Premier University 1/A, O.R. Nizam Road, Panchlaish, Chittagong, Bangladesh.

Report Name: GSM TECHNOLOGY

Report Submitted toMiss Amina AkterLecturer of Premier UniversityDept. of Computer Science and Engineering

Report Authors Rahul Roy Chowdhury (ID-035120069)Manna Dey ( ID- 035120070)Md Rashed Ali (ID-035120072)Ripan Das ( ID-035120075)

Date of SubmissionJune 20th, 2007

INDEX

1. Introduction

2. History of GSM

2.1 Core technology of GSM

2.1.1 2G technologies

2.1.2 2.5G technologies

2.1.3 3G technologies

2.1.4 4G technologies

3. Radio Interface

4. GSM frequency bands

4.1 GSM-900 and GSM-1800

4.2 GSM-850

4.3 GSM-1900

4.4 GSM-400

5. Network structure and Architectural elements

5.1 Base Station Subsystem of GSM

5.2 Sectorisation

5.3 Base Station Controller

5.4 Transcoder

5.5 Packet Control Unit

5.6 Accessing a GSM network

5.7 Data transmission

5.8 Circuit-switched data protocols

5.9 Authentication and Key generation work in a GSM network

6. Voice calls

6.1 How outgoing calls are made from a mobile

6.2 How incoming calls are made to a mobile 6.2.1 Step One: Contact the Gateway MSC 6.2.2 Step Two: Determine how to route the call

6.2.3 Step Three: Ringing the phone

7. Voice charges

8. How speech is encoded during mobile phone calls9. Some features of GSM

9.1 General Packet Radio Service (GPRS)

9.2 Short Message Service (SMS)

9.3 Supplementary Services

10. Subscriber identity module

11. GSM security

12. GSM vs. CDMA

12.1 The Origins

12.2 Coverage

12.3 Data transfer

12.4 Phone Identification (SIM cards)

12.5 Which technology is the best

13. GSM cell phone Advantages & Disadvantages

13.1 Advantages

13.2 GSM cell phone disadvantages

1. Introduction

The Global System for Mobile Communications (GSM: originally from Groupe Spécial Mobile) is the most popular standard for mobile phones in the world. GSM service is used by over 2 billion people across more than 212 countries and territories.[1][2] The ubiquity of the GSM standard makes international roaming very common between mobile phone operators, enabling subscribers to use their phones in many parts of the world. GSM differs significantly from its predecessors in that both signaling and speech channels are Digital call quality, which means that it is considered a second generation (2G) mobile phone system. This fact has also meant that data communication was built into the system from the 3rd Generation Partnership Project (3GPP).

Fig 1: The GSM logo is used to identify compatible handsets and equipment

From the point of view of the consumers, the key advantage of GSM systems has been higher digital voice quality and low cost alternatives to making calls such as text messaging. The advantage for network operators has been the ability to deploy equipment from different vendors because the open standard allows easy inter-operability.[3] Like other cellular standards GSM allows network operators to offer roaming services which mean subscribers can use their phones all over the world.

As the GSM standard continued to develop, it retained backward compatibility with the original GSM phones; for example, packet data capabilities were added in the Release '97 version of the standard, by means of GPRS. Higher speed data transmission has also been introduced with EDGE in the Release '99 version of the standard.

2. History of GSM

Europeans quickly realized the disadvantages of each European country operating on their mobile network. It prevents cell phone use from country to country within Europe. With the emerging European Union and high travel volume between countries in Europe this was seen as a problem. Rectifying the situation the Conference of European Posts and Telegraphs (CEPT) assembled a research group with intentions of researching the mobile phone system in Europe. This group was called Group Special Mobile (GSM).

For the next ten years the GSM group outlined standards, researched technology and designed a way to implement a pan-European mobile phone network. In 1989 work done by the GSM group was transferred to the European Telecommunication Standards Institute (ETSI). The name GSM was transposed to name the type of service invented. The acronym GSM had been changed from Group Special Mobile to Global Systems Mobile Telecommunications.

The first GSM network was launched in 1991 by Radiolinja in Finland.[4]Just a year and half later in 1993 there were already 36 GSM networks in over 22 countries. Several other countries were on the rise to adopt this new mobile phone network and participate in what was becoming a worldwide standard. At the same time, GSM also became widely used in the Middle East, South Africa and Australia.

While the European Union had developed a sophisticated digital cell phone system, the United States was still operating primarily on the old, analog AMPS network and TDMA. In the end of October 2001, Cingular was the first to announce their switch to the 3G GSM network. This involved switching more then 22 million customers from TDMA to GSM.

In 2005 Cingular stopped new phone activation on the TDMA network and began only selling GSM service.

Most of the world external to the United States uses GSM technology. However, operate on different frequencies then the United States GSM phones.

There are five major GSM frequencies that have become standard worldwide. They include GSM-900, GSM-1800, GSM-850, GSM-1900 and GSM-400.

2.1 Core technology of GSM

2.1.1 2G technologies

2G technologies can be divided into TDMA-based and CDMA-based standards depending on the type of multiplexing used. The main 2G standards are:

• GSM (TDMA-based), originally from Europe but used worldwide (Time Division Multiple Access)

• iDEN (TDMA-based), proprietary network used by Nextel in the United States and Telus Mobility in Canada

• IS-136 aka D-AMPS, (TDMA-based, commonly referred as simply TDMA in the US), used in the Americas

• IS-95 aka cdmaOne, (CDMA-based, commonly referred as simply CDMA in the US), used in the Americas and parts of Asia

• PDC (TDMA-based), used exclusively in Japan

2G services are frequently referred as Personal Communications Service, or PCS, in the United States.

2.5G services enable high-speed data transfer over upgraded existing 2G networks. Beyond 2G, there's 3G, with higher data speeds, and 4G, with even higher data speeds, to enable new services for subscribers, such as picture messaging and video telephony.

2.1.2 2.5G technologies

2.5G is a stepping stone between 2G and 3G cellular wireless technologies. The term "second and a half generation" is used to describe 2G-systems that have implemented a packet switched domain in addition to the circuit switched domain. It does not necessarily provide faster services because bundling of timeslots is used for circuit switched data services (HSCSD) as well.

While the terms "2G" and "3G" are officially defined, "2.5G" is not. It was invented for marketing purposes only.

2.5G provides some of the benefits of 3G (e.g. it is packet-switched) and can use some of the existing 2G infrastructure in GSM and CDMA networks. GPRS is a 2.5G technology used by GSM operators. Some protocols, such as EDGE for GSM and CDMA2000 1x-RTT for CDMA, can qualify as "3G" services (because they have a data rate of above 144 kbit/s), but are considered by most to be 2.5G services (or 2.75G which sounds even more sophisticated) because they are several times slower than "true" 3G services.

2.1.3 3G technologies

3G is third-generation technology in the context of mobile phone standards. The services associated with 3G provide the ability to transfer simultaneously both voice data (a telephone call) and non-voice data (such as downloading information, exchanging email, and instant messaging). In marketing 3G services, video telephony has often been suggested as the killer application for 3G.

Roll-out of 3G networks was delayed in some countries by the enormous costs of additional spectrum licensing fees. In many parts of the world 3G networks do not use the same radio frequencies as 2G, requiring mobile operators to build entirely new networks and license entirely new frequencies; a notable exception is the United States where carriers operate 3G service in the same frequencies as other services. The license fees in some European countries were particularly high, bolstered by initial excitement over 3G's potential. Other delays were as a result of the expenses related to upgrading equipment for the new systems.

The first country that introduced 3G on a large commercial scale was Japan. In 2005, about 40% of subscribers used 3G networks only, with 2G being on the way out. It was expected that the transition from 2G to 3G would be largely completed during 2006, and upgrades to the next 3.5G stage with 3 Mbit/s data rates were under way.

The successful 3G introduction in Japan showed that video telephony was not the killer application for 3G networks after all. The real-life usage of video telephony on 3G networks was found to be a small fraction of all services. On the other hand, downloading of music found strong acceptance by customers. Music download services in Japan were pioneered by KDDI with the EZchakuuta and Chaku Uta Full services.

3G networks are not IEEE 802.11 networks. IEEE 802.11 networks are short range, higher-bandwidth (primarily) data networks, while 3G networks are wide area cellular telephone networks which evolved to incorporate high-speed internet access and video telephony.

2.1.4 4G technologies

4G is short for fourth-generation cellular communication system. There is no set definition to what 4G is, however the features that are predicted for 4G can be summarized in a single sentence:

The 4G will be a fully IP-based integrated system of systems and network of networks achieved after the convergence of wired and wireless networks as well as computer, consumer electronics, communication technology, and several other convergences that will be capable of providing 100 Mbps and 1Gbps, respectively, in outdoor and indoor environments with end-to-end QoS and high security, offering any kind of services anytime, anywhere, at affordable cost and one billing.

The Wireless World Research Forum (WWRF) defines 4G as a network that operates on Internet technology, combines it with other applications and technologies such as Wi-Fi and WiMAX, and runs at speeds ranging from 100 Mbps (in cell-phone networks) to 1 Gbps (in local Wi-Fi networks).[6] 4G is not just one defined technology or standard, but rather a collection of technologies and protocols to enable the highest throughput, lowest cost wireless network possible.[7]

3.Radio Interface

GSM is a cellular network, which means that mobile phones connect to it by searching for cells in the immediate vicinity. GSM networks operate in four different frequency ranges. Most GSM networks operate in the 900 MHz or 1800 MHz bands. Some countries in the Americas (including the United States and Canada) use the 850 MHz and 1900 MHz bands because the 900 and 1800 MHz frequency bands were already allocated.

The rarer 400 and 450 MHz frequency bands are assigned in some countries, notably Scandinavia, where these frequencies were previously used for first-generation systems.

In the 900 MHz band the uplink frequency band is 890-915 MHz, and the downlink frequency band is 935-960 MHz. This 25 MHz bandwidth is subdivided into 124 carrier frequency channels, each spaced 200 kHz apart. Time division multiplexing is used to allow eight full-rate or sixteen half-rate speech channels per radio frequency channel. There are eight radio timeslots (giving eight burst periods) grouped into what is called a TDMA frame. Half rate channels use alternate frames in the same timeslot. The channel data rate is 270.833 kbit/s, and the frame duration is 4.615 ms.

The transmission power in the handset is limited to a maximum of 2 watts in GSM850/900 and 1 watt in GSM1800/1900.

GSM has used a variety of voice codecs to squeeze 3.1kHz audio into between 6 and 13kbps. Originally, two codecs, named after the types of data channel they were allocated, were used, called "Full Rate" (13kbps) and "Half Rate" (6kbps). These used a system based upon linear predictive coding (LPC). In addition to being efficient with bitrates, these codecs also made it easier to identify more important parts of the audio, allowing the air interface layer to prioritize and better protect these parts of the signal.

GSM was further enhanced in 1997[7] with the GSM-EFR codec, a 12.2kbps codec that uses a full rate channel. Finally, with the development of UMTS, EFR was refactored into a variable-rate codec called AMR-Narrowband, which is high quality and robust against interference when used on full rate channels, and less robust but still relatively high quality when used in good radio conditions on half-rate channels.

There are four different cell sizes in a GSM network - macro, micro, pico and umbrella cells. The coverage area of each cell varies according to the implementation environment. Macro cells can be regarded as cells where the base station antenna is installed on a mast or a building above average roof top level. Micro cells are cells whose antenna height is under average roof top level; they are typically used in urban areas. Picocells are small cells whose diameter is a few dozen meters; they are mainly used indoors. Umbrella cells are used to cover shadowed regions of smaller cells and fill in gaps in coverage between those cells.

Cell horizontal radius varies depending on antenna height, antenna gain and propagation conditions from a couple of hundred meters to several tens of kilometers. The longest distance the GSM specification supports in practical use is 35 km or 22 miles. There are also several implementations of the concept of an extended cell, where the cell radius could be double or even more, depending on the antenna system, the type of terrain and the timing advance.

Indoor coverage is also supported by GSM and may be achieved by using an indoor picocell base station, or an indoor repeater with distributed indoor antennas fed through power splitters, to deliver the radio signals from an antenna outdoors to the separate indoor distributed antenna system. These are typically deployed when a lot of call capacity is needed indoors, for example in shopping centers or airports. However, this is not a prerequisite, since indoor coverage is also provided by in-building penetration of the radio signals from nearby cells.

The modulation used in GSM is Gaussian minimum shift keying (GMSK), a kind of continuous-phase frequency shift keying. In GMSK, the signal to be modulated onto the carrier is first smoothed with a Gaussian low-pass filter prior to being fed to a frequency modulator, which greatly reduces the interference to neighboring channels (adjacent channel interference).

A nearby GSM handset is usually the source of the "dit dit dit, dit dit dit, dit dit dit" signal that can be heard from time to time on home stereo systems, televisions, computers, and personal music devices. When these audio devices are in the near field of the GSM handset, the radio signal is strong enough that the solid state amplifiers in the audio chain function as a detector. The clicking noise itself represents the power bursts that carry the TDMA signal. These signals have been known to interfere with other electronic devices, such as car stereos and portable audio players. This is a form of RFI, and could be mitigated or eliminated by use of additional

shielding and/or bypass capacitors in these audio devices[citation needed], however, the increased cost of doing so is difficult for a designer to justify. [5]

4.GSM frequency bands

There are eight frequency bands defined in 3GPP TS 05.05:

• Standard or primary GSM 900 Band, P GSM• GSM 450 Band• GSM 480 Band• GSM 850 Band• Extended GSM 900 Band, E GSM (includes Standard GSM 900 band)• Railways GSM 900 Band, R GSM (includes Standard and Extended GSM 900 band)• DCS 1 800 Band• PCS 1 900 Band

System Band Uplink Downlink Channel NumberGSM 400 450 450.4 - 457.6 460.4 - 467.6 259 - 293GSM 400 480 478.8 - 486.0 488.8 - 496.0 306 - 340GSM 850 850 824.0 - 849.0 869.0 - 894.0 128 - 251GSM 900 (P-GSM) 900 890.0 - 915.0 935.0 - 960.0 1 - 124GSM 900 (E-GSM) 900 880.0 - 915.0 925.0 - 960.0 975 - 1023, (0, 1-124)GSM-R (R-GSM) 900 876.0 - 915.0 921.0 - 960.0 955 - 973, (0, 1-124, 975 - 1023)DCS 1800 1800 1710.0 – 1785.0 1805.0 - 1880.0 512 - 885PCS 1900 1900 1850.0 – 1910.0 1930.0 - 1990.0 512 - 810

Note: The table shows the extents of the band and not center frequency.

4.1 GSM-900 and GSM-1800

GSM-900 and GSM-1800 are used in most parts of the world: Europe, Middle East, Africa and most of Asia.

• GSM-900 uses 890 - 915 MHz to send information from the Mobile Station to the Base Transceiver Station (uplink) and 935 - 960 MHz for the other direction (downlink), providing 124 RF channels (channel numbers 1 to 124) spaced at 200 kHz. Duplex spacing of 45 MHz is used.

In some countries the GSM-900 band has been extended to cover a larger frequency range. This 'extended GSM', E-GSM, uses frequency range 880 - 915 MHz (uplink) and 925 - 960 MHz (downlink), adding 50 channels (channel numbers 975 to 1023 and 0) to the original GSM-900 band. The GSM specifications also describe 'railways GSM',

GSM-R, which uses frequency range 876 - 915 MHz (uplink) and 921 - 960 MHz (downlink). Channel numbers 955 to 1023. GSM-R provides additional channels and specialized services for use by railway personnel.All these variants are included in the GSM-900 specification.

• GSM-1800 uses 1710 - 1785 MHz to send information from the Mobile Station to the Base Transceiver Station (uplink) and 1805 - 1880 MHz for the other direction (downlink), providing 374 channels (channel numbers 512 to 885). Duplex spacing is 95 MHz.

GSM-1800 is also called PCS in Hong Kong and the United Kingdom. Most of the GSM operators in India use the 900 MHz band. Operators like , Airtel, Idea, and some others, use 900MHz in rural areas as well as in urban areas.where as hutch uses 1800mhz everywhere except in its bpl network

4.2 GSM-850

GSM-850 and GSM-1900 are used in the United States, Canada, and many other countries in the Americas. GSM-850 is also sometimes erroneously called GSM-800.

In Australia, GSM 850 is the frequency allocated to Telstra's NextG Network which was switched on in October 2006. The NextG Network is a step up from the 3G Network and is available at faster speeds Australia wide compared to the 3G Network which is limited to only major population centres.

• GSM-850 uses 824 - 849 MHz to send information from the Mobile Station to the Base Transceiver Station (uplink) and 869 - 894 MHz for the other direction (downlink). Channel numbers 128 to 251.

Cellular is the term used to describe the 850 MHz band, as the original analog cellular mobile communication system was allocated in this spectrum. Providers commonly operate in one or both frequency ranges.

4.3 GSM-1900

GSM-850 and GSM-1900 are used in the United States, Canada, and many other countries in the Americas.

• GSM-1900 uses 1850 - 1910 MHz to send information from the Mobile Station to the Base Transceiver Station (uplink) and 1930 - 1990 MHz for the other direction (downlink). Channel numbers 512 to 810.

PCS is an initialism for Personal Communications Service and merely represents the original name in North America for the 1900 MHz band.

4.4 GSM-400

Another less common GSM version is GSM-400. It uses the same frequency as and can co-exist with old analog NMT systems. NMT is a first generation (1G) mobile phone system which was primarily used in Nordic countries, Eastern Europe and Russia prior to the introduction of GSM. It operates in either 450.4 - 457.6 MHz paired with 460.4 - 467.6 MHz (channel numbers 259 to 293), or 478.8 - 486 MHz paired with 488.8 - 496 MHz (channel numbers 306 to 340). There is currently one GSM-400 network in Tanzania

5. Network structure and Architectural elements

The GSM mobile telephony service is based on a series of contiguous radio cells which provide complete coverage of the service area and allow the subscriber operation anywhere within it. Prior to this cellular concept, radiophones were limited to just the one transmitter covering the whole service area. Cellular telephony differs from the radiophone service because instead of one large transmitter, many small ones are used to cover the same area. The basic problem is to handle the situation where a person using the phone in one cell moves out of range of that cell. In the radiophone service there was no solution and the call was lost, which is why the service area was so large. In cellular telephony, handing the call over to the next cell solves the problem. This process is totally automatic and requires no special intervention by the user, but it is a complex technical function requiring significant processing power to achieve a quick reaction.

The functional architecture of a GSM system can be broadly divided into the Mobile Station, the Base Station Subsystem, and the Network Subsystem. Each subsystem is comprised of functional entities that communicate through the various interfaces using specified protocols. The subscriber carries the mobile station; the base station subsystem controls the radio link with the Mobile Station. The network subsystem, which is the main part of which is the Mobile services Switching Center, performs the switching of calls between the mobile and other fixed or mobile network users, as well as management of mobile services, such as authentication.

Fig 5: The structure of a GSM network

A GSM Base Transceiving Station ( BTS) houses the transmit and receive equipment for one or more cells. It constitutes the interface between the network provider and the mobile phone. The Base Station Controller ( BSC) administers the transmit and receive resources of the connected base stations.

5.1 Base Station Subsystem of GSM

The Base Station Subsystem (BSS) is the section of a GSM network which is responsible for handling traffic and signaling between a mobile phone and the Network Switching Subsystem. The BSS carries out transcoding of speech channels, allocation of radio channels to mobile phones, paging, quality management of transmission and reception over the Air interface and many other tasks related to the radio network.

Fig 5.1: A typical GSM Base Station

The Base Transceiver Station, or BTS, contains the equipment for transmitting and receiving of radio signals (transceivers), antennas, and equipment for encrypting and decrypting communications with the Base Station Controller (BSC). Typically a BTS for anything other than a picocell will have several transceivers (TRXs) which allow it to serve several different frequencies and different sectors of the cell (in the case of sectorised base stations). A BTS is controlled by a parent BSC via the Base Station Control Function (BCF). The BCF is implemented as a discrete unit or even incorporated in a TRX in compact base stations. The BCF provides an Operations and Maintenance (O&M) connection to the Network Management System (NMS), and manages operational states of each TRX, as well as software handling and alarm collection.

Even though GSM is a standard, the reality is that the functions of a BTS vary from vendor to vendor. There are vendors in which the BTS is a plain transceiver which receives information from the MS (Mobile Station) through the Um (Air Interface) and then converts it to a TDM ("PCM") based interface, the Abis, and sends it towards the BSC. There are vendors which build their BTSs so the information is preprocessed, target cell lists are generated and even intracell handover (HO) can be fully handled. The advantage in this case is less load on the expensive Abis interface.

The BTSs are equipped with radios that are able to modulate layer 1 of interface Um; for GSM 2G+ the modulation type is GMSK, while for EDGE-enabled networks it is GMSK and 8-PSK.

Antenna combiners are implemented to use the same antenna for several TRXs (carriers), the more TRXs are combined the greater the combiner loss will be. Up to 8:1 combiners are found in micro and pico cells only.

Frequency hopping is often used to increase overall BTS performance, this involves the rapid switching of voice traffic between TRXs in a sector. A hopping sequence is followed by the TRXs and handsets using the sector. Several hopping sequences are available, the sequence in use for a particular cell is continually broadcast by that cell so that it is known to the handsets.

A TRX transmits and receives according to the GSM standards, which specify eight TDMA timeslots per radio frequency. A TRX may lose some of this capacity as some information is required to be broadcast to handsets in the area that the BTS serves. This information allows the handsets to identify the network and gain access to it. This signalling makes use of a channel known as the BCCH (Broadcast Control Channel).

Fig 5.1.1:- Base Transceiver Station Antenna in Paris

5.2 Sectorisation

By using directional antennas on a base station, each pointing in different directions, it is possible to sectorise the base station so that several different cells are served from the same location. Typically these directional antennas have a beamwidth of 65 to 85 degrees. This increases the traffic capacity of the base station (each frequency can carry eight voice channels) whilst not greatly increasing the interference caused to neighboring cells (in any given direction, only a small number of frequencies are being broadcast). Typically two antennas are used per sector, at spacing of ten or more wavelengths apart. This allows the operator to overcome the effects of fading due to physical phenomena such as multipath reception. Some amplification of the received signal as it leaves the antenna is often used to preserve the balance between uplink and downlink signal.

5.3 Base Station Controller

The Base Station Controller (BSC) provides, classically, the intelligence behind the BTSs. Typically a BSC has 10s or even 100s of BTSs under its control. The BSC handles allocation of radio channels, receives measurements from the mobile phones, controls handovers from BTS to BTS (except in the case of an inter-BSC handover in which case control is in part the responsibility of the Anchor MSC). A key function of the BSC is to act as a concentrator where many different low capacity connections to BTSs (with relatively low utilisation) become reduced to a smaller number of connections towards the Mobile Switching Center (MSC) (with a high level of utilisation). Overall, this means that networks are often structured to have many

BSCs distributed into regions near their BTSs which are then connected to large centralised MSC sites.

The BSC is undoubtedly the most robust element in the BSS as it is not only a BTS controller but, for some vendors, a full switching center, as well as an SS7 node with connections to the MSC and SGSN (when using GPRS). It also provides all the required data to the Operation Support Subsystem (OSS) as well as to the performance measuring centers.

A BSC is often based on a distributed computing architecture, with redundancy applied to critical functional units to ensure availability in the event of fault conditions. Redundancy often extends beyond the BSC equipment itself and is commonly used in the power supplies and in the transmission equipment providing the A-ter interface to PCU.

The databases for all the sites, including information such as carrier frequencies, frequency hopping lists, power reduction levels, receiving levels for cell border calculation, are stored in the BSC. This data is obtained directly from radio planning engineering which involves modelling of the signal propagation as well as traffic projections.

5.4 Transcoder

Although the Transcoding (compressing/decompressing) function is as standard defined as a BSC function, there are several vendors which have implemented the solution in a stand-alone rack using a proprietary interface. This subsystem is also referred to as the TRAU (Transcoder and Rate Adaptation Unit). The transcoding function converts the voice channel coding between the GSM (Regular Pulse Excited-Long Term Prediction, also known as RPE-LPC) coder and the CCITT standard PCM (G.711 A-law or u-law). Since the PCM coding is 64 kbit/s and the GSM coding is 13 kbit/s, this also involves a buffering function so that PCM 8-bit words can be recoded to construct GSM 20 ms traffic blocks, to compress voice channels from the 64 kbit/s PCM standard to the 13 kbit/s rate used on the air interface. Some networks use 32 kbit/s ADPCM on the terrestrial side of the network instead of 64 kbit/s PCM and the TRAU converts accordingly. When the traffic is not voice but data such as fax or email, the TRAU enables its Rate Adaptation Unit function to give compatibility between the BSS data rates and the MSC capability.

However, at least in Siemens' and Nokia's architecture, the Transcoder is an identifiable separate sub-system which will normally be co-located with the MSC. In some of Ericsson's systems it is integrated to the MSC rather than the BSC. The reason for these designs is that if the compression of voice channels is done at the site of the MSC, fixed transmission link costs can be reduced.

5.5 Packet Control Unit

The Packet Control Unit (PCU) is a late addition to the GSM standard. It performs some of the processing tasks of the BSC, but for packet data. The allocation of channels between voice and data is controlled by the base station, but once a channel is allocated to the PCU, the PCU takes full control over that channel.

The PCU can be built into the base station, built into the BSC or even, in some proposed architectures, it can be at the SGSN site.

5.6 Accessing a GSM network

In order to gain access to GSM services, a user needs three things:

• A subscription with a mobile phone operator. This is usually either a Pay As You Go arrangement, where all GSM services are paid for in advance, or a Pay Monthly option where a bill is issued each month for line rental, normally paid for a month in advance, and for services used in the previous month.

• A mobile phone which is GSM compliant and operates at the same frequency as the operator. Most phone companies sell phones from third-party manufacturers.

• A SIM card which is issued by the operator once the subscription is granted. The card comes pre-programmed with the subscriber's phone "identity" and will be used to store personal information (like contact numbers of friends and family).

After subscribers sign up, information about their phone's identity and what services they are allowed to access are stored in a "SIM record" in the Home Location Register (HLR). The Home Location Register is a database maintained by the "home" phone company for all of its subscribers. It is used to answer queries like, "Where on the mobile phone network is the device associated with this phone number?" and "What services is this subscriber paying for?"

Once the SIM card is loaded into the phone and it is powered on, it will search for the nearest mobile phone mast, also called a Base Transceiver Station or BTS. If a mast can be successfully contacted, then there is said to be coverage in the area.

Stationary phones are always connected to the same part of the phone network, but mobile phones can "visit" any part of the network, whether across town or in another country via a foreign provider. Each geographic area has a database called the Visitors Location Register (VLR) which contains details of all the local mobiles. Whenever a phone attaches, or visits, a new area, the Visitors Location Register must contact the Home Location Register.

The Visitors LR will tell the Home LR where the phone is connected to the network (which VLR), and will ask it for a copy of the SIM record (which includes, for example, what services the phone is allowed to access). The current cellular location of the phone (i.e. which BTS it is at) is entered into the VLR record and will be used during a process called paging when the GSM network wishes to locate the mobile phone.

Every SIM card contains a secret key, called the Ki, which it uses to prove its identity to the phone network (to prevent theft of services) upon first contact. The network does this by consulting the Authentication Center of the "home" phone company, which also has a copy of the secret key. (Though the authentication is accomplished without transmitting the key directly.)

Every phone contains a unique identifier (different from the phone number, which is associated at the HLR with the removable SIM card), called the International Mobile Equipment Identity

(IMEI). When a phone contacts the network, its IMEI is supposed to be checked against the global Equipment Identity Register to locate stolen phones and facilitate monitoring.

5.7 Data transmission

The Public Switched Telephone Network (PSTN) is essentially a collection of interconnected systems for taking an audio signal from one place and delivering it to another. Older analogue phone networks simply converted sound waves into electrical pulses and back again. The modern phone system digitally encodes audio signals so that they can be combined and transmitted long distances over fiber optic cables and other means, without losing signal quality in the process. When someone uses a computer with a traditional modem, they are encoding a (relatively slow) data stream into a series of audio chirps, which are then relayed by the PSTN in the same way as regular voice calls. This means that computer data is being encoded as phone audio, which is then being re-encoded as phone system data, and then back to phone quality audio, which is finally converted back to computer data at the destination.

GSM voice calls are essentially an extension of the PSTN, dealing only with audio signals. Behind the scenes, we know these audio channels happen to be transmitted as digital radio signals.

The GSM standard also provides separate facilities for transmitting digital data directly, without any of the inefficient conversions back and forth to audio form. This allows a mobile "phone" to act like any other computer on the Internet, sending and receiving data via the Internet Protocol or X.25.

The mobile may also be connected to a desktop computer, laptop, or PDA, for use as a network interface. (Like a modem or ethernet card, but using a GSM-compatible data protocol instead of a PSTN-compatible audio channel or an ethernet link to transmit data.) Newer GSM phones can be controlled by a standardised Hayes AT command set through a serial cable or a wireless link (using IrDA or Bluetooth). The AT commands can control anything from ring tones to data compression algorithms.

In addition to general Internet access, other special services may be provided by the mobile phone operator, such as SMS.

5.8 Circuit-switched data protocols

A circuit-switched data connection reserves a certain amount of bandwidth between two points for the life of a connection, just as a traditional phone call allocates an audio channel of a certain quality between two phones for the duration of the call. (But remember that in the GSM system, there is no need to use audio signals to create data connections, even circuit-switched ones. The idea of a circuit-switched data connection being like a phone call is just an analogy to help explain the idea.)

Two circuit-switched data protocols are defined in the GSM standard, and they have not-very-creative names: Circuit Switched Data (CSD) and High-Speed Circuit-Switched Data (HSCSD).

These types of connections are typically charged on a per-second basis, regardless of the amount of data sent over the link. This is because a certain amount of bandwidth is dedicated to the connection regardless of whether or not it is needed.

Circuit-switched connections do have the advantage of providing a constant, guaranteed quality of service, which is useful for real-time applications like video conferencing.

5.9 Authentication and Key generation work in a GSM network

Encryption in the GSM network utilizes a Challenge/Response mechanism.

1. The Mobile Station (MS) signs into the network.2. The Mobile Services Switching Center (MSC) requests 5 triples from the Home Location

Register (HLR).3. The Home Location Register creates five triples utilizing the A8 algorithm. These five

triples each contain:o A 128-bit random challenge (RAND)o A 32-bit matching Signed Response (SRES)o A 64-bit ciphering key used as a Session Key (Kc).

4. The Home Location Register sends the Mobile Services Switching Center the five triples.5. The Mobile Services Switching Center sends the random challenge from the first triple to

the Base Transceiver Station (BTS).6. The Base Transceiver Station sends the random challenge from the first triple to the

Mobile Station.7. The Mobile Station receives the random challenge from the Base Transceiver Station and

encrypts it with the Individual Subscriber Authentication Key (Ki) assigned to the Mobile Station utilizing the A3 algorithm.

8. The Mobile Station sends the Signed Response to the Base Transceiver Station.9. The Base Transceiver Station sends the Signed Response to the Mobile Services

Switching Center.10. The Mobile Services Switching Center verifies the Signed Response.11. The Mobile Station generates a Session Key (Kc) utilizing the A8 algorithm, the

Individual Subscriber Authentication Key (Ki) assigned to the Mobile Station, and the random challenge received from the Base Transceiver Station.

12. The Mobile Station sends the Session Key (Kc) to the Base Transceiver Station.13. The Mobile Services Switching Center sends the Session Key (Kc) to the Base

Transceiver Station.14. The Base Transceiver Station receives the Session Key (Kc) from the Mobile Services

Switching Center.15. The Base Transceiver Station receives the Session Key (Kc) from the Mobile Station.16. The Base Transceiver Station verifies the Session Keys from the Mobile Station and the

Mobile Services switching Center.17. The A5 algorithm is initialized with the Session Key (Kc) and the number of the frame to

be encrypted.18. Over-the-air communication channel between the Mobile Station and Base Transceiver

Station can now be encrypted utilizing the A5 algorithm.

This process authenticates the GSM Mobile Station (MS) to the GSM network. One known security limitation of GSM networks is that the GSM network is never authenticated by the GSM Mobile Station (MS).

This one-way authentication makes it possible for an attacker to pretend to be a GSM network provider.

6. Voice calls

6.1 How outgoing calls are made from a mobile

Once a mobile phone has successfully attached to a GSM network as described above, calls may be made from the phone to any other phone on the global Public Switched Telephone Network assuming the subscriber has an arrangement with their "home" phone company to allow the call.

The user dials the telephone number, presses the send or talk key, and the mobile phone sends a call setup request message to the mobile phone network via the mobile phone mast (BTS) it is in contact with.

The element in the mobile phone network that handles the call request is the Visited Mobile Switching Center (Visited MSC). The MSC will check against the subscriber's temporary record held in the Visitor Location Register to see if the outgoing call is allowed. If so, the MSC then routes the call in the same way that a telephone exchange does in a fixed network.

If the subscriber is on a Pay As You Go tariff, then an additional check is made to see if the subscriber has enough credit to proceed. If not, the call is rejected. If the call is allowed to continue, then it is continually monitored and the appropriate amount is decremented from the subscriber's account. When the credit reaches zero, the call is cut off by the network. The systems that monitor and provide the prepaid services are not part of the GSM standard services, but instead an example of intelligent network services that a mobile phone operator may decide to implement in addition to the standard GSM ones.

6.2 How incoming calls are made to a mobile

6.2.1 Step One: Contact the Gateway MSC

When someone places a call to a mobile phone, they dial the telephone number (also called a MSISDN) associated with the phone user and the call is routed to the mobile phone operator's Gateway Mobile Switching Centre. The Gateway MSC, as the name suggests, acts as the "entrance" from exterior portions of the Public Switched Telephone Network onto the provider's network.

As noted above, the phone is free to roam anywhere in the operator's network or on the networks of roaming partners, including in other countries. So the first job of the Gateway MSC is to determine the current location of the mobile phone in order to connect the call. It does this by

consulting the Home Location Register (HLR), which, as described above, knows which Visitor Location Register (VLR) the phone is associated with, if any.

6.2.2 Step Two: Determine how to route the call

When the HLR receives this query message, it determines whether the call should be routed to another number (called a divert), or if it is to be routed directly to the mobile.

• If the owner of the phone has previously requested that all incoming calls be diverted to another number, known as the Call Forward Unconditional (CFU) Number, then this number is stored in the Home Location Register. If that is the case, then the CFU number is returned to the Gateway MSC for immediate routing to that destination.

• If the mobile phone is not currently associated with a Visited Location Register (because the phone has been turned off or is not in range) then the Home Location Register returns a number known as the Call Forward Not Reachable (CFNRc) number to the Gateway MSC, and the call is forwarded there. Many operators may set this value automatically to the phone's voice mail number, so that callers may leave a message. The mobile phone may sometimes override the default setting.

• Finally, if the Home Location Register knows that the phone is in the jurisdiction of a particular Visited Location Register, then it will request a temporary number (called an MSRN) from that VLR. This number is relayed to the Gateway MSC, which uses it to route the call to another Mobile Switching Center, called the Visiting MSC.

6.2.3 Step Three: Ringing the phone

When the call is received by the Visiting MSC, the MSRN is used to find the phone's record in the Visited Location Register. This record identifies the phone's location area. Paging occurs to all mobile phone masts in that area. When the subscriber's mobile responds, the exact location of the mobile is returned to the Visited MSC. The VMSC then forwards the call to the appropriate phone mast, and the phone rings. If the subscriber answers, a speech path is created through the Visiting MSC and Gateway MSC back to the network of the person making the call, and a normal telephone call follows.

It is also possible that the phone call is not answered. If the subscriber is busy on another call (and call waiting is not being used) the Visited MSC routes the call to a pre-determined Call Forward Busy (CFB) number. Similarly, if the subscriber does not answer the call after a period of time (typically 30 seconds) then the Visited MSC routes the call to a pre-determined Call Forward No Reply (CFNRy) number. Once again, the operator may decide to set this value by default to the voice mail of the mobile so that callers can leave a message.

7. Voice charges

In the United States and Canada, callers pay the cost of connecting to the Gateway MSC of the subscriber's phone company, regardless of the actual location of the phone. As mobile numbers

are given standard geographic numbers according to the North American Numbering Plan, callers pay the same to reach fixed phones and mobile phones in a given geographic area. Mobile subscribers pay for the connection time (typically using in-plan or prepaid minutes) for both incoming and outgoing calls. For outgoing calls, any long distance charges are billed as if they originate at the GMSC, even though it is the Visiting MSC which completes the connection to the PSTN. Plans that include nationwide long distance and/or nationwide roaming at no additional charge over "local" outgoing calls are popular.

Mobile networks in Europe, Asia and Australia only charge their subscribers for outgoing calls. Incoming calls are free to the mobile subscriber; however, callers typically pay a higher rate when calling mobile phones. Special prefixes are used to designate mobile numbers so that callers are aware they are calling a mobile phone and therefore will be charged a higher rate.

From the caller's point of view, it does not matter where the mobile subscriber is, as the technical process of connecting the call is the same. If a subscriber is roaming on a different company's network, the subscriber, instead of the caller, may pay a surcharge for the connection time. International roaming calls are often quite expensive, and as a result some companies require subscribers to grant explicit permission to receive calls while roaming to certain countries.

When a subscriber is roaming internationally and a call is forwarded to his or her voice mail, such as when his or her phone is off, busy, or not answered, he or she may actually be charged for two simultaneous international phone calls—the first to get from the GMSC to the VMSC and the second to get from the VMSC to the Call Forward Busy or Call Forward No Reply number (typically the voice mailbox) in the subscriber's country. However, some networks' GMSCs connect unanswered calls directly, keeping the voice signal entirely within the home country and thus avoiding the double charge.

8. How speech is encoded during mobile phone calls

During a GSM call, speech is converted from analogue sound waves to digital data by the phone itself, and transmitted through the mobile phone network by digital means. (Though older parts of the fixed Public Switched Telephone Network may use analog transmission.)

The digital algorithm used to encode speech signals is called a codec. The speech codecs used in GSM are called Half-Rate (HR), Full-Rate (FR), Enhanced Full-Rate (EFR) and Adaptive Multirate (AMR). All codecs except AMR operate with a fixed data rate and error correction level.

9. Some features of GSM

9.1 General Packet Radio Service (GPRS)

A packet-switched connection chops data into distinct chunks, known as packets, which may arrive at their destination via different routes, at different times, out of sequence, or (hopefully only occasionally) not at all. An intermediate protocol, like TCP, might be used to ensure the

original data stream is reassembled at the destination (by putting packets in order and retransmitting missing ones, if necessary).

The General Packet Radio Service (GPRS) is a packet-switched data transmission protocol which was incorporated into the GSM standard in 1997. It is backwards-compatible with systems that use pre-1997 versions of the standard. GPRS does this by sending packets to the local mobile phone mast (BTS) on channels not being used by circuit-switched voice calls or data connections. Multiple GPRS users can share a single unused channel because each of them uses it only for occasional short bursts.

The advantage of packet-switched connections is that bandwidth is only used when there is actually data to transmit. This type of connection is thus generally billed by the kilobyte instead of by the second, and is usually a cheaper alternative for applications that only need to send and receive data sporadically, like instant messaging.

GPRS is usually described as a 2.5G technology; see the main article for more information.

9.2 Short Message Service (SMS)

The GSM standards first defined the structure of a Short Message, and provide a means of transmitting messages between mobile devices and Short Message Service Centres via the Short Message Service (SMS). SMS messages may be carried between phones and SMSCs by any of the circuit-switched or packet-switched methods described above or, more typically, by the MAP protocol through the SS7 signaling channel used for call setup.

SMSCs can be thought of as central routing hubs for Short Messages. Many mobile service operators use their SMSCs as gateways to external systems, including the Internet, incoming SMS news feeds, and each other (often using the de facto SMPP standard).

The SMS standard is also used outside of the GSM system; see the main article for details.

9.3 Supplementary Services

GSM supports a comprehensive set of supplementary services that complement and support the telephony and data services described above. They are all defined in GSM standards. (See GSM codes for supplementary services) 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 calls.

• Barring of Incoming Calls. This function allows the subscriber to prevent incoming calls. The following two conditions for incoming call barring exist: 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 telecommunications 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 PBX. They are a group of subscribers who are capable of only calling themselves and certain numbers.

• Explicit Call Transfer (ECT). This service allows a user who has two calls to connect these two calls together and release its connections to both other parties.

10. Subscriber identity module

One of the key features of GSM is the Subscriber Identity Module (SIM), commonly known as a SIM card. The SIM is a detachable smart card containing the user's subscription information and phonebook. This allows the user to retain his or her information after switching handsets. Alternatively, the user can also change operators while retaining the handset simply by changing the SIM. Some operators will block this by allowing the phone to use only a single SIM, or only a SIM issued by them; this practice is known as SIM locking, and is illegal in some countries. In the United States, Canada, Europe and Australia, many operators lock the mobiles they sell. This is done because the price of the mobile phone is typically subsidised with revenue from subscriptions and operators want to try to avoid subsidising competitor's mobiles. A subscriber can usually contact the provider to remove the lock for a fee, utilize private services to remove the lock, or make use of ample software and websites available on the Internet to unlock the

handset themselves. While most web sites offer the unlocking for a fee, some do it for free. The locking applies to the handset, identified by its International Mobile Equipment Identity (IMEI) number, not to the account (which is identified by the SIM card). It is always possible to switch to another (non-locked) handset if such other handset is available. Some providers will unlock the phone for free if the customer has held an account for a certain period. Third party unlocking services exist that are often quicker and lower cost than that of the operator. In most countries removing the lock is legal. Cingular and T-Mobile provide free unlock services to their customers after 3 months of subscription. In countries like India, Pakistan, Indonesia, Belgium, etc., all phones are sold unlocked. However, in Belgium, it is unlawful for operators there to offer any form of subsidy on the phone's price. This was also the case in Finland until April 1, 2006, when selling subsidized combinations of handsets and accounts became legal though operators have to unlock phone free of charge after a certain period (at most 24 months).

11. GSM security

GSM was designed with a moderate level of security. The system was designed to authenticate the subscriber using shared-secret cryptography. Communications 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 give greater security, as well as mutually authenticating the network and the user - whereas GSM only authenticated the user to the network (and not vice versa). The security model therefore offers confidentiality and authentication, but limited authorization no and capabilities, non-repudiation.

GSM uses several cryptographic algorithms for security. The A5/1 and A5/2 stream ciphers are used for ensuring over-the-air voice privacy. A5/1 was developed first and is a stronger algorithm used within Europe and the United States; A5/2 is weaker and used in other countries. A large security advantage of GSM over earlier systems is that the Ki, the crypto variable stored on the SIM card that is the key to any GSM ciphering algorithm, is never sent over the air interface. Serious weaknesses have been found in both algorithms, and it is possible to break A5/2 in real-time in a ciphertext-only attack.

12. GSM vs. CDMA

The two major network technologies are fighting each other worldwide. The CDMA vs. GSM debate has many advocates on both sides, since CDMA and GSM both have their advantages. When choosing a cell phone carrier you may have to figure out which of these two technologies is the best for you. Hopefully, this article will help you pick a side in the CDMA vs. GSM war.

12.1 The Origins

CDMA: Code Division Multiple Access (CDMA) is a technology developed by Qualcomm in the United States, and it is currently the dominant network standard in North America.

GSM: Global System for Mobile communications (GSM) was invented in 1987 by the GSM Association, an international organization dedicated to developing the GSM standard worldwide.

There is no clear winner in the CDMA vs. GSM debate here; it all depends on your needs. CDMA was established earlier in North America and thus has a bit more coverage than GSM. GSM on the other hand is an international standard backed by an international organization.

12.2 Coverage

CDMA: CDMA is mostly used in America and some parts of Asia. It is currently making progress in other parts of the world, but the coverage is still limited compared to the GSM technology. Its support is currently non-existent in Europe because the European Union mandates the sole use of GSM. In North America however, CDMA generally offers a better coverage than GSM in some rural areas because it was deployed earlier. The CDMA network reaches over 270 millions users worldwide.

GSM: GSM being an international standard, it is better suited for international roaming, provided you own a quad-band cell phone (850/900/1800/1900 MHz). The GSM network is also well established in North America, but not as much as the CDMA network yet. The GSM network reaches over a billion users worldwide.

CDMA is prominent in North America, but GSM reaches a lot more users worldwide (about 270 millions for CDMA and 1 billion for GSM). In the CDMA against GSM debate, GSM wins if you plan to travel to foreign countries but CDMA might have a better coverage in your area.

12.3 Data transfer

CDMA: The best data transfer technology CDMA has to offer is the EVDO technology, allowing for a maximum download speed of about 2mb/s (about 700kbps in practice), which is similar to what a DSL line has to offer. EVDO is not available everywhere yet and requires a cell phone that is EVDO ready.

GSM: GSM on the other hand offers EDGE, allowing for a maximum download speed of 384kbps (around 140kbps in practice). More technologies are being developed on top of EDGE such as HSDPA to boost the transfer rate to over 384kbps in practice. This technology requires an EDGE-ready cell phone.

CDMA offers faster data download. GSM is catching up fast, but its EDGE technology is subject to interferences. CDMA would therefore be the favored choice for data transfer.

12.4 Phone Identification (SIM cards)

CDMA: On a CDMA phone, your account information is programmed into your cellular phone. If you want to change your phone, you have to contact your carrier and have them reprogram your new phone. You will also need to re-enter your contact list and calendar information into

your new phone. If you have a lot of contacts, your carrier might be able to help you perform this task.

GSM: On a GSM phone your account information along with your contact list and other personal data are stored on a SIM card (Subscriber Identity Module) which is a small chip you can freely remove from your phone. When you get a new mobile device, you can simply insert your SIM card into it and it will work with your current account information and contact list. If you travel to another country, it might even be possible to purchase a prepaid SIM card which you can use to avoid roaming fees.

GSM is a clear winner here. The SIM card technology offers many advantages.

12.5 Which technology is the best

When asking which technology to choose between CDMA and GSM, first we have to consider the following topics:

1. Is international roaming important ? If we travel a lot to foreign countries, we might want to opt for a GSM phone for a better coverage.

2. Transfer a lot of data using the phone? Currently, CDMA offers the best data transfer speed with its EVDO technology and is the clear winner for now. If we intend to use the phone for mobile web browsing, watching television or downloading MP3s, we might be better off with a CDMA phone.

3. Do we plan to change phone often? If we do, a GSM phone is best for us since we can swap the SIM card to a new device without having to re-enter our personal data.

Once we have established our basic needs, we have to compare CDMA and GSM signals in our region. Also we have to compare the carriers offering both services in our area. When choosing between CDMA vs. GSM, preferring a network technology over another also means limiting our choice in carriers.

13. GSM cell phone Advantages & Disadvantages

13.1 Advantages

GSM phones offer superior sound quality. Background sounds, disturbances, and static are vastly reduced and crossed-line conversations are also almost eliminated in gsm cellular phones . GSM phone sound quality is more like that of a fixed telephone. The main advantage of GSM mobiles are International Roaming ability, good sound quality, small cheaper handsets and ability to handle high volumes of users. Unlike analogue phones, your conversations using a gsm cell phone within the digital network are safe and secure.

With superior battery life & digital technology, you get twice as much talk time from each battery charge, compared with analogue phones. In addition the digital service allows more calls to be handled at any one time, therefore reducing congestion in areas of dense population and high usage.

GSM mobile phones offer roaming facility, which means, you can use your mobile phone, and same mobile number in other countries around the world who operate through GSM network or you can just use your SIM card into another GSM cell phone.

13.2 GSM cell phone disadvantages

Intellectual property is concentrated among a few industry participants, creating barriers to entry for new entrants and limiting competition among phone manufacturers.

GSM has a fixed maximum cell site range of 35 km, which is imposed by technical limitations.

The GSM system is the most widely used mobile telecommunications system in use in the world today. The letters GSM originally stood for the words Group Special Mobile, but as it became clear this standard was to be used world wide the meaning of GSM was changed to Global System for Mobile Communications.

REFERENCE

1. About GSM Association. GSM Association. Retrieved on 2007-01-08

2. Two Billion GSM Customers Worldwide. 3G Americas (June 13, 2006). Retrieved on 2007-01-08.

3. Texas Instruments Executive Meets with India Government Official to outline Benefits of Open Standards to drive mobile phone penetration. Texas Instruments (July 12, 2006). Retrieved on 2007-01-08.

4. Nokia delivers first phase GPRS core network solution to Radiolinja, Finland. Nokia (January 24, 2000). Retrieved on 2006-01-08.

5. www.wikipedia.com

6. Suk Yu Hui; Kai Hau Yeung (December 2003). "Challenges in the migration to 4G mobile systems". Communications Magazine, IEEE.

7. 4G Mobile. Alcatel-Lucent (June 13, 2005).8. www.wikipedia.com