System for Mobile communications

What is GSM?Evolution of GSM.Fundamental of GSMArchitecture of GSMEstablishment of NetworkFundamental of Radio PropagationCall flow

What is GSM?

The Global System for Mobile communications is a digital cellular communications system. It was developed in order to create a common European mobile telephone standard but it has been rapidly accepted worldwide. Formerly it was Groupe Spéciale Mobile

(founded 1982)now: Global System for Mobile Communication

Evolution of GSM.

The idea of cell-based mobile radio systems appeared at Bell Laboratories (in USA) in the early 1970s. However, mobile cellular systems were not introduced for commercial use until the 1980s.

But in the beginnings of cellular systems, each country developed its own system, which was an undesirable situation for the following reasons:  

The equipment was limited to operate only within the boundaries of each country.

The market for each mobile equipment was limited.

In order to overcome these problems, the Conference of European Posts and telecommunications (CEPT) formed, in 1982 ,the Group Special Mobile (GSM) in order to develop a pan-European mobile cellular radio system . The standardized system had to meet certain criteria:

Spectrum efficiency International roaming Low mobile and base stations costs Good subjective voice quality Compatibility with other systems such as ISDN

(Integrated Services Digital Network) Ability to support new services

YEAR EVENT TOOK PLACE

1982 CEPT establishes a GSM group in order to develop the standards for a pan-European cellular mobile system

CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile

GSM created to set standard

1985 Adoption of a list of recommendations to be generated by the group

1987 Digital Technology standards set for TDMA, speech coding, channel: and modulation method.

Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install system in

1991

1988 Industrial development started

1989 Acceptance of GSM-1800 system, with GSM as standard

1991 First system deployed (July)

1992 First GSM terminals receive interim type approval

1993 First GSM-1800 network launched

1995 First GSM-1900 network in US & Canada

GENERATION OF GSM

1st generation:- Analog mobile technologies :- AMPS , TACS & NMT.

2nd generation:- digital mobile technologies :- GSM , CDMA

2.5generation:- Enhancement of GSM:- GPRS

3rd generation:- Technologies coursed by ITU-IMT

The following table lists the key events in the GSM evolution

AMPS—Advanced Mobile Phone System TACS----Total Access Communication System. NMT----Nordic Mobile Telephones

GSM

EDGE

2.5 G

GPRS

2G

2.5 g+

UMTS 3G WCDMA

GPRS-Wireless Data Services .EDGE—Provides 3 times the data capacity of

GPRS.3G---Uses WCDMA technologies Over Air

interface (5MHz).

GSM STANDARDSGSM-900 Standard

The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band. This so called “primary band" includes two sub bands of 25 MHz

GSM-1800 Standard

In GSM-1800, 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz. This was three times the bandwidth allocated for GSM-900.

GSM-1900 Standard

GSM-1900 is the standard for the 1900MHz band. It includes the same network component as the GSM-900 or GSM-1800. The band width of this standard is 60 MHz.

GSM Channels/CarriersThe following table will show the channels and carriers of different

GSM models

P-GSM(Primary-GSM)

900

E-GSM

900(Extended-GSM)

R-GSM

900(Railways-GSM)

GSM 1800

GSM 1900

Uplink frequency 890-915 MHz

880-915 MHz

886-915 MHz

1710-1785 MHz

1850-1910 MHz

Down link frequency

935-960 MHz

925-960 MHz

931-960 MHz

1805-1855 MHz

1930-1990 MHz

Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz

200 kHz

Carrier Frequency

124 174 144 374 299

Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ

80 MHZ

Type of access TechnologyFDMA (Frequency division Multiple Access):- In

FDMA, signals from various users are assigned different frequencies. Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels.

TDMA (TIME DIVISION MULTIPLE ACCESS):- In a TDMA system, data from each user is conveyed in time intervals called “Time slots”. Several slots make up a frame . Each slot is made up of a preamble plus information bits addressed to various stations .the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver .Guard times are used between each user’s transmission to minimize crosstalk between channels.

GSM NUMBERING STRUCTURE:

MSISDN :- The real telephone number of a mobile station is the mobile subscriber ISDN number (MSISDN). It is assigned to the subscriber (his or her SIM, respectively), such that a mobile station set can have several MSISDNs depending on the SIM. - CC+NDC+SN - 12digits

IMSI - Each registered user is uniquely identified by its international mobile subscriber identity (IMSI). It is stored in the subscriber identity module (SIM) A mobile station can only be operated if a SIM with a valid IMSI is inserted into equipment with a valid IMEI. MCC (3)+MNC (2)+MSIN (10) - 15 digits

TMSI - The VLR, which is responsible for the current location of a subscriber, can assign a temporary mobile subscriber identity (TMSI) which has only local significance in the area handled by the VLR. It is stored on the network side only in the VLR and is not passed to the HLR. 4 octets

IMEI - The international mobile station equipment identity (IMEI) uniquely identifies a mobile station internationally. It is a kind of serial number. The IMEI is allocated by the equipment manufacturer and registered by the network operator and registered by the network operator who stores it in the EIR. By means of IMEI one recognizes obsolete, stolen or nonfunctional equipment. TAC+FAC+SNR+Spare - 15 digits

LAI - Each LA of an PLMN has its own identifier. The Location Area Identifier (LAI) is also structured hierarchically and internationally unique as follows: MCC+MNC+LAC

GSM Multiplex structure

GSM works on a combination of frequency-division multiplexing and time-division multiplexing for providing multiple access by mobile stations.

  GSM utilizes a cellular concept, in which a

geographical area is divided into planned radio cells , with one BTS per cell with which a mobile can make contact. The radio cells, each having the exclusive use of specific FDM channels. The same frequencies are only used after sufficiently long distances in neighboring cell clusters. The cell radius can vary according to user density from 300 mtrs to 35 Kms.

 

Cells

Different interfaces used in Mobile Um interface   The "air" or radio interface standard that is used for exchanges

between a mobile (ME) and a base station (BTS / BSC).

Abis interface   This is a BSS internal interface linking the BSC and a BTS, and it has not been totally standardised. The Abis interface allows control of the radio equipment and radio frequency allocation in the BTS.

A interface   The A interface is used to provide communication between the BSS

and the MSC. The interface carries information to enable the channels, timeslots and the like to be allocated to the mobile equipments being serviced by the BSSs. The messaging required within the network to enable handover etc to be undertaken is carried over the interface.

B interface   The B interface exists between the MSC and the VLR . As most VLRs are collocated with an MSC, this makes the interface purely an "internal" interface. The interface is used whenever the MSC needs access to data regarding a MS located in its area.

C interface   The C interface is located between the HLR and a GMSC or a SMS-G. When a call originates from outside the network, i.e. from the PSTN or another mobile network it ahs to pass through the gateway so that routing information required to complete the call may be gained. In addition to this, the MSC may optionally forward billing information to the HLR after the call is completed and cleared down.

D interface   The D interface is situated between the VLR and HLR.

E interface   The E interface provides communication between two MSCs. The E interface exchanges data related to handover between the anchor .

F interface   The F interface is used between an MSC and EIR. The communications along this interface are used to confirm the status of the IMEI of the ME gaining access to the network.

G interface   The G interface interconnects two VLRs of different MSCs and uses to transfer subscriber information, during e.g. a location update procedure.

H interface   The H interface exists between the MSC the SMS-G. It transfers short messages

I interface   The I interface can be found between the MSC and the ME. Messages exchanged over the I interface are relayed transparently through the BSS.

BASIC PARTS OF GSM

BSS/RSS:BTS and BSCNSS:MSC(HLR,VLR ,EIR,AUC)OSS : OMCR, OMCS

Mobile station(MS) The mobile station (MS) represents the terminal equipment used by the

wireless subscriber supported by the GSM Wireless system. Man machine interface. The SIM may be a removable module, while the equipment identity is not linked to a particular subscriber.

  Functions of a Mobile Station :- Radio transmission termination. Radio Channel Management. Speech Encoding/Decoding .Radio Link error Protection. Flow control of data. Mobility Management.

SIM (Subscriber identity Module) - It is basically a removable smart card in compliance with the ISO 7816 standard and a plug-in module (25 x 15 mm) .It includes a microprocessor with all the subscriber-related information . SIM (and consequently MS) is protected by a Personal Identification Number (PIN). It has a PIN Unblocking Key (PUK) used to unblock it.

  Information stored in a SIM card- Serial number International Mobile Subscriber Identity (IMSI) Security authentication and cyphering information Temporary Network information (LAI, TMSI) List of services subscribed by the user

BSSThe BSS is responsible for communicating with mobile stations in cell areas.

 BTS is a network element with transmission and reception devices

(transceivers) to and from the MS, including antennas signal processing specific devices for the Air interface management It can be considered as a complex radio modem controlled by the BSC It is involved also in the transmission and reception with the BSC

through the A-bis interface It has just executive functions (no management Function Broadcast/receive to/from the MS either signalling and traffic signals Perform source and channel coding Modulate/Demodulate signals to be broadcasted/received through the

Air interface radio channel Multiplex the information to be transmitted over each carrier.

BSC One BSC controls one or more BTS’s and can perform inter-BTS and intra-BTS handover.

 BSC FunctionControl and supervise the BTSsConfigure each cell with the allocation and the release

of traffic and signalling channelsManage the paging operationCollect the signals quality measures acquired by the

BTSs over the downlink and uplink channels Manage all the radio interfacesManage the handover procedures Transcode and Sub-multiplex the bit stream

Function of NSS Call controlidentification of the subscriberestablishing a call and release of the connection after the

call is overMobility management taking care of the location of the subscribers before,

during and after a callCollecting the charging information about a callnumber of the caller and of the called subscriberlength and type of the provided servicesTransfer the acquired charging information to the Billing

centreSignalling with other networks and BSS through the

different interfacesSubscriber data handlingData storage permanently or temporarily in some

databasesOMCR:- It is used to monitor and maintain the alarms of

the system.

The following logical channels are defined in GSM   : Traffic Channel TCHf - Full rate traffic channel. TCH h - Half rate traffic channel. Broadcast Channels BCCH - Broadcast Network information, e.g. for describing the current

control channel structure. The BCCH is a point-to-multipoint channel (BSS-to-MS).

SCH - Synchronisation of the MSs. FCHMS - frequency correction. Common Control Channels AGCH - Acknowledge channel requests from MS and allocate a SDCCH. PCHMS - terminating call announcement. RACHMS - access requests, response to call announcement, location

update, etc. CBCH: Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages, for example road traffic reports or network engineering messages.

NCH: Used for GSM Phase II voice services such as Voice Broadcast

Service (VBS) or Voice Group Calling Service (VGCSDedicated Control ChannelsDCCH comprise the following bi-directional (uplink /

downlink) point to point control channels:SDCCH - For signalling exchanges, e.g. during call setup,

registration / location updates.  FACCHs - FACCH for the SDCCH. The SDCCH burst is

stolen for a full signalling burst. Function not clear in the present version of GSM (could be used for e.g. handover of an eight-rate channel, i.e. using a "SDCCH-like" channel for other purposes than signalling).

SACCHs - SDCCH in-band signalling, e.g. for link monitoring

Types of GSM handover

Within the GSM system there are four types of handover that can be performed for GSM only systems:

Intra-BTS handover:   This form of GSM handover occurs if it is required to change the frequency or slot being used by a mobile because of interference, or other reasons. In this form of GSM handover, the mobile remains attached to the same base station transceiver, but changes the channel or slot.

Inter-BTS Intra BSC handover:   This for of GSM handover or GSM handoff occurs when the mobile moves out of the coverage area of one BTS but into another controlled by the same BSC. In this instance the BSC is able to perform the handover and it assigns a new channel and slot to the mobile, before releasing the old BTS from communicating with the mobile.

Inter-BSC handover:   When the mobile moves out of the range of cells controlled by one BSC, a more involved form of handover has to be performed, handing over not only from one BTS to another but one BSC to another. For this the handover is controlled by the MSC.

Inter-MSC handover:   This form of handover occurs when changing between networks. The two MSCs involved negotiate to control the handover