Nokia Project Report

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GSM

The GSM logo is used to identify compatible handsets and equipment

GSM (Global System for Mobile Communications, originally Groupe Spcial Mobile), is a standard set

developed by the European Telecommunications Standards Institute (ETSI) to describe technologies for

second generation (or "2G") digital cellular networks. Developed as a replacement for first generation analogcellular networks, the GSM standard originally described a digital, circuit switched network optimized for full

duplex voice telephony. The standard was expanded over time to include first circuit switched data transport,

then packet data transport via GPRS. Packet data transmission speeds were later increased via EDGE. The

GSM standard is succeeded by the third generation (or "3G")UMTS standard developed by the 3GPP. GSM

networks will evolve further as they begin to incorporate fourth generation (or "4G") LTE Advanced standards.

"GSM" is a trademark owned by the GSM Association.

The GSM Association estimates that technologies defined in the GSM standard serve 80% of the world's

population, encompassing more than 5 billion people across more than 212 countries and territories, making

GSM the most ubiquitous of the many standards for cellular networks.

Contents

1 History

2 Technical details

o 2.1 GSM carrier frequencies

o 2.2 Voice codecs

o 2.3 Network structure

o 2.4 Subscriber Identity Module (SIM)

o 2.5 Phone locking

o 2.6 GSM service security

3 Standards information
http://en.wikipedia.org/wiki/European_Telecommunications_Standards_Institutehttp://en.wikipedia.org/wiki/2Ghttp://en.wikipedia.org/wiki/Cellular_networkhttp://en.wikipedia.org/wiki/Duplex_(telecommunications)#Full_duplexhttp://en.wikipedia.org/wiki/Duplex_(telecommunications)#Full_duplexhttp://en.wikipedia.org/wiki/Telephonyhttp://en.wikipedia.org/wiki/GPRShttp://en.wikipedia.org/wiki/EDGEhttp://en.wikipedia.org/wiki/3Ghttp://en.wikipedia.org/wiki/UMTShttp://en.wikipedia.org/wiki/3GPPhttp://en.wikipedia.org/wiki/4Ghttp://en.wikipedia.org/wiki/LTE_Advancedhttp://en.wikipedia.org/wiki/Trademarkhttp://en.wikipedia.org/wiki/GSM_Associationhttp://en.wikipedia.org/wiki/Gsm#Historyhttp://en.wikipedia.org/wiki/Gsm#Technical_detailshttp://en.wikipedia.org/wiki/Gsm#GSM_carrier_frequencieshttp://en.wikipedia.org/wiki/Gsm#Voice_codecshttp://en.wikipedia.org/wiki/Gsm#Network_structurehttp://en.wikipedia.org/wiki/Gsm#Subscriber_Identity_Module_.28SIM.29http://en.wikipedia.org/wiki/Gsm#Phone_lockinghttp://en.wikipedia.org/wiki/Gsm#GSM_service_securityhttp://en.wikipedia.org/wiki/Gsm#Standards_informationhttp://en.wikipedia.org/wiki/File:GSMLogo.svghttp://en.wikipedia.org/wiki/Gsm#Standards_informationhttp://en.wikipedia.org/wiki/Gsm#GSM_service_securityhttp://en.wikipedia.org/wiki/Gsm#Phone_lockinghttp://en.wikipedia.org/wiki/Gsm#Subscriber_Identity_Module_.28SIM.29http://en.wikipedia.org/wiki/Gsm#Network_structurehttp://en.wikipedia.org/wiki/Gsm#Voice_codecshttp://en.wikipedia.org/wiki/Gsm#GSM_carrier_frequencieshttp://en.wikipedia.org/wiki/Gsm#Technical_detailshttp://en.wikipedia.org/wiki/Gsm#Historyhttp://en.wikipedia.org/wiki/GSM_Associationhttp://en.wikipedia.org/wiki/Trademarkhttp://en.wikipedia.org/wiki/LTE_Advancedhttp://en.wikipedia.org/wiki/4Ghttp://en.wikipedia.org/wiki/3GPPhttp://en.wikipedia.org/wiki/UMTShttp://en.wikipedia.org/wiki/3Ghttp://en.wikipedia.org/wiki/EDGEhttp://en.wikipedia.org/wiki/GPRShttp://en.wikipedia.org/wiki/Telephonyhttp://en.wikipedia.org/wiki/Duplex_(telecommunications)#Full_duplexhttp://en.wikipedia.org/wiki/Duplex_(telecommunications)#Full_duplexhttp://en.wikipedia.org/wiki/Cellular_networkhttp://en.wikipedia.org/wiki/2Ghttp://en.wikipedia.org/wiki/European_Telecommunications_Standards_Institute


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4 GSM open-source software

o 4.1 Issues with patents and open source

History

Early European analogue cellular networks employed an uncoordinated mix of technologies and protocols

that varied from country to country, preventing interoperability of subscriber equipment and increasing

complexity for equipment manufacturers who had to contend with varying standards from a fragmented

market. The work to develop a European standard for digital cellular voice telephony began in 1982 when

the European Conference of Postal and Telecommunications Administrations (CEPT) created the Group

Special Mobile committee and provided a permanent group of technical support personnel, based in

Paris. In 1987, 15 representatives from 13 European countries signed a memorandum of

understanding to develop and deploy a common cellular telephone system across Europe. The foresight

of deciding to develop a continental standard paid off, eventually resulting in a unified, open, standard-

based network larger than that in the United States.[1][2][3][4]
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France and Germany signed a joint development agreement in 1984 and were joined by Italy and the UK

in 1986. In 1986 the European Commission proposed to reserve the 900 MHz spectrum band for GSM.

By 1987, basic parameters of the GSM standard had been agreed upon and 15 representatives from 13

European nations signed a memorandum of understanding in Copenhagen, committing to deploy GSM.

In 1989, the Groupe Spcial Mobile committee was transferred from CEPT to the European

Telecommunications Standards Institute (ETSI).[3]

Phase I of the GSM specifications were published in 1990. The historic world's first GSM call was made

by the Finnish prime minister Harri Holkeri to Kaarina Suonio (mayor in city ofTampere) in July 1 1991.

The first network was built by Telenokia and Siemens and operated by Radiolinja.[5]

1992, the first short

messaging service (SMS or "text message") message was sent and Vodafone UK and Telecom Finland

signed the first international roaming agreement. Work had begun in 1991 to expand the GSM standard

to the 1800 MHz frequency band and the first 1800 MHz network became operational in the UK in 1993.

Also in 1993, Telecom Australia became the first network operator to deploy a GSM network outside of

Europe and the first practical hand-held GSM mobile phone became available. In 1995, fax, data and

SMS messaging services became commercially operational, the first 1900 MHz GSM network in the world

became operational in the United States and GSM subscribers worldwide exceeded 10 million. In this

same year, the GSM Association was formed. Pre-paid GSM SIM cards were launched in 1996 and

worldwide GSM subscribers passed 100 million in 1998.[3]

In 2000, the first commercial GPRS services were launched and the first GPRS compatible handsets

became available for sale. In 2001 the first UMTS (W-CDMA) network was launched and worldwide GSM

subscribers exceeded 500 million. In 2002 the first multimedia messaging services (MMS) wereintroduced and the first GSM network in the 800 MHz frequency band became

operational. EDGE services first became operational in a network in 2003 and the number of worldwide

GSM subscribers exceeded 1 billion in 2004.[3]

By 2005, GSM networks accounted for more than 75% of the worldwide cellular network market, serving

1.5 billion subscribers. In 2005, the first HSDPA capable network also became operational. The

first HSUPA network was launched in 2007 and worldwide GSM subscribers exceeded two billion in

2008.[3]

The GSM Association estimates that technologies defined in the GSM standard serve 80% of the global

mobile market, encompassing more than 1.5 billion people across more than 212 countries and

territories, making GSM the most ubiquitous of the many standards for cellular networks.[6]
http://en.wikipedia.org/wiki/European_Telecommunications_Standards_Institutehttp://en.wikipedia.org/wiki/European_Telecommunications_Standards_Institutehttp://en.wikipedia.org/wiki/GSM#cite_note-gsmworld_history-2http://en.wikipedia.org/wiki/GSM#cite_note-gsmworld_history-2http://en.wikipedia.org/wiki/GSM#cite_note-gsmworld_history-2http://en.wikipedia.org/wiki/Harri_Holkerihttp://en.wikipedia.org/wiki/Tamperehttp://en.wikipedia.org/wiki/Nokia_Siemens_Networkshttp://en.wikipedia.org/wiki/Mobile_network_operatorhttp://en.wikipedia.org/wiki/Radiolinjahttp://en.wikipedia.org/wiki/GSM#cite_note-4http://en.wikipedia.org/wiki/GSM#cite_note-4http://en.wikipedia.org/wiki/GSM#cite_note-4http://en.wikipedia.org/wiki/Short_messaging_servicehttp://en.wikipedia.org/wiki/Short_messaging_servicehttp://en.wikipedia.org/wiki/GSM_Associationhttp://en.wikipedia.org/wiki/GSM#cite_note-gsmworld_history-2http://en.wikipedia.org/wiki/GSM#cite_note-gsmworld_history-2http://en.wikipedia.org/wiki/GSM#cite_note-gsmworld_history-2http://en.wikipedia.org/wiki/GPRShttp://en.wikipedia.org/wiki/EDGEhttp://en.wikipedia.org/wiki/GSM#cite_note-gsmworld_history-2http://en.wikipedia.org/wiki/GSM#cite_note-gsmworld_history-2http://en.wikipedia.org/wiki/GSM#cite_note-gsmworld_history-2http://en.wikipedia.org/wiki/HSDPAhttp://en.wikipedia.org/wiki/HSUPAhttp://en.wikipedia.org/wiki/GSM#cite_note-gsmworld_history-2http://en.wikipedia.org/wiki/GSM#cite_note-gsmworld_history-2http://en.wikipedia.org/wiki/GSM#cite_note-gsmworld_history-2http://en.wikipedia.org/wiki/GSM_Associationhttp://en.wikipedia.org/wiki/GSM#cite_note-gsma_stats-5http://en.wikipedia.org/wiki/GSM#cite_note-gsma_stats-5http://en.wikipedia.org/wiki/GSM#cite_note-gsma_stats-5http://en.wikipedia.org/wiki/GSM#cite_note-gsma_stats-5http://en.wikipedia.org/wiki/GSM_Associationhttp://en.wikipedia.org/wiki/GSM#cite_note-gsmworld_history-2http://en.wikipedia.org/wiki/HSUPAhttp://en.wikipedia.org/wiki/HSDPAhttp://en.wikipedia.org/wiki/GSM#cite_note-gsmworld_history-2http://en.wikipedia.org/wiki/EDGEhttp://en.wikipedia.org/wiki/GPRShttp://en.wikipedia.org/wiki/GSM#cite_note-gsmworld_history-2http://en.wikipedia.org/wiki/GSM_Associationhttp://en.wikipedia.org/wiki/Short_messaging_servicehttp://en.wikipedia.org/wiki/Short_messaging_servicehttp://en.wikipedia.org/wiki/GSM#cite_note-4http://en.wikipedia.org/wiki/Radiolinjahttp://en.wikipedia.org/wiki/Mobile_network_operatorhttp://en.wikipedia.org/wiki/Nokia_Siemens_Networkshttp://en.wikipedia.org/wiki/Tamperehttp://en.wikipedia.org/wiki/Harri_Holkerihttp://en.wikipedia.org/wiki/GSM#cite_note-gsmworld_history-2http://en.wikipedia.org/wiki/European_Telecommunications_Standards_Institutehttp://en.wikipedia.org/wiki/European_Telecommunications_Standards_Institute


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Technical details

GSM cell site antennas in the Deutsches Museum, Munich, Germany

GSM is a cellular network, which means that mobile phones connect to it by searching for cells in the

immediate vicinity. There are five different cell sizes in a GSM network

macro, micro, pico, femto 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

coverage diameter is a few dozen metres; they are mainly used indoors. Femtocells are cells designed

for use in residential or small business environments and connect to the service providers network via a

broadband internet connection. 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 metres to several tens of kilometres. The longest distance the GSM specification

supports in practical use is 35 kilometres (22 mi). There are also several implementations of the concept

of an extended cell,[7]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.
http://en.wikipedia.org/wiki/Cell_sitehttp://en.wikipedia.org/wiki/Deutsches_Museumhttp://en.wikipedia.org/wiki/Cellular_networkhttp://en.wikipedia.org/wiki/Mobile_phonehttp://en.wikipedia.org/wiki/Macrocellhttp://en.wikipedia.org/wiki/Microcellhttp://en.wikipedia.org/wiki/Picocellhttp://en.wikipedia.org/wiki/Femtocellhttp://en.wikipedia.org/w/index.php?title=Umbrella_cells&action=edit&redlink=1http://en.wikipedia.org/wiki/Base_stationhttp://en.wikipedia.org/wiki/Antenna_(electronics)http://en.wikipedia.org/wiki/GSM#cite_note-6http://en.wikipedia.org/wiki/GSM#cite_note-6http://en.wikipedia.org/wiki/GSM#cite_note-6http://en.wikipedia.org/wiki/Timing_advancehttp://en.wikipedia.org/wiki/Cellular_repeaterhttp://en.wikipedia.org/wiki/File:Deutsches_Museum_-_GSM_cell_site_antennas.jpghttp://en.wikipedia.org/wiki/Cellular_repeaterhttp://en.wikipedia.org/wiki/Timing_advancehttp://en.wikipedia.org/wiki/GSM#cite_note-6http://en.wikipedia.org/wiki/Antenna_(electronics)http://en.wikipedia.org/wiki/Base_stationhttp://en.wikipedia.org/w/index.php?title=Umbrella_cells&action=edit&redlink=1http://en.wikipedia.org/wiki/Femtocellhttp://en.wikipedia.org/wiki/Picocellhttp://en.wikipedia.org/wiki/Microcellhttp://en.wikipedia.org/wiki/Macrocellhttp://en.wikipedia.org/wiki/Mobile_phonehttp://en.wikipedia.org/wiki/Cellular_networkhttp://en.wikipedia.org/wiki/Deutsches_Museumhttp://en.wikipedia.org/wiki/Cell_site


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However, this is not a prerequisite, since indoor coverage is also provided by in-building penetration of

the radio signals from any nearby cell.

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 smoothened

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).

GSM carrier frequencies

GSM networks operate in a number of different carrier frequency ranges (separated into GSM frequency

ranges for 2G and UMTS frequency bands for 3G), with most 2G GSM networks operating in the

900 MHz or 1800 MHz bands. Where these bands were already allocated, the 850 MHz and 1900 MHz
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bands were used instead (for example in Canada and the United States). In rare cases the 400 and

450 MHz frequency bands are assigned in some countries because they were previously used for first-

generation systems.

Most 3G networks in Europe operate in the 2100 MHz frequency band.

Regardless of the frequency selected by an operator, it is divided into timeslots for individual phones to

use. This allows eight full-rate or sixteen half-rate speech channels per radio frequency. These eight radio

timeslots (or eight burst periods) are grouped into a TDMA frame. Half rate channels use alternate frames

in the same timeslot. The channel data rate for all 8 channels 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.

Voice codecs

GSM has used a variety of voice codecs to squeeze 3.1 kHz audio into between 5.6 and 13 kbit/s.

Originally, two codecs, named after the types of data channel they were allocated, were used, called Half

Rate (5.6 kbit/s) and Full Rate (13 kbit/s). 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[8]

with the Enhanced Full Rate (EFR) codec, a 12.2 kbit/s 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
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Network structure

The structure of a GSM network

The network is structured into a number of discrete sections:

TheBase Station Subsystem(the base stations and their controllers).

TheNetwork and Switching Subsystem(the part of the network most similar to a fixed network).

This is sometimes also just called the core network.

TheGPRS Core Network(the optional part which allows packet based Internet connections).

TheOperations support system(OSS)for maintenance of the network
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Base station subsystem

The hardware of GSM base station displayed in Deutsches Museum

The base station subsystem (BSS) is the section of a traditional cellular telephone 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, transmission and reception over the air interface and many other tasks related to the radio

network.

Contents

[hide]

1 Base transceiver station

o 1.1 Sectorisation

2 Base station controller

o 2.1 Transcoder

3 Packet control unit

4 BSS interfaces

Base transceiver station
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Two GSM base station antennas disguised as trees in Dublin, Ireland.

A solar-powered GSM base station on top of a mountain in the wilderness of Lapland

The base transceiver station, or BTS, contains the equipment for transmitting and receiving radio signals

(transceivers), antennas, and equipment for encrypting and decrypting communications with the base

station controller (BSC). Typically a BTS for anything other than apicocell 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.

The functions of a BTS vary depending on the cellular technology used and the cellular telephone

provider. 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,
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the Abis interface, 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, and 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 Broadcast Control

Channel (BCCH).
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Sectorisation

By using directional antennae 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

Base station controller
http://en.wikipedia.org/wiki/Directional_antennahttp://en.wikipedia.org/wiki/Co-channel_interferencehttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Fadinghttp://en.wikipedia.org/wiki/Multipath_receptionhttp://en.wikipedia.org/wiki/Amplifierhttp://en.wikipedia.org/wiki/Amplifierhttp://en.wikipedia.org/wiki/Multipath_receptionhttp://en.wikipedia.org/wiki/Fadinghttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Co-channel_interferencehttp://en.wikipedia.org/wiki/Directional_antenna


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GSM transmitter

The base station controller (BSC) provides, classically, the intelligencebehind the BTSs. Typically a BSC

has tens or even hundreds of BTSs under its control. The BSC handles allocation of radio channels,

receives measurements from the mobile phones, and 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 serving

GPRS support node (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.
http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Descriptionhttp://en.wikipedia.org/wiki/Concentratorhttp://en.wikipedia.org/wiki/Network_Switching_Subsystem#Mobile_services_Switching_Centre_.28MSC.29http://en.wikipedia.org/wiki/Signalling_System_7http://en.wikipedia.org/wiki/SGSNhttp://en.wikipedia.org/wiki/SGSNhttp://en.wikipedia.org/wiki/General_Packet_Radio_Servicehttp://en.wikipedia.org/wiki/File:Nokia_GSM_transmittor.jpghttp://en.wikipedia.org/wiki/General_Packet_Radio_Servicehttp://en.wikipedia.org/wiki/SGSNhttp://en.wikipedia.org/wiki/SGSNhttp://en.wikipedia.org/wiki/Signalling_System_7http://en.wikipedia.org/wiki/Network_Switching_Subsystem#Mobile_services_Switching_Centre_.28MSC.29http://en.wikipedia.org/wiki/Concentratorhttp://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Description


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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.

Transcoder
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The transcoder is responsible for transcoding the voice channel coding between the coding used in the

mobile network, and the coding used by the world's terrestrial circuit-switched network, the Public

Switched Telephone Network. Specifically, GSM uses a regular pulse excited-long term prediction (RPE-

LTP) coder for voice data between the mobile device and the BSS, but pulse code modulation (A-

law or-law standardized in ITU G.711) upstream of the BSS. RPE-LPC coding results in a data rate for

voice of 13 kbit/s where standard PCM coding results in 64 kbit/s. Because of this change in data rate for

the same voice call, the transcoder also has a buffering function so that PCM 8-bit words can be recoded

to construct GSM 20 ms traffic blocks.

Although transcoding (compressing/decompressing) functionality is defined as a base station function by

the relevant standards, there are several vendors which have implemented the solution outside of the

BSC. Some vendors have implemented it in a stand-alone rack using a proprietary interface. 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, the

number of fixed transmission links between the BSS and MSC can be reduced, decreasing network

infrastructure costs.

This subsystem is also referred to as the transcoder and rate adaptation unit(TRAU). Some networks use

32 Kbit/s ADPCM on the terrestrial side of the network instead of 64 kbit/sPCM 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 and MSC data rates.

Packet control unit
http://en.wikipedia.org/wiki/Transcodinghttp://en.wikipedia.org/wiki/Public_Switched_Telephone_Networkhttp://en.wikipedia.org/wiki/Public_Switched_Telephone_Networkhttp://en.wikipedia.org/wiki/Regular_Pulse_Excited-Long_Term_Predictionhttp://en.wikipedia.org/wiki/Pulse_code_modulationhttp://en.wikipedia.org/wiki/A-lawhttp://en.wikipedia.org/wiki/A-lawhttp://en.wikipedia.org/wiki/%CE%9C-lawhttp://en.wikipedia.org/wiki/%CE%9C-lawhttp://en.wikipedia.org/wiki/%CE%9C-lawhttp://en.wikipedia.org/wiki/G.711http://en.wikipedia.org/wiki/Siemenshttp://en.wikipedia.org/wiki/Nokiahttp://en.wikipedia.org/wiki/Ericssonhttp://en.wikipedia.org/wiki/ADPCMhttp://en.wikipedia.org/wiki/PCMhttp://en.wikipedia.org/wiki/PCMhttp://en.wikipedia.org/wiki/ADPCMhttp://en.wikipedia.org/wiki/Ericssonhttp://en.wikipedia.org/wiki/Nokiahttp://en.wikipedia.org/wiki/Siemenshttp://en.wikipedia.org/wiki/G.711http://en.wikipedia.org/wiki/%CE%9C-lawhttp://en.wikipedia.org/wiki/A-lawhttp://en.wikipedia.org/wiki/A-lawhttp://en.wikipedia.org/wiki/Pulse_code_modulationhttp://en.wikipedia.org/wiki/Regular_Pulse_Excited-Long_Term_Predictionhttp://en.wikipedia.org/wiki/Public_Switched_Telephone_Networkhttp://en.wikipedia.org/wiki/Public_Switched_Telephone_Networkhttp://en.wikipedia.org/wiki/Transcoding


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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. In most of the cases, the PCU is a separate node communicating extensively

with the BSC on the radio side and the SGSN on the Gb side.

BSS interfaces

Image of the GSM network, showing the BSS interfaces to the MS, NSS and GPRS Core Network

Um

The air interface between the mobile station (MS) and the BTS. This interface uses LAPDm

protocol for signaling, to conduct call control, measurement reporting, handover, power

control, authentication,authorization, location update and so on. Traffic and signaling are sent in

bursts of 0.577 ms at intervals of 4.615 ms, to form data blocks each 20 ms.

Abis

The interface between the BTS and BSC. Generally carried by a DS-1, ES-1, or E1 TDM circuit.

Uses TDM subchannels for traffic (TCH), LAPD protocol for BTS supervision and telecom

signaling, and carries synchronization from the BSC to the BTS and MS.

A
http://en.wikipedia.org/wiki/Um_Interfacehttp://en.wikipedia.org/wiki/Mobile_stationhttp://en.wikipedia.org/wiki/Handoffhttp://en.wikipedia.org/wiki/Power_controlhttp://en.wikipedia.org/wiki/Power_controlhttp://en.wikipedia.org/wiki/Authenticationhttp://en.wikipedia.org/wiki/Authorizationhttp://en.wikipedia.org/wiki/Link_Access_Procedures,_D_channelhttp://en.wikipedia.org/wiki/File:Gsm_structures.svghttp://en.wikipedia.org/wiki/Link_Access_Procedures,_D_channelhttp://en.wikipedia.org/wiki/Authorizationhttp://en.wikipedia.org/wiki/Authenticationhttp://en.wikipedia.org/wiki/Power_controlhttp://en.wikipedia.org/wiki/Power_controlhttp://en.wikipedia.org/wiki/Handoffhttp://en.wikipedia.org/wiki/Mobile_stationhttp://en.wikipedia.org/wiki/Um_Interface


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The interface between the BSC and MSC. It is used for carrying traffic channels and the BSSAP

user part of the SS7 stack. Although there are usually transcoding units between BSC and MSC,

the signaling communication takes place between these two ending points and the transcoder

unit doesn't touch the SS7 information, only the voice or CS data are transcoded or rate adapted.

Ater

The interface between the BSC and transcoder. It is a proprietary interface whose name depends

on the vendor (for example Ater by Nokia), it carries the A interface information from the BSC

leaving it untouched.

Gb

Connects the BSS to the SGSN in the GPRS core network.

Network switching subsystem
http://en.wikipedia.org/wiki/Signalling_System_7http://en.wikipedia.org/wiki/GPRS_core_networkhttp://en.wikipedia.org/wiki/GPRS_core_networkhttp://en.wikipedia.org/wiki/Signalling_System_7


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Network switching subsystem (NSS) (or GSM core network) is the component of a GSM system that

carries out call switching and mobility management functions for mobile phonesroaming on the network of base

stations. It is owned and deployed by mobile phone operators and allows mobile devices to communicate with

each other and telephones in the widerPublic Switched Telephone Network or (PSTN). The architecture

contains specific features and functions which are needed because the phones are not fixed in one location.

The NSS originally consisted of the circuit-switched core network, used for traditional GSM services such as

voice calls, SMS, and circuit switched data calls. It was extended with an overlay architecture to provide

packet-switched data services known as the GPRS core network. This allows mobile phones to have access to

services such as WAP, MMS, and theInternet.

All mobile phones manufactured today have both circuit and packet based services, so most operators have a

GPRS network in addition to the standard GSM core network.

Contents

[hide]

1 Mobile switching center (MSC)

o 1.1 Description

o 1.2 Mobile switching centre server (MSCS)

o 1.3 Other GSM core network elements connected to the MSC
http://en.wikipedia.org/wiki/GSMhttp://en.wikipedia.org/wiki/Telephone_exchangehttp://en.wikipedia.org/wiki/Mobility_managementhttp://en.wikipedia.org/wiki/Mobile_phoneshttp://en.wikipedia.org/wiki/Base_Station_subsystemhttp://en.wikipedia.org/wiki/Base_Station_subsystemhttp://en.wikipedia.org/wiki/Mobile_phone_operatorhttp://en.wikipedia.org/wiki/Telephonehttp://en.wikipedia.org/wiki/Public_Switched_Telephone_Networkhttp://en.wikipedia.org/wiki/Core_networkhttp://en.wikipedia.org/wiki/GSM_serviceshttp://en.wikipedia.org/wiki/Short_message_servicehttp://en.wikipedia.org/wiki/Circuit_Switched_Datahttp://en.wikipedia.org/wiki/GPRS_core_networkhttp://en.wikipedia.org/wiki/Wireless_Application_Protocolhttp://en.wikipedia.org/wiki/Multimedia_Messaging_Servicehttp://en.wikipedia.org/wiki/Internethttp://en.wikipedia.org/wiki/Network_and_Switching_Subsystemhttp://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Mobile_switching_center_.28MSC.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Descriptionhttp://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Mobile_switching_centre_server_.28MSCS.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Other_GSM_core_network_elements_connected_to_the_MSChttp://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Other_GSM_core_network_elements_connected_to_the_MSChttp://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Mobile_switching_centre_server_.28MSCS.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Descriptionhttp://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Mobile_switching_center_.28MSC.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystemhttp://en.wikipedia.org/wiki/Internethttp://en.wikipedia.org/wiki/Multimedia_Messaging_Servicehttp://en.wikipedia.org/wiki/Wireless_Application_Protocolhttp://en.wikipedia.org/wiki/GPRS_core_networkhttp://en.wikipedia.org/wiki/Circuit_Switched_Datahttp://en.wikipedia.org/wiki/Short_message_servicehttp://en.wikipedia.org/wiki/GSM_serviceshttp://en.wikipedia.org/wiki/Core_networkhttp://en.wikipedia.org/wiki/Public_Switched_Telephone_Networkhttp://en.wikipedia.org/wiki/Telephonehttp://en.wikipedia.org/wiki/Mobile_phone_operatorhttp://en.wikipedia.org/wiki/Base_Station_subsystemhttp://en.wikipedia.org/wiki/Base_Station_subsystemhttp://en.wikipedia.org/wiki/Mobile_phoneshttp://en.wikipedia.org/wiki/Mobile_phoneshttp://en.wikipedia.org/wiki/Mobility_managementhttp://en.wikipedia.org/wiki/Telephone_exchangehttp://en.wikipedia.org/wiki/GSM


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o 1.4 Procedures implemented

2 Home location register (HLR)

o 2.1 Other GSM core network elements connected to the HLR

o

2.2 Procedures implemented3 Authentication centre (AUC)

o 3.1 Description

o 3.2 Other GSM core network elements connected to the AUC

o 3.3 Procedures implemented

4 Visitor location register (VLR)

o 4.1 Description

o 4.2 Other GSM core network elements connected to the VLR

o

4.3 Procedures implemented

5 Equipment identity register (EIR)

6 Other support functions

o 6.1 Billing centre (BC)

o 6.2 Short message service centre (SMSC)

o 6.3 Multimedia messaging service centre (MMSC)

o 6.4 Voicemail system (VMS)

o 6.5 Lawful interception functions

Mobile switching center (MSC)
http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Procedures_implementedhttp://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Home_location_register_.28HLR.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Other_GSM_core_network_elements_connected_to_the_HLRhttp://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Procedures_implemented_2http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Authentication_centre_.28AUC.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Description_2http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Other_GSM_core_network_elements_connected_to_the_AUChttp://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Procedures_implemented_3http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Visitor_location_register_.28VLR.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Description_3http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Other_GSM_core_network_elements_connected_to_the_VLRhttp://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Procedures_implemented_4http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Equipment_identity_register_.28EIR.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Other_support_functionshttp://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Billing_centre_.28BC.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Short_message_service_centre_.28SMSC.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Multimedia_messaging_service_centre_.28MMSC.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Voicemail_system_.28VMS.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Lawful_interception_functionshttp://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Lawful_interception_functionshttp://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Voicemail_system_.28VMS.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Multimedia_messaging_service_centre_.28MMSC.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Short_message_service_centre_.28SMSC.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Billing_centre_.28BC.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Other_support_functionshttp://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Equipment_identity_register_.28EIR.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Procedures_implemented_4http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Other_GSM_core_network_elements_connected_to_the_VLRhttp://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Description_3http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Visitor_location_register_.28VLR.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Procedures_implemented_3http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Other_GSM_core_network_elements_connected_to_the_AUChttp://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Description_2http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Authentication_centre_.28AUC.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Procedures_implemented_2http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Other_GSM_core_network_elements_connected_to_the_HLRhttp://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Home_location_register_.28HLR.29http://en.wikipedia.org/wiki/Network_and_Switching_Subsystem#Procedures_implemented


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Description


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The mobile switching center (MSC) is the primary service delivery node for GSM/CDMA, responsible

for routing voice calls and SMS as well as other services (such as conference calls, FAX and circuit

switched data).

The MSC sets up and releases the end-to-end connection, handles mobility and hand-over

requirements during the call and takes care of charging and real time pre-paid account monitoring.

In the GSM mobile phone system, in contrast with earlier analogue services, fax and data information is

sent directly digitally encoded to the MSC. Only at the MSC is this re-coded into an "analogue" signal

(although actually this will almost certainly mean sound encoded digitally as PCM signal in a 64-kbit/s

timeslot, known as a DS0 in America).

There are various different names for MSCs in different contexts which reflects their complex role in the

network, all of these terms though could refer to the same MSC, but doing different things at different

times.

The Gateway MSC (G-MSC) is the MSC that determines which visited MSC the subscriber who is being

called is currently located at. It also interfaces with the PSTN. All mobile to mobile calls and PSTN to

mobile calls are routed through a G-MSC. The term is only valid in the context of one call since any MSC

may provide both the gateway function and the Visited MSC function, however, some manufacturers

design dedicated high capacity MSCs which do not have any BSSs connected to them. These MSCs will

then be the Gateway MSC for many of the calls they handle.
http://en.wikipedia.org/wiki/Routing_in_the_PSTNhttp://en.wikipedia.org/wiki/Pulse-code_modulationhttp://en.wikipedia.org/wiki/DS0http://en.wikipedia.org/wiki/Base_Station_Subsystemhttp://en.wikipedia.org/wiki/Base_Station_Subsystemhttp://en.wikipedia.org/wiki/DS0http://en.wikipedia.org/wiki/Pulse-code_modulationhttp://en.wikipedia.org/wiki/Routing_in_the_PSTN


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The visited MSC (V-MSC) is the MSC where a customer is currently located. The VLR associated with

this MSC will have the subscriber's data in it.

The anchor MSC is the MSC from which a handover has been initiated. The target MSC is the MSC

toward which a Handover should take place. A mobile switching centre server is a part of the redesigned

MSC concept starting from 3GPP Release 4.

Mobile switching centre server (MSCS)

The mobile switching centre server is a soft-switch variant of the mobile switching centre, which

provides circuit-switched calling, mobility management, and GSM services to the mobile

phones roaming within the area that it serves. MSS functionality enables split between control (signalling)

and user plane (bearer in network element called as media gateway/MG), which guarantees better

placement of network elements within the network.

MSS and MGW media gateway makes it possible to cross-connect circuit switched calls switched by

using IP, ATM AAL2 as well as TDM. More information is available in 3GPP TS 23.205.

Other GSM core network elements connected to the MSC

The MSC connects to the following elements:

The home location register (HLR) for obtaining data about the SIM and mobile services ISDN number

(MSISDN; i.e., the telephone number).

The base station subsystem which handles the radio communication with 2G and 2.5G mobile

phones.

The UMTS terrestrial radio access network (UTRAN) which handles the radio communication

with 3G mobile phones.

The visitor location register (VLR) for determining where other mobile subscribers are located.

Other MSCs for procedures such as handover.

Procedures implementedTasks of the MSC include:

Delivering calls to subscribers as they arrive based on information from the VLR.

Connecting outgoing calls to other mobile subscribers or the PSTN.

Delivering SMSs from subscribers to the short message service centre (SMSC) and vice versa.
http://en.wikipedia.org/wiki/Visitors_Location_Registerhttp://en.wikipedia.org/wiki/Handoffhttp://en.wikipedia.org/wiki/Mobile_switching_centre_serverhttp://en.wikipedia.org/w/index.php?title=3GPP_Release_4&action=edit&redlink=1http://en.wikipedia.org/wiki/Roaminghttp://en.wikipedia.org/wiki/Media_Gatewayhttp://en.wikipedia.org/wiki/Network_switching_subsystem#Home_location_register_.28HLR.29http://en.wikipedia.org/wiki/Subscriber_Identity_Modulehttp://en.wikipedia.org/wiki/MSISDNhttp://en.wikipedia.org/wiki/Base_Station_Subsystemhttp://en.wikipedia.org/wiki/2Ghttp://en.wikipedia.org/wiki/2.5Ghttp://en.wikipedia.org/wiki/UTRANhttp://en.wikipedia.org/wiki/3Ghttp://en.wikipedia.org/wiki/Visitor_Location_Registerhttp://en.wikipedia.org/wiki/Handoffhttp://en.wikipedia.org/wiki/GSM_services#Call_set_up_process_for_incoming_callshttp://en.wikipedia.org/wiki/SMSChttp://en.wikipedia.org/wiki/SMSChttp://en.wikipedia.org/wiki/GSM_services#Call_set_up_process_for_incoming_callshttp://en.wikipedia.org/wiki/Handoffhttp://en.wikipedia.org/wiki/Visitor_Location_Registerhttp://en.wikipedia.org/wiki/3Ghttp://en.wikipedia.org/wiki/UTRANhttp://en.wikipedia.org/wiki/2.5Ghttp://en.wikipedia.org/wiki/2Ghttp://en.wikipedia.org/wiki/Base_Station_Subsystemhttp://en.wikipedia.org/wiki/MSISDNhttp://en.wikipedia.org/wiki/Subscriber_Identity_Modulehttp://en.wikipedia.org/wiki/Network_switching_subsystem#Home_location_register_.28HLR.29http://en.wikipedia.org/wiki/Media_Gatewayhttp://en.wikipedia.org/wiki/Roaminghttp://en.wikipedia.org/w/index.php?title=3GPP_Release_4&action=edit&redlink=1http://en.wikipedia.org/wiki/Mobile_switching_centre_serverhttp://en.wikipedia.org/wiki/Handoffhttp://en.wikipedia.org/wiki/Visitors_Location_Register


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Arranging handovers from BSC to BSC.

Carrying out handovers from this MSC to another.

Supporting supplementary services such as conference calls or call hold.

Generating billing information.

Home location register (HLR)
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The home location register (HLR) is a central database that contains details of each mobile phone

subscriber that is authorized to use the GSM core network. There can be several logical, and physical,

HLRs per public land mobile network (PLMN), though one international mobile subscriber

identity (IMSI)/MSISDN pair can be associated with only one logical HLR (which can span several

physical nodes) at a time.

The HLRs store details of every SIM card issued by the mobile phone operator. Each SIM has a unique

identifier called an IMSI which is the primary key to each HLR record.

The next important items of data associated with the SIM are the MSISDNs, which are the telephone

numbers used by mobile phones to make and receive calls. The primary MSISDN is the number used for

making and receiving voice calls and SMS, but it is possible for a SIM to have other secondary MSISDNs

associated with it for fax and data calls. Each MSISDN is also a primary key to the HLR record. The HLR

data is stored for as long as a subscriber remains with the mobile phone operator.

Examples of other data stored in the HLR against an IMSI record is:

GSM services that the subscriber has requested or been given.

GPRS settings to allow the subscriber to access packet services.

Current location of subscriber (VLR and serving GPRS support node/SGSN).

Call divert settings applicable for each associated MSISDN.

The HLR is a system which directly receives and processes MAP transactions and messages from

elements in the GSM network, for example, the location update messages received as mobile phones

roam around.

Other GSM core network elements connected to the HLR

The HLR connects to the following elements:

The G-MSC for handling incoming calls

The VLR for handling requests from mobile phones to attach to the network

The SMSC for handling incoming SMSs

The voice mail system for delivering notifications to the mobile phone that a message is waiting

The AUC for authentication and ciphering and exchange of data (triplets)
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Procedures implemented

The main function of the HLR is to manage the fact that SIMs and phones move around a lot. The

following procedures are implemented to deal with this:

Manage the mobility of subscribers by means of updating their position in administrative areas called

'location areas', which are identified with a LAC. The action of a user of moving from one LA to

another is followed by the HLR with a Location area update procedure.

Send the subscriber data to a VLR or SGSN when a subscriber first roams there.

Broker between the G-MSC or SMSC and the subscriber's current VLR in order to allow incoming

calls or text messages to be delivered.

Remove subscriber data from the previous VLR when a subscriber has roamed away from it.

Authentication centre (AUC)

Description

The authentication centre (AUC) is a function to authenticate each SIM card that attempts to connect to

the GSM core network (typically when the phone is powered on). Once the authentication is successful,

the HLR is allowed to manage the SIM and services described above. An encryption key is also
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generated that is subsequently used to encrypt all wireless communications (voice, SMS, etc.) between

the mobile phone and the GSM core network.

If the authentication fails, then no services are possible from that particular combination of SIM card and

mobile phone operator attempted. There is an additional form of identification check performed on the

serial number of the mobile phone described in the EIR section below, but this is not relevant to the AUC

processing.

Proper implementation of security in and around the AUC is a key part of an operator's strategy to

avoid SIM cloning.

The AUC does not engage directly in the authentication process, but instead generates data known

as tripletsfor the MSC to use during the procedure. The security of the process depends upon a shared

secret between the AUC and the SIM called the Ki. The Ki is securely burned into the SIM during

manufacture and is also securely replicated onto the AUC. ThisKi is never transmitted between the AUCand SIM, but is combined with the IMSI to produce a challenge/response for identification purposes and

an encryption key called Kc for use in over the air communications.

Other GSM core network elements connected to the AUC

The AUC connects to the following elements:

the MSC which requests a new batch of triplet data for an IMSI after the previous data have been

used. This ensures that same keys and challenge responses are not used twice for a particularmobile.


The AUC stores the following data for each IMSI:

the Ki

Algorithm id. (the standard algorithms are called A3 or A8, but an operator may choose a proprietary

one).

When the MSC asks the AUC for a new set of triplets for a particular IMSI, the AUC first generates a

random number known as RAND. This RANDis then combined with the Ki to produce two numbers as

follows:

The Kiand RANDare fed into the A3 algorithm and the signed response (SRES) is calculated.
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The Kiand RANDare fed into the A8 algorithm and a session key called Kc is calculated.

The numbers (RAND, SRES, Kc) form the triplet sent back to the MSC. When a particular IMSI requests

access to the GSM core network, the MSC sends the RANDpart of the triplet to the SIM. The SIM then

feeds this number and the Ki(which is burned onto the SIM) into the A3 algorithm as appropriate and an

SRES is calculated and sent back to the MSC. If this SRES matches with the SRES in the triplet (which it

should if it is a valid SIM), then the mobile is allowed to attach and proceed with GSM services.

After successful authentication, the MSC sends the encryption key Kc to the base station controller (BSC)

so that all communications can be encrypted and decrypted. Of course, the mobile phone can generate

the Kc itself by feeding the same RAND supplied during authentication and the Ki into the A8 algorithm.

The AUC is usually collocated with the HLR, although this is not necessary. Whilst the procedure is

secure for most everyday use, it is by no means crack proof. Therefore a new set of security methods

was designed for 3G phones.

A3 Algorithm is used to encrypt Global System for Mobile Communications (GSM) cellular

communications. In practice, A3 and A8 algorithms are generally implemented together (known as

A3/A8). An A3/A8 algorithm is implemented in Subscriber Identity Module (SIM) cards and in GSM

network Authentication Centres. It is used to authenticate the customer and generate a key for encrypting

voice and data traffic, as defined in 3GPP TS 43.020 (03.20 before Rel-4). Development of A3 and A8

algorithms is considered a matter for individual GSM network operators, although example

implementations are available.

Visitor location register (VLR)
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Description

The visitor location register is a database of the subscribers who have roamed into the jurisdiction of

the MSC (Mobile Switching Center) which it serves. Each base station in the network is served by exactly

one VLR, hence a subscriber cannot be present in more than one VLR at a time.
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The data stored in the VLR has either been received from the HLR, or collected from the MS (Mobile

station). In practice, for performance reasons, most vendors integrate the VLR directly to the V-MSC and,

where this is not done, the VLR is very tightly linked with the MSC via a proprietary interface. Whenever

an MSC detects a new MS in its network, in addition to creating a new record in the VLR, it also updates

the HLR of the mobile subscriber, apprising it of the new location of that MS. If VLR data is corrupted it

can lead to serious issues with text messaging and call services.

Data stored include:

IMSI (the subscriber's identity number).

Authentication data.

MSISDN (the subscriber's phone number).

GSM services that the subscriber is allowed to access.

Access point (GPRS) subscribed.

The HLR address of the subscriber.

Other GSM core network elements connected to the VLR

The VLR connects to the following elements:

The V-MSC to pass required data for its procedures; e.g., authentication or call setup.

The HLR to request data for mobile phones attached to its serving area.

Other VLRs to transfer temporary data concerning the mobile when they roam into new VLR areas.

For example, the temporal mobile subscriber identity (TMSI).


The primary functions of the VLR are:

To inform the HLR that a subscriber has arrived in the particular area covered by the VLR.

To track where the subscriber is within the VLR area (location area) when no call is ongoing.

To allow or disallow which services the subscriber may use.

To allocate roaming numbers during the processing of incoming calls.

To purge the subscriber record if a subscriber becomes inactive whilst in the area of a VLR. The VLR

deletes the subscriber's data after a fixed time period of inactivity and informs the HLR (e.g., when
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the phone has been switched off and left off or when the subscriber has moved to an area with no

coverage for a long time).

To delete the subscriber record when a subscriber explicitly moves to another, as instructed by the

HLR.

Equipment identity register (EIR)

Theequipment identity registeris often integrated to the HLR. The EIR keeps a list of mobile phones

(identified by their IMEI) which are to be banned from the network or monitored. This is designed to allow

tracking of stolen mobile phones. In theory all data about all stolen mobile phones should be distributed to

all EIRs in the world through a Central EIR. It is clear, however, that there are some countries where this

is not in operation. The EIR data does not have to change in real time, which means that this function can

be less distributed than the function of the HLR. The EIR is a database that contains information about

the identity of the mobile equipment that prevents calls from stolen, unauthorized or defective mobile

stations. Some EIR also have the capability to log Handset attempts and store it in a log file.

Other support functions

Connected more or less directly to the GSM core network are many other functions.
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Billing centre (BC)

The billing centre is responsible for processing the toll tickets generated by the VLRs and HLRs and

generating a bill for each subscriber. It is also responsible for generating billing data of roaming

subscriber.

Short message service centre (SMSC)

Theshort message service centresupports the sending and reception of text messages.

Multimedia messaging service centre (MMSC)

Themultimedia messaging service centre supports the sending of multimedia messages (e.g.,

images, audio, video and their combinations) to (or from) MMS-enabled Handsets.

Voicemail system (VMS)

Thevoicemail system records and stores voicemails.

Lawful interception functions

According to U.S. law, which has also been copied into many other countries, especially in Europe, all

telecommunications equipment must provide facilities for monitoring the calls of selected users. There

must be some level of support for this built into any of the different elements. The concept of lawful

interceptionis also known, following the relevant U.S. law, asCALEA. Generally Lawful Interception

implementation is similar to the implementation of conference call. While A and B are talking with each

other, C can join the call and listen silently.

GPRS Core Network

The GPRS core network is the central part of the General Packet Radio Service which

allows 2G, 3G and WCDMA mobile networks to transmit IP packets to external networks such as the Internet.

The GPRS system is an integrated part of the GSM network switching subsystem.

Contents
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[hide]

1 General support functions

2 GPRS tunnelling protocol (GTP)

3 GPRS support nodes (GSN)

o 3.1 Gateway GPRS Support Node (GGSN)

o 3.2 Serving GPRS Support Node (SGSN)

3.2.1 Common SGSN Functions

3.2.2 GSM/EDGE specific SGSN functions

3.2.3 WCDMA specific SGSN functions

4 Access point

5 PDP Context

6 Reference Points and Interfaces

o 6.1 Interfaces in the GPRS network

General support functions

GPRS core structure

The GPRS core network provides mobility management, session management and transport for Internet

Protocol packet services in GSM and WCDMA networks. The core network also provides support for other

additional functions such as billing and lawful interception. It was also proposed, at one stage, to support
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packet radio services in the US D-AMPS TDMA system, however, in practice, all of these networks have been

converted to GSM so this option has become irrelevant.

Like GSM in general, GPRS module is an open standards driven system. The standardization body is

the 3GPP.

GPRS tunnelling protocol (GTP)
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GPRS tunneling protocolis the defining IP protocol of the GPRS core network. Primarily it is the protocol which

allows end users of a GSM or WCDMA network to move from place to place while continuing to connect to the

Internet as if from one location at the Gateway GPRS Support Node (GGSN). It does this by carrying the

subscriber's data from the subscriber's currentServing GPRS Support Node (SGSN) to the GGSN which is

handling the subscriber's session. Three forms of GTP are used by the GPRS core network.

GTP-U

for transfer of user data in separated tunnels for each Packet Data Protocol (PDP) context

GTP-C

for control reasons including:

setup and deletion of PDP contexts

verification of GSN reach ability

Updates; e.g., as subscribers move from one SGSN to another.

GTP'

for transfer of charging data from GSNs to the charging function.

GGSNs and SGSNs (collectively known as GSNs) listen for GTP-C messages on UDP port 2123

and for GTP-U messages on port 2152. This communication happens within a single network or

may, in the case of international roaming, happen internationally, typically across a GPRS

roaming exchange (GRX).

The Charging Gateway Function(CGF) listens to GTP' messages sent from the GSNs on TCP or

UDP port 3386. The core network sends charging information to the CGF, typically including PDP

context activation times and the quantity of data which the end user has transferred. However,

this communication which occurs within one network is less standardized and may, depending on

the vendor and configuration options, use proprietary encoding or even an entirely proprietary

system.

GTP version zero supports both signalling and user data under one generic header. It can be

used with UDP (User Datagram Protocol) or TCP (Transmission Control Protocol) on the

registered port 3386. GTP version one is used only on UDP. The control plane protocol GTP-C

(Control) using registered port 2123 and the user plane protocol GTP-U (User) using registered

port 2152.

GPRS support nodes (GSN)
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A GSN is a network node which supports the use of GPRS in the GSM core network. All GSNs

should have a Gninterface and support the GPRS tunneling protocol. There are two key variants

of the GSN, namely Gateway and Serving GPRS Support Node.

Gateway GPRS Support Node (GGSN)

The Gateway GPRS Support Node (GGSN) is a main component of the GPRS network. The

GGSN is responsible for the interworking between the GPRS network and external packet

switched networks, like the Internet and X.25 networks.

From an external network's point of view, the GGSN is a router to a sub-network, because the

GGSN hides the GPRS infrastructure from the external network. When the GGSN receives data

addressed to a specific user, it checks if the user is active. If it is, the GGSN forwards the data to

the SGSN serving the mobile user, but if the mobile user is inactive, the data is discarded. On the

other hand, mobile-originated packets are routed to the right network by the GGSN.

The GGSN is the anchor point that enables the mobility of the user terminal in the

GPRS/UMTS networks. In essence, it carries out the role in GPRS equivalent to the Home

Agent inMobile IP. It maintains routing necessary to tunnel the Protocol Data Units (PDUs) to the

SGSN that service a particular MS (Mobile Station).

The GGSN converts the GPRS packets coming from the SGSN into the appropriate packet data

protocol (PDP) format (e.g., IP or X.25) and sends them out on the corresponding packet data

network. In the other direction, PDP addresses of incoming data packets are converted to the

GSM address of the destination user. The readdressed packets are sent to the responsible

SGSN. For this purpose, the GGSN stores the current SGSN address of the user and his or her

profile in its location register. The GGSN is responsible for IP address assignment and is the
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default router for the connected user equipment (UE). The GGSN also performs authentication

and charging functions.

Other functions include subscriber screening, IP Pool management and address

mapping, QoS and PDP context enforcement.

With LTE scenario the GGSN functionality moves to SAE gateway (with SGSN functionality

working in MME).

Serving GPRS Support Node (SGSN)

A Serving GPRS Support Node (SGSN) is responsible for the delivery of data packets from and

to the mobile stations within its geographical service area. Its tasks include packet routing and

transfer, mobility management (attach/detach and location management), logical link

management, and authentication and charging functions. The location register of the SGSN

stores location information (e.g., current cell, current VLR) and user profiles (e.g., IMSI,

address(es) used in the packet data network) of all GPRS users registered with this SGSN....

Common SGSN Functions

Detunnel GTP packets from the GGSN (downlink)

Tunnel IP packets toward the GGSN (uplink)

Carry out mobility management as Standby mode mobile moves from one Routing Area to

another Routing Area

Billing user data
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GSM/EDGE specific SGSN functions

Enhanced Data Rates for GSM Evolution (EDGE) specific SGSN functions and characteristics

are:

Maximum data rate of approx. 60 kbit/s (150 kbit/s for EDGE) per subscriber

Connect via frame relay or IP to the Packet Control Unit using the Gb protocol stack

Accept uplink data to form IP packets

Encrypt down-link data, decrypt up-link data

Carry out mobility management to the level of a cell for connected mode mobiles

WCDMA specific SGSN functions

Carry up to about 42 Mbit/s traffic downlink and 5.8 Mbit/s traffic uplink (HSPA+)

Tunnel/detunnel downlink/uplink packets toward the radio network controller (RNC)

Carry out mobility management to the level of an RNC for connected mode mobiles

These differences in functionality have led some manufacturers to create specialist SGSNs for

each of WCDMA and GSM which do not support the other networks, whilst other manufacturers

have succeeded in creating both together, but with a performance cost due to the compromises

required.

Access point

An access point is:

An IP network to which a mobile can be connected

A set of settings which are used for that connection

A particular option in a set of settings in a mobile phone
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When a GPRS mobile phone sets up a PDP context, the access point is selected. At this point

an access point name (APN) is determined

Example: aricenttechnologies.mnc012.mcc345.gprs

Example: Internet

Example: mywap

Example: hcl.cisco.ggsn

This access point is then used in a DNS query to a private DNS network. This

process (called APN resolution) finally gives the IP address of the GGSN which

should serve the access point. At this point a PDP context can be activated.

PDP Context

The packet data protocol (PDP; e.g., IP, X.25, FrameRelay) context is a data

structure present on both the Serving GPRS Support Node(SGSN) and

the Gateway GPRS Support Node(GGSN) which contains the subscriber's

session information when the subscriber has an active session. When a mobile

wants to use GPRS, it must first attach and then activate a PDP context. This

allocates a PDP context data structure in the SGSN that the subscriber is

currently visiting and the GGSN serving the subscriber's access point. The data

recorded includes

Subscriber's IP address

Subscriber's IMSI

Subscriber's

Tunnel Endpoint ID (TEID) at the GGSN

Tunnel Endpoint ID (TEID) at the SGSN

The Tunnel Endpoint ID (TEID) is a number allocated by the GSN which

identifies the tunnelled data related to a particular PDP context.

Several PDP contexts may use the same IP address. The Secondary PDP

Context Activation procedure may be used to activate a PDP context while

reusing the PDP address and other PDP context information from an already

active PDP context, but with a different QoS profile.[1]Note that the procedure

is called secondary, not the resulting PDP contexts that have no such

relationship with the one the PDP address of which they reused.
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