for data calls is provided on the basis of time measurements,

Call hold : This service enables the subscriber to interrupt an ongoing call and then subsequently reestablish the call. The call hold service is only applicable to normal telephony.

Call waiting : This service enables the mobile subscriber to be notified of an incoming call during a conversation. The subscriber can answer, reject, or ignore the incoming call. Call waiting is applicable to all GSM telecommunication services using a circuit switched connection.

Multiparty service : The multiparty service enables a mobile subscriber to establish a multiparty conversation-that is, a simultaneous conversation between three and six subscribers. This service is only applicable to normal telephony.

Calling line identification presentation/restriction : These services supply the called party with the integrated services digital network (ISDN) number of the calling party. The restriction service enables the calling party to restrict the presentation. The restriction overrides the presentation.

Closed user groups (CUGs) : CUGs are generally comparable to a PBS. They are a group of subscribers who are capable of only calling themselves and certain numbers.

Providing voice or data transmission quality over the radio link is only part of the function of cellular mobile network. A GSM mobile can seamlessly roam nationally and internationally, requiring standardized call routing and location updating functions in GSM networks. A public communications system also needs solid security mechanisms to prevent misuse by third parties. Security functions such as authentication, encryption and the use of Temporary Mobile Subscriber Identities (TMSIs) are an absolute must.

            Within a GSM network, different protocols are needed to enable the flow of data and signaling between different GSM subsystems. Figure shows the intervals that link the different GSM subsystems and the protocols used to communicate on each interface.

GSM protocols are basically divided into three layers :

Layer 1 : Physical layer

Enables physical transmission (TDMA, FDMA, etc.)

Assessment of channel quality

Except on the air interface (GSM Rec.04.04) PCM 30 or ISDN links are used (GSM Rec.08.54 on A interface and 08.04 on A to F interface)

Layer 2 : Data link layer

Multiplexing of one or more layer 2 connections on control/signaling channels

Error detection (based on HDLC)

Flow control

Transmission quality assurance

Routing

Layer 3 : Network layer

Connection management (air interface)

Management of location data

Subscriber identification

Management of added services (SMS,  call forwarding, conference calls , etc.)

System Features

The section provides a brief description of the GSM network features :

Roaming  : The roaming feature allows a user to make and receive calls in any GSM network and to use the same user specific services worldwide. This requires a roaming agreement between the individual operators. With worldwide roaming the MS is accessible under the same phone number everywhere.

Handover : In a cellular network, the radio and fixed voice connections are not permanently allocated for the duration of a call. Hanover, or handoff as it is called in North America, means switching and ongoing call to a different channel or cell. The execution and measurements require for handover are a basic function of the RR protocol layer.

Multipath equalization : At the 900 MHz range, radio waves bounce off everything- buildings, hills, cars, airoplanes, etc. Many reflected signals, each with a different phase, can reach an antenna (also known as “multipath propagation”) Equalization is used to extract the desired signal from the unwanted reflections. It works by finding out how a known transmitted signal is 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. The actual implementation of the equalizer is not specified in the GS< specifications.

Discontinuous Transmission (DTX) : To reduce the MS’s power consumption and minimize interference on the air interface, user signal transmission is interrupted during pauses in speech. “Comfort noise” is artificially generated by the MS to avoid disruption due to an abrupt interruption in speech.

Discontinuous Reception (DRS) : Another method used to conserve power at the mobile station is discontinuous reception. The paging channel, used by the base station to signal an incoming call, is structured into sub-channels. Each mobile station 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.

Short Message Service(SMS) : SMS offers message delivery (similar to “two-way-paging”) that is guaranteed to reach the MS. If the GSM telephone is not turned on, the message is held for later delivery. Each time a message is delivered to an MS, the network expects to receive an acknowledgement from this MS that the message was correctly received. Without a positive acknowledgement the network will re-send the message or store it for later delivery. SMS supports messages up to 160 characters in length that can be delivered by any GSM network around the world wherever the MS is able to roam.

Call Waiting(CW) : CW is a network-based feature that must also be supported by the GSM telephone(MS). With CW, GSM users with a call in progress will receive an audible beep to alert them that there is an incoming call for the MS. The incoming call can be accepted, sent to voice mail or rejected. If the incoming call is rejected, the dealer will receive a busy signal. Once the call is accepted, the original call is put on hold to allow a connection to the new incoming call.

Call Hold (CH) : CH must be supported by the MS and the network. It allows the MS to “park” and “in progress call”, to make additional calls or to receive incoming calls.

Call Forwarding(CF) : This is a network-based feature that can be activated by the MS. CF allows calls to be sent to other numbers under conditions defined by the user. Theses conditions can be either unconditional or dependent on certain criteria. (no answer, busy, not reachable)

Calling Line ID : Calling Line ID must be supported by the GSM network and the telephone. The GSM telephone displays the originating telephone number of incoming calls. This feature requires the caller’s network to deliver the calling line ID (telephone no.) to the GSM network.

GSM Security

            GSM was designed with a moderate level of  security. The system was designed to authenticate the subscriber using shared secret cryptography communication between the subscriber and the base station can be encrypted. The development of UMTs introduces an optional USIM, that uses a longer authentication key to five greater security, as well as mutually authenticating the network and the user whereas GSM only authenticated the user to network (can not vise versa). The security model therefore offers confidentiality and authentication but limited authorization capabilities and no reproduction.

            GSM uses several cryptographic algorithms for security The AS/1 stream ciphers are used for ensuring over air voice privacy. A large security advantage of GSM is that the kit, the crypto variable stored on SIM card that is the key to any GSM ciphering algorithms is never send over the air interface.

 CONCLUSION

             In the modern world where faster common is needed the wireless communication systems are best to use. For mobile communication various systems are used in which GSM is most reliable and accepted digital system.