Gsm

INTRODUCTION TO GSM

INTRODUCTION

The Global System for Mobile Communications (GSM) is a set of recommendations and specifications for a digital cellular telephone network (known as a Public Land Mobile Network, or PLMN).

These recommendations ensure the compatibility of equipment from different GSM manufacturers, and interconnectivity between different administrations, including operation across international boundaries.

GSM networks are digital and can cater for high system capacities. They are consistent with the world-wide digitization of the telephone

network, and are an extension of the Integrated Services Digital Network (ISDN), using a digital radio interface between the cellular network and the mobile subscriber equipment.

INTRODUCTION TO GSM

CELLULAR TELEPHONY

A cellular telephone system links mobile subscribers into the public telephone system or to another cellular subscriber.

Information between the mobile unit and the cellular network uses radio communication. Hence the subscriber is able to move around and become fully mobile.

The service area in which mobile communication is to be provided is divided into regions called cells.

Each cell has the equipment to transmit and receive calls from any subscriber located within the borders of its radio coverage area.

Radio

Mobile subscriber

Cell

INTRODUCTION TO GSM

GSM FREQUENCIESGSM systems use radio frequencies between 890-915 MHz for receive

and between 935-960 MHz for transmit.RF carriers are spaced every 200 kHz, allowing a total of 124 carriers for

use.An RF carrier is a pair of radio frequencies, one used in each direction.Transmit and receive frequencies are always separated by 45 MHz.

890 960935915

UPLINK FREQUENCIES DOWNLINK FREQUENCIES

UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 45MHZ

INTRODUCTION TO GSM


890 915 935 960880 925


Extended GSM (EGSM)EGSM has 10MHz of bandwidth on both transmit and receive.Receive bandwidth is from 880 MHz to 890 MHz.Transmit bandwidth is from 925 MHz to 935 MHz.Total RF carriers in EGSM is 50.

INTRODUCTION TO GSM

1710 MHz 1880 MHz1805 MHz1785 MHz



DCS1800 FREQUENCIESDCS1800 systems use radio frequencies between 1710-1785 MHz for

receive and between 1805-1880 MHz for transmit.RF carriers are spaced every 200 kHz, allowing a total of 373 carriers.There is a 100 kHz guard band between 1710.0 MHz and 1710.1 MHz

and between 1784.9 MHz and 1785.0 MHz for receive, and between 1805.0 MHz and 1805.1 MHz and between 1879.9 MHz and 1880.0 MHz for transmit.

Transmit and receive frequencies are always separated by 95 MHz.

INTRODUCTION TO GSM

FEATURES OF GSM

INCREASED CAPACITY

The GSM system provides a greater subscriber capacity than analogue systems.

GSM allows 25 kHz per user, that is, eight conversations per 200 kHz channel pair (a pair comprising one transmit channel and one receive channel).

Digital channel coding and the modulation used makes the signal resistant to interference from cells where the same frequencies are re-used (co-channel interference); a Carrier to Interference Ratio (C/I) level of 12 dB is achieved, as opposed to the 18 dB typical with analogue cellular.

This allows increased geographic reuse by permitting a reduction in the number of cells in the reuse pattern.

FEATURES OF GSM

AUDIO QUALITY

Digital transmission of speech and high performance digital signal processors provide good quality speech transmission.

Since GSM is a digital technology, the signals passed over a digital air interface can be protected against errors by using better error detection and correction techniques.

In regions of interference or noise-limited operation the speech quality is noticeably better than analogue.

USE OF STANDARDISED OPEN INTERFACESStandard interfaces such as C7 and X25 are used throughout the system.

Hence different manufacturers can be selected for different parts of the PLMN.

There is a high flexibilty in where the Network components are situated.

FEATURES OF GSM

IMPROVED SECURITY AND CONFIDENTIALITY

GSM offers high speech and data confidentiality.Subscriber authentication can be performed by the system to check if a

subscriber is a valid subscriber or not.The GSM system provides for high degree of confidentiality for the

subscriber. Calls are encoded and ciphered when sent over air. The mobile equipment can be identified independently from the mobile

subscriber. The mobile has a identity number hard coded into it when it is manufactured. This number is stored in a standard database and whenever a call is made the equipment can be checked to see if it has been reported stolen.

FEATURES OF GSM

CLEANER HANDOVERS

GSM uses Mobile assisted handover techique.The mobile itself carries out the signal strength and quality

measurement of its server and signal strength measurement of its neighbors.

This data is passed on the Network which then uses sophisticated algorithms to determine the need of handover.

SUBSCRIBER IDENTIFICATIONIn a GSM system the mobile station and the subscriber are identified

separately.The subscriber is identified by means of a smart card known as a SIM. This enables the subscriber to use different mobile equipment while

retaining the same subscriber number.

FEATURES OF GSM

ENHANCED RANGE OF SERVICES

Speech services for normal telephony.Short Message Service for point ot point transmission of text

message.Cell broadcast for transmission of text message from the cell to all

MS in its coverage area. Message like traffic information or advertising can be transmitted.

Fax and data services are provided. Data rates available are 2.4 Kb/s, 4.8 Kb/s and 9.6 Kb/s.

Supplementary services like number identification , call barring, call forwarding, charging display etc can be provided.

FEATURES OF GSM

FREQUENCY REUSEThere are total 124 carriers in GSM ( additional 50 carriers are available if

EGSM band is used).Each carrier has 8 timeslots and if 7 can be used for traffic then a

maximum of 868 ( 124 X 7 ) calls can be made. This is not enough and hence frequencies have to be reused.

The same RF carrier can be used for many conversations in several different cells at the same time.

6

43

72

The radio carriers available are allocated according to a regular pattern which repeats over the whole coverage area.

The pattern to be used depends on traffic requirement and spectrum availability.

Some typical repeat patterns are 4/12, 7/21 etc.5

12

1

FEATURES OF GSM

NETWORK COMPONENTS

Mobile Switching Centre (MSC)The Mobile services Switching Centre (MSC) co-ordinates the setting up of

calls to and from GSM users. It is the telephone switching office for MS originated or terminated traffic and

provides the appropriate bearer services, teleservices and supplementary services.

It controls a number of Base Station Sites (BSSs) within a specified geographical coverage area and gives the radio subsystem access to the subscriber and equipment databases.

The MSC carries out several different functions depending on its position in the network.

When the MSC provides the interface between PSTN and the BSS in the GSM network it is called the Gateway MSC.

Some important functions carried out by MSC are Call processing including control of data/voice call setup, inter BSS & inter MSC handovers, control of mobility management, Operation & maintenance support including database management, traffic metering and man machine interface & managing the interface between GSM & PSTN N/W.

NETWORK COMPONENTS

Mobile Switching Centre (MSC)

NETWORK COMPONENTS

Mobile Station (MS)The Mobile Station consists of the Mobile Equipment (ME) and the

Subscriber Identity Module (SIM). Mobile EquipmentThe Mobile Equipment is the hardware used by the subscriber to

access the network. The mobile equipment can be Vehicle mounted, with the antenna

physically mounted on the outside of the vehicle or portable mobile unit, which can be handheld.

Mobiles are classified into five classes according to their power rating.

CLASS POWER OUTPUT1 20W2 8W3 5W4 2W5 0.8W

NETWORK COMPONENTS

SIMThe SIM is a removable card that plugs into the ME.It identifies the mobile subscriber and provides information about the service

that the subscriber should receive.The SIM contains several pieces of information

– International Mobile Subscribers Identity ( IMSI ) - This number identifies the mobile subscriber. It is only transmitted over the air during initialising.

– Temporary Mobile Subscriber Identity ( TMSI ) - This number also identifies the subscriber. It can be alternatively used by the system. It is periodically changed by the system to protect the subscriber from being identified by someone attempting to monitor the radio interface.

– Location Area Identity ( LAI ) - Identifies the current location of the subscriber.

– Subscribers Authentication Key ( Ki ) - This is used to authenticate the SIM card.

– Mobile Station International Standard Data Number ( MSISDN ) - This is the telephone number of the mobile.

NETWORK COMPONENTS

SIMMost of the data contained within the SIM is protected against reading (eg

Ki ) or alterations after the SIM is issued.Some of the parameters ( eg. LAI ) will be continously updated to reflect the

current location of the subscriber.The SIM card can be protected by use of Personal Identity Number ( PIN )

password.The SIM is capable of storing additional information such as accumulated

call charges.

G S M

FULL SIZE SIM CARD MINI SIM CARD

NETWORK COMPONENTS

98 XXX 12345

CCNDCSN

CC NDC SN

= Country code= National Destination Code= Subscriber Number

Mobile Station International Subscribers Dialling Number ( MSISDN ) :Human identity used to call a MSThe Mobile Subscriber ISDN (MSISDN) number is the telephone number

of the MS.This is the number a calling party dials to reach the subscriber.It is used by the land network to route calls toward the MSC.

NETWORK COMPONENTS

MCC MNC MSIN

404 XX 12345..10

MCCMNCMSIN

= Mobile Country Code ( 3 Digits )= Mobile Network Code ( 2 Digits )= Mobile Subscriber Identity Number

International Mobile Subscribers Identity ( IMSI ) :Network Identity Unique to a MSThe International Mobile Subscriber Identity (IMSI) is the primary identity of the subscriber within the mobile network and is permanently assigned to that subscriber. The IMSI can be maximum of 15 digits.

NETWORK COMPONENTS

Temporary Mobile Subscribers Identity ( TMSI ) : The GSM system can also assign a Temporary Mobile

Subscriber Identity (TMSI). After the subscriber's IMSI has been initialized on the

system, the TMSI can be used for sending messages backwards and forwards across the network to identify the subscriber.

The system automatically changes the TMSI at regular intervals, thus protecting the subscriber from being identified by someone attempting to monitor the radio channels.

The TMSI is a local number and is always allocated by the VLR.

The TMSI is maximum of 4 octets.

NETWORK COMPONENTS

Equipment Identity Register ( EIR ) The Equipment Identity Register (EIR) contains a centralized

database for validating the international mobile station equipment identity, the IMEI.

The database contains three lists: The white list contains the number series of equipment identities

that have been allocated in the different participating countries. This list does not contain individual numbers but a range of numbers by identifying the beginning and end of the series.

The grey list contains IMEIs of equipment to be monitored and observed for location and correct function.

The black list contains IMEIs of MSs which have been reported stolen or are to be denied service.

The EIR database is remotely accessed by the MSC’s in the Network and can also be accessed by an MSC in a different PLMN..

NETWORK COMPONENTS

Equipment Identity Register ( EIR )

White List

All Valid assigned ID’s

Range 1Range 2

Range n

Black List

Service denied

MS IMEI 1MS IMEI 2

MS IMEI n

Grey List

Service allowedbut noted

MS IMEI 1MS IMEI 2

MS IMEI n

EIR

NETWORK COMPONENTS

TAC FAC SNR

6 2 6 1

TACFACSNRSP

SP

= Type Approval Code= Final Assembly Code= Serial Number= Spare

International Mobile Equipment Identity ( IMEI ) :IMEI is a serial number unique to each mobileEach MS is identified by an International Mobile station Equipment Identity

(IMEI) number which is permanently stored in the Mobile Equipment. On request, the MS sends this number over the signalling channel to the

MSC.The IMEI can be used to identify MSs that are reported stolen or operating

incorrectly.

NETWORK COMPONENTS

HOME LOCATION REGISTER( HLR ) The HLR contains the master database of all subscribers in the PLMN. This data is remotely accessed by the MSC´´s and VLRs in the network.

The data can also be accessed by an MSC or a VLR in a different PLMN to allow inter-system and inter-country roaming.

A PLMN may contain more than one HLR, in which case each HLR contains a portion of the total subscriber database. There is only one database record per subscriber.

The subscribers data may be accessed by the IMSI or the MSISDN. The parameters stored in HLR are

– Subscribers ID (IMSI and MSISDN )– Current subscriber VLR.– Supplementary services subscribed to.– Supplementary services information (eg. Current forwarding

address ).– Authentication key and AUC functionality.– TMSI and MSRN

NETWORK COMPONENTS

VISITOR LOCATION REGISTER ( VLR ) The Visitor Location Register (VLR) is a local subscriber database,

holding details on those subscribers who enter the area of the network that it covers.

The details are held in the VLR until the subscriber moves into the area serviced by another VLR.

The data includes most of the information stored at the HLR, as well as more precise location and status information.

The additional data stored in VLR are– Mobile status ( Busy / Free / No answer etc. )– Location Area Identity ( LAI )– Temporary Mobile Subscribers Identity ( TMSI )– Mobile Station Roaming Number ( MSRN )

The VLR provides the system elements local to the subscriber, with basic information on that subscriber, thus removing the need to access the HLR every time subscriber information is required.

NETWORK COMPONENTS

The AUC is a processor system that perform authentication function.

It is normally co-located with the HLR. The authentication process usually takes place each

time the subscriber initialises on the system. Each subscriber is assigned an authentication key (Ki)

which is stored in the SIM and at the AUC.

Authentication Centre ( AUC )

NETWORK COMPONENTS

Base Station Sub-System ( BSS ) : The BSS is the fixed end of the radio interface that provides

control and radio coverage functions for one or more cells and their associated MSs.

It is the interface between the MS and the MSC. The BSS comprises one or more Base Transceiver Stations

(BTSs), each containing the radio components that communicate with MSs in a given area, and a Base Site Controller (BSC) which supports call processing functions and the interfaces to the MSC.

Digital radio techniques are used for the radio communications link, known as the Air Interface, between the BSS and the MS.

The BSS consists of two basic Network Elements (NEs). . Base Station Controller (BSC). Base Transceiver Stations (BTSs) assigned to the BSC. .

NETWORK COMPONENTS

Base Station Controller (BSC) The BSC network element provides the control for the BSS. It controls and manages the associated BTSs, and interfaces

with the Operations and Maintenance Centre (OMC). The purpose of the BSC is to perform a variety of functions.

The following comprise the functions provided by the BSC:– Controls the BTS components.-– Performs Call Processing. – Performs Operations and Maintenance (O & M). – Provides the O & M link (OML) between the BSS and the

OMC. – Provides the A Interface between the BSS and the MSC. – Manages the radio channels. – Transfers signalling information to and from MSs.

NETWORK COMPONENTS

插框插框

Base Station Controller (BSC) – Huawei BSCNETWORK COMPONENTS

subrack

Air inlet and Fan subrack

Power distribution box

Air defence subrack

Base Transceiver Station (BTS) The BTS network element consists of the hardware

components, such as radios, interface modules and antenna systems that provide the Air Interface between the BSS and the MSs.

The BTS provides radio channels (RF carriers) for a specific RF coverage area.

The radio channel is the communication link between the MSs within an RF coverage area and the BSS.

The BTS also has a limited amount of control functionality which reduces the amount of traffic between the BTS and BSC.

NETWORK COMPONENTS

Base Transceiver Station (BTS)

NETWORK COMPONENTS

MSC BSC BTS12

BTS1

BTS2

BTS4

BTS3BTS11

BTS13 BTS14

BTS5

BTS6

BTS7

BTS8

BTS9

BTS11

Open ended Daisy Chain

Daisy Chain with a fork. Fork has a return loop back to the chain

Star

Daisy Chain with a fork. Fork has a return loop back to the chain

BTS Connectivity

NETWORK COMPONENTS

The OMC controls and monitors the Network elements within a region. The OMC also monitors the quality of service being provided by the

Network.The following are the main functions performed by the OMC-R

– The OMC allows network devices to be manually removed for or restored to service. The status of network devices can be checked from the OMC and tests and diagnostics invoked.

– The alarms generated by the Network elements are reported and logged at the OMC. The OMC-R Engineer can monitor and analyse these alarms and take appropriate action like informing the maintenance personal.

– The OMC keeps on collecting and accumulating traffic statistics from the network elements for analysis.

– Software loads can be downloaded to network elements or uploaded to the OMC.

Operation And Maintenance Centre For Radio (OMC-R)

NETWORK COMPONENTS

Operation And Maintenance Centre For Radio (OMC-R)

NETWORK COMPONENTS

TERRESTERIAL INTERFACE

The terrestrial interfaces comprises all the connections between the GSM system entities ,apart from the Um or air interface.

The terrestrial interfaces transport the traffic across the system and allows the passage of thousands of data messages to make the system function.

INTERFACE NAMES

Each interface specified in GSM has a name associated with it.

NAME INTERFACE

Um MS ----- BTS

Abis BTS ----- BSC

A MSC ------ BSC

B MSC ------ VLR

C MSC ------ HLR

D VLR ----- HLR

E MSC ------ MSC

F MSC ------ EIR

G VLR ------ VLR

H HLR ------ AUC

Gsm

Engineering

Transcript of Gsm