31.4 GSM ARCHITECTURE

< Day Day Up >

To facilitate roaming from one country to another within Europe using the same mobile terminal, in 1983 the European Telecommunications Standards Institute (ETSI) formed Groupe Speciale Mobile (GSM) to develop standards for mobile communication system. Because the standard has been adapted widely by many countries in Asia, Africa, and the Middle East in addition to Europe, GSM is now known as Global System for Mobile Communications.

The Global System for Mobile Communications (GSM) standard was developed by the European Standards Telecommunications Institute (ETSI). GSM systems are now installed in Asia, Africa, and the Middle East, in addition to Europe.

The Memorandum of Understanding for GSM was signed by 17 European operators and manufacturers in 1987. The first trial version of GSM was developed in 1991, and the commercial systems were launched in 1992. During the initial days of GSM, many people were skeptical about its commercial viability because of the complex protocol architecture. GSM used to be called "God, Send Mobiles" because of the complexity involved in developing handsets. Thanks to the advances in microelectronics, GSM handsets now go into pockets.

31.4.1 Salient Features of GSM

The salient features of GSM are:

It is based on digital technology, so security can be built into the system easily and has all the advantages of the digital communication systems.
Since the interfaces are standardized, network elements manufactured by different equipment vendors can work with one another, paving the way for competition, and hence the network operator and the subscriber will be benefited.
A higher calling capacity per cell (about 125 calls per cell) as compared to analog systems.
Support for international roaming.
In addition to voice services, data services are also supported.

The broad specifications of the GSM system are:

Frequency band: 900MHz band (890-915 MHz for uplink and 935-960 MHz for downlink). As the 900MHz band become congested, 1800MHz band has been allocated with 1710-1785 MHz for uplink and 1805-1880 MHz for downlink. The systems operating in 1800MHz band are referred to as DCS 1800.
Duplex distance (distance between uplink and downlink frequencies): 45MHz.
Channel spacing (between adjacent carrier frequencies): 200kHz.
Modulation: Gaussian minimum shift keying (GMSK). GMSK is a special form of FSK, ones and zeros are represented by shifting the RF carrier plus or minus 67.708kHz. FSK modulation in which the bit rate is exactly four times the frequency shift is called minimum shift keying (MSK). As the modulation spectrum is reduced by applying a Gaussian premodulation filter to avoid spreading energy into adjacent channels, the modulation is called Gaussian MSK (GMSK).
Transmit data rate (over the air bit rate): 270.833kbps, which is exactly four times the RF frequency shift.
Access method: TDMA with eight time slots.
Speech codec: 13kbps using residually excited linear prediction (RELP), a derivative of the LPC coding technique.
Signaling: Signaling System No. 7 (SS7) is used to carry out signaling. Hence, the radio channels are used efficiently for speech transmission.

GSM is a digital cellular system operating in the 900MHz band. It uses TDMA access technology. A 13kbps voice codec is used to transmit digitized voice.

31.4.2 GSM Services

GSM services are divided into telephony services (referred to as teleservices) and data services (referred to as bearer services). In addition to the normal telephony services, the following services are also supported:

Group 3 facsimile transmission through a special interface.
Short messaging service (SMS) to transmit a maximum of 160 alphanumeric characters. If the handset is turned off or is out of the coverage area, the message will be stored in a message center and sent to the handset when it is turned on or when it is within the coverage area.
Cell broadcast to transmit maximum of 93 characters to all the handsets in a particular cell. This service can be used to transmit information regarding traffic congestion, accident information, and so on.
Voice mail.
Fax mail.

In addition to the voice service, GSM supports the following services: short messaging service (SMS), with which messages with a maximum of 160 characters can be sent, Group 3 fax, voice mail, and fax mail.

The GSM system also supports the following supplementary services:

Call forwarding, to forward a call to another mobile handset or a land line.
Barring outgoing calls.
Barring incoming calls. All incoming calls can be barred or only incoming calls when roaming outside a PLMN can be barred.
Advice of charge, which gives an estimate of the call charges based on the call duration.
Call hold, to interrupt a call and then reestablish it again.
Call waiting, to announce an incoming call when a conversation is in progress.
Multiparty service to provide conferencing facility.
Calling Line Identification Presentation (CLIP) to display the telephone number of the calling party.
Closed user groups (CUGs), which emulate the function of a PBX. A predefined group of mobile terminals will form the equivalent of a PBX.

The supplementary services supported by GSM are call forwarding, call barring, call holding, call waiting, calling line identification, advice of charge, and closed user groups.

Note

In Closed user group service, a number of mobile subscribers can communicate among themselves at reduced charges.

31.4.3 GSM System Architecture

The GSM architecture is shown in Figure 31.2.

click to expand
Figure 31.2: GSM architecture.

A mobile communications service provider operates in a given geographical region. The mobile network of the entire region is known as a public land mobile network (PLMN). The PLMN will be in the administrative control of one operator. The PLMN consists of mobile stations (MS), base station subsystems (BSS), and network switching subsystem (NSS).

The MS can be handheld or car mounted. The BSS consists of base station controller (BSC) and base transceiver subsystem (BTS). The NSS consists of mobile switching center (MSC), home location register (HLR), equipment identity register (EIR), authentication center (AuC) and visitor location register (VLR). In addition to these elements, there will be an operation and management center (OMC) that provides the man-machine interface to carry out administrative functionality such as subscriber management, network management, billing, and so on.

The GSM system consists of mobile stations, base transceiver subsystem (BTS), base station controller (BSC), and mobile switching center (MSC). Public networks such as PSTN and ISDN are connected to the MSC.

The PLMN is connected to the Public Switched Telephone Network (PSTN) or Public Data Network (PDN) or Integrated Services Digital Network (ISDN) at the MSC. The functions of each element of the GSM system are described in the following:

Note

The home location register (HLR), visitor location register (VLR), equipment identity register (EIR), and authentication center (AuC) are databases that will be accessed by the MSC. These databases are implemented using an RDBMS engine such as Oracle or Informix.

Mobile station (MS): Also known as mobile handset or hand phone, this is the subscriber terminal. Nowdays, mobile terminals are coming with many features-voice dialing, whereby one can use voice to dial out a number, powerful batteries to provide at least 6 hours of talk time and 4 to 5 days of standby time, and such. The power transmitted by the MS is in the range 0.8- 20 watts.

BSC contains the transcoders that convert the PCM-coded speech into 13kbps data for sending it to the BTS. In the reverse direction, the 13kbps-coded speech is converted into 64kbps PCM data to send it to the MSC.

The MS is identified by a number known as MS-ISDN (the mobile phone number). Each MS is also uniquely identified by IMSI (international mobile subscriber identity). The MS contains a SIM (subscriber identity module). SIM is a smart card inserted in the handset. It is protected by a personal identity number (PIN). PIN is checked locally and not transmitted over the radio link. SIM contains IMSI. To identify a handset hardware uniquely, IMEI (international mobile equipment identity) is used, which is a number given by the manufacturer to the handset.

BSS: The BSS acts as a relay between the NSS and the mobile stations. The BSS consists of BSC and BTSs. The service area is arranged into cells, and each cell will have a BTS. Each cell can vary from 350 meters to 35 kilometers, depending on the terrain and the subscriber density. Multiple BSSs can be controlled by one BSC.

BSC: BSC handles radio management functions. BSC arranges new radio link connections to mobile stations when handover is required. It is connected to the MSC through landlines, normally 2Mbps links using PCM for voice transmission. To reduce radio bandwidth, GSM uses low bit rate coding of speech at 13kbps between the MS and the BSC. BSC does the transcoding-conversion of the 13kbps speech to PCM and vice versa. Each BSC controls a number of BTSs, typically up to 40.

BTS: BTS is the radio interface between the MS and the BSC. Communication between the MS and the BTS is through one channel consisting of a pair of frequencies-one for uplink and one for downlink. The frequency allocation for GSM in 900MHz band is depicted in Figure 31.3. Carriers are separated by 200kHz with 16kbps as the aggregate data rate per channel. To overcome signal fading and reflections due to highrise buildings and such, frequency hopping is used. Depending on the shape of the cell, omnidirectional or sectoral antennas are used at each BTS. The maximum power transmitted by the BTS is in the range 0.25-320 watts. To conserve radio spectrum, 13kbps low bit rate voice coding is used. BTS uses TDMA for multiple access, with 8 slots per channel. The TDMA frame format is shown in Figure 31.4. This is a very simplified format-TDMA slots are also used to carry out all the signaling between the BSC and the MS.

click to expand
Figure 31.3: Frequency allocation for GSM.

click to expand
Figure 31.4: TDMA frame format in GSM.

TDMA format: Each data bit is of 3.692 microseconds duration. Each time slot has a time period equal to 156.25 data bits. There are eight time slots per frame, and hence frame period is 4.615 milliseconds. Twenty-six or 51 frames are grouped together to make a multiframe. A superframe consists of 51 or 26 multiframes. This complex frame and multiframe structure is used to transmit control information, to carry out synchronization, and of course, to carry speech data.

Some slots in the TDMA frame are exclusively for transmitting the signaling information. Short messages are also sent in the signaling slots.

TCH (traffic channel) carries the bidirectional speech data between the mobile station and the base station. Each base station produces a BCH (broadcast channel), which acts as a beacon signal to find service and decode network information. BCH occupies time slot zero. Each cell is given a number of frequency pairs (channels) denoted by ARFCN (absolute radio frequency channel numbers). If a cell has one ARFCN, there will be one BCH and seven time slots for TCH. If there are two ARFCN's in one cell, there will be one BCH and 15 time slots for TCH.

Home location register (HLR): HLR is a centralized database to manage the subscriber data. It is a standalone system connected to GSM network subsystems with Signaling System No. 7. This database contains:

Subscriber information
Subscriber rights and privileges (what types of calls are permitted)
Location information
Activity status

HLR permanently knows the location of the subscriber. When an MS receives a call, the HLR is consulted, and the database translates the MS-ISDN number to IMSI number. HLR reroutes incoming calls to the MSC target or ISDN number when call forwarding is requested.

Authentication center (AuC): AuC contains security functions such as IMSI, the encryption key, and the algorithm to be used for encryption. AuC provides the data to verify the identity of each user and to provide confidentiality of the conversation/data.

HLR/AuC are administered by man-machine interface (MMI) commands from the OMC.

Visitor location register (VLR): VLR contains information about all the mobile subscribers currently located in the MSC service area. VLR is generally integrated into MSC. When a mobile station roams into a new MSC service area, the VLR connected to that MSC gets the data about the mobile station from the HLR and stores it. Temporary mobile subscriber identity (TMSI) is assigned by VLR and is used for call establishment. VLR is responsible for the current location of the user.

Equipment identity register (EIR): EIR contains information about the mobile equipment. Each MS is uniquely identified by IMEI (international mobile equipment identity). When a mobile handset is lost, the subscriber informs the customer support center, and this information is stored in the EIR. When the lost mobile is used for making a call, the EIR will not permit the call. Because EIR also provides the security, EIR and AuC can be combined into one computer.

Note

The mobile phone hardware is uniquely identified by the international mobile equipment identity (IMEI). Hence, it is possible to trace a stolen mobile phone if the thief is using it.

Mobile switching center (MSC): MSC provides complete switching functionality for the entire network, so all call control functions are built in the MSC. It also provides the interface to the rest of the world-PSTN, PDN, ISDN, and so on. MSC is also connected to the OMC through which the configuration of the network, entry and modification of the subscriber data, traffic analysis, billing, and other network management functions can be carried out.

Note

A service area can have more than one MSC. In such a case, one of the MSCs is designated as gateway MSC. Public networks such as PSTN are connected to the gateway MSC.

Operation and maintenance center (OMC): The OMC is used to carry out network management activities such as fault diagnosis of various network elements, traffic analysis, billing, performance management, configuration management (such as adding a new BSC and cell splitting), as well as managing subscriber information.

The message center is a computer that is used to store and forward SMS, voice mail, fax mail, and so on. Many value-added services such as examination results announcements and astrological services can be provided through the message center.

The communication between the MSC and the databases (HLR, EIR, AuC) is through an SS7 network, because only signaling information is exchanged between these entities. The communication between the OMC and the MSC/BSC is through a packet switching network based on X.25 standards.

In addition, the GSM system can contain the following network elements:

Message center: Message center is a node that provides voice, data, and fax messaging. It handles the SMS, cell broadcast, voice mail, fax mail, and e-mail messaging. Separate servers are required to handle these messaging systems, which are connected to the MSC.

Gateway MSC: When a PLMN contains more than one MSC, one of the MSCs is designated as a gateway MSC to interconnect with other networks such as PSTN and ISDN. If the PLMN contains only one MSC, that MSC itself can act as a gateway MSC.

31.4.4 GSM Network Areas

In a GSM network, the following areas are defined: cell, location area, service area, and PLMN.

Cell: Cell is the basic service area; one BTS covers one cell. Each cell is given a cell global identity (CGI), a number that uniquely identifies the cell.

The area covered by one network operator is called the PLMN. The area covered by one MSC is called the MSC/VLR area. The area paged to locate a subscriber is called location area. The area covered by one BTS is called a cell.

Location area: A group of cells form a location area. This is the area that is paged when a subscriber gets an incoming call. Each location area is assigned a location area identity (LAI). Each location area is served by one or more BSCs.

MSC/VLR service area: The area covered by one MSC is called the MSC/VLR service area.

PLMN: The area covered by one network operator is called PLMN. A PLMN can contain one or more MSCs.

Figure 31.5 shows a GSM PLMN serving two cities. Each city will have a number of BTSs and one BSC. The two BSCs are connected to the MSC. MSC also acts as the gateway to the PSTN. The number of BTSs in a city depends on the subscriber density and the terrain. Presently, each Indian city is covered by about 100 to 200 BTSs. SingTel, the cellular operator in Singapore, installed 1800 BTSs-this is because of the very high subscriber density in Singapore.

click to expand
Figure 31.5: GSM PLMN serving two cities.

31.4.5 GSM Operation

The operation of the GSM system can be understood by studying the sequence of events that takes place when a call is initiated.

Call from Mobile Station

When a mobile subscriber makes a call to a PSTN telephone subscriber, the following sequence of events takes place:

The MSC/VLR receives the message of a call request.
The MSC/VLR checks if the mobile station is authorized to access the network. If so, the mobile station is activated.
MSC/VLR analyzes the number and initiates a call setup with the PSTN.
MSC/VLR asks the corresponding BSC to allocate a traffic channel (a radio channel and a time slot).
BSC allocates the traffic channel and passes the information to the mobile station.
Called party answers the call, and the conversation takes place.
The mobile station keeps on taking measurements of the radio channels in the present cell and neighboring cells and passes the information to the BSC. BSC decides if handover is required and, if so, a new traffic channel is allocated to the mobile station and the handover is performed.

When a mobile phone initiates a call, the MSC verifies the authenticity of the mobile device and initiates call setup with the PSTN. A frequency and a voice slot are allocated to the mobile device.

Call to a Mobile Station

When a PSTN subscriber calls a mobile station, the sequence of events is as follows:

The gateway MSC receives the call and queries the HLR for the information needed to route the call to the serving MSC/VLR.
The GMSC routes the call to the MSC/VLR.
MSC checks the VLR for the location area of the MS.
MSC contacts the MS via the BSC by sending a pager request.
MS responds to the page request.
BSC allocates a traffic channel and sends a message to the MS to tune to the channel. The MS generates a ringing signal and, when the subscriber answers, the speech connection is established.
Handover, if required, takes place, as discussed in the earlier case.

Note that the MS codes the speech at 13kbps for transmission over the radio channel in the given time slot. The BSC transcodes the speech to 64kbps and sends it over a land link or radio link to the MSC. MSC then forwards the speech data to the PSTN. In the reverse direction, the speech is received at 64kbps rate at the BSC, and the BSC does the transcoding to 13kbps for radio transmission.

< Day Day Up >