ObjectivesUnderstand the functional blocks in a GSM NetworkUnderstand Concept of Location UpdateUnderstand the Message flow for Location Update procedures

Basics

Cellular network has Location areas of different sizes. Bigger LA are used where subscriber density is low. Smaller LA are used where subscriber density is high.Location areas structure LA 1LA 2LA 3

As a convention and for simplicity in representation, Cells in Cellular system are shown as Hexagons. The actual cell covered by a Base station takes a very different shape depending upon the terrain, obstuctions and transmitting antenna charecteristics. The cells are of different sizes and shapes. To cover a densly populated areas, smaller cells are used. Where as large cells cover a low subscriber density areas. On the highways and main roads, cell size is optimized to cover larger length of the cell along the road. This is made possible by using a highly directive antenna.

MSBTSBTSBTSBTSBTSBTSBTSBTSBSCBSCPSTNHLROMCSMSCBCVMSCMSCAbisAOMLGSM Network Architecture

BTS : Base Transceiver StationBSC : Base Station ControllerMS : Mobile StationTRAU : Transcoder Rate Adaptation UnitMSC : Mobile Switching CenterHLR : Home Location RegisterVLR : Visiting Location RegisterAUC : Authentication CentreEIR : Equipment Identity RegisterSMSC : Short Message Service CenterVMSC : Voice Mail Service CenterBC : Billing CenterOMC : Operation

HGSM Signaling InterfacesGMSCAbisABCFEGDSS7/R2MSCBSCBTSOMLOMCA,B,C,D,E,F,G,H,I = SS7Abis = LAPDOML = X.25IH

HSIM - Subscriber Identity Module Removable Module inserted when the subscriber wants to use the MS

IMSI : Network Identity unique to a SIMMCCMNCMSIN45204123453 digits2 digits10 digitsTwo SizesCredit CardStamp Size

4-8 digits PIN code3 false entries - blocks8 digit PUK10 false entries - disabled

MSROM = 6kb to 16kbRAM = 128 byte to 256 byteEEPROM = 3 kb to 8 kb

SIM card as defined by GSM is a removable module which can be inserted inside the Mobile Phone when the user wants to use the MS. It is available in two sizes the Credit Card size ; known as ISO SIM or the Stamp size ; known as Pug in SIM. SIM card has a unique information on it which is its identity the IMSI ( International Mobile Subscriber Identity ). This identity is unique to each SIM used in any of the GSM Networks. this identity is used by the Network to accomplish Signaling with the MS and also to d signaling with other GSM networks for information transfer related to particular connection. The IMSI is a 15 digit identity which consists of MCC ( Mobile Country Code ) which is unique to each GSM Operations country, the MNC( Mobile Network Code) which discriminates the networks within a MCC , further the MSIN ( Mobile Subscriber Identity Number ) which discriminates individual subscribers in network.SIM also has a two level protection to prevent misuse. SIM can be optionally by the user be set for a PIN code . Whenever the Mobile is turned ON or the SIM is inserted , the Mobile will ask the user to enter the PIN code. If three false entries of SIM card are entered then the SIM is blocked . The the user needs to enter a Pin Unblocking Key ( PUK) . Ten false entries of PUK will permanently disable the SIM card permanently.

HSIM - Subscriber Identity Module Contents of SIMSerial NumberIMSI, Subscriber Key ( Ki )Algorithms for Authentication, Ciphering Network CodePIN, PUKCharging InformationAbbreviated DiallingSupplementary Features ( e.g. call barring )

SIM features and contents are personalized by the Service ActivatorMS also stores some temporary data on SIM during operation

Apart from the IMSI, SIM also contains some other essential information which is required to establish successful communication with the network like the subscriber key and algorithms are used for authentication and Ciphering. SIM also contains a list of preferred networks which the Mobile hunts for when it turns on. Ther are several other features and contents on the SIM which are personalized by the Service Provider which activates the SIM card.MS also stores some information on the SIM card EEPROM like its location information and some timer values.

HHLR - Home Location RegisterHLR stores the most critical database of subscribers, including a subscribers service profile, its location and activity status.

HLR will have the series of all subscriber numbers, which may not be activated or issued .

When a subcriber takes service from an operator, it is issued with a SIM card, this SIM card as we discussed earlier has an IMSI corresponding to which is the MSISDN ( 98xxx.... ) which is known to the subscriber. Now IMSI is an identity which is used by the network to communicate with the mobile. So it can be easily be captured and a bogus or fraud SIM can be manufactured easily. TO avoid this the process of Authentication is used, which is the verification of SIM before a service is alloted to a subscriber. This is done with the help of a Secret Key ( code ) which is avialble on the SIM. This Key associated with every SIM which means with every IMSI and is different for each SIM. This pair of IMSI and Key is also available at the HLR -AUC. The AUC associated with the HLR does this job of verification of SIM. The Mobile is asked to send its secret key by performing some operations on it by using some number send by AUC. The secret key is then send in this coded form to the MSC , where it is verified.

HVLR - Visiting Location RegisterA subscription when activated is registered in VLRVLR has all the subscriber no's which are activated VLR also has temporary database of all activated subscribers ( on/off, location )

MSC communicates with HLR for subscribers coming from different MSC'sand if found valid, then registers them in its VLR

EIR is the records of the valid IMEI's . Valid IMEI's mean those mobile equipments which are type approved. MSC may ask a mobile to send its IMEI, which it checks with the EIR, to find its validity. In the EIR, the IMEI's are classifiefd into three categories.White List : This contians the IMEI of type approved mobiles.Black List : List of IMEI's which should be barred because either they are stolen or are not functioning properlyGrey List : List of IMEI's who are to evaluated before they are put in black list.

HAUC - Authentication CentreAuthentification is a process by which a SIM is verifiedSecret data and the verification process algorithm are stored at AUCAUC is the element which carries out the verification of SIMAUC is associated with the HLR

HEIR : Equipment Identity RegisterEIR is the Mobile Equipment Database which has a series of IMEI'sMSC asks the Mobile to send its IMEIMSC then checks the validity of IMEI with the EIRAll IMEI are stored in EIR with relevant classfications

EIRMSCClassifications of IMEI( Mobile Stations )White List

Grey List

Black List

Once a mobile turns on power ,it camps to the BCCH .On the BCCH is broadcasted the Location area identity (LAI) of the cell to in which the mobile is.Mobile reads this LAI and compares this LAI with the LAI which it has store d when it switched off.If this new Lai is different form the one stored,it does a normal location update.

HMobile Searches for Broadcast Channels (BCH)

Synchronises Frequency and Timing

Decodes BCH sub-channels (BCCH)

Checks if Network Allowed by SIM

Location Update

Authentication

Mobile Turn-On

BSCNetwork Service Areas

BSCLocation AreaLocation Area Identity

Location Area is the area covered by one or more BTS' s where a mobile can move freely without updating the system.One Location Area can be covered by more than one BSC,but only by one MSC.

HIMPORTANCE OF LOCATION AREAReduce Paging Load

Resource Planning

By dividing a PLMN area into various location areas,paging load reduce since whenever there is page for a mobile it is send by the MSC to those BSC's which have cells with location areas same as that of the current location area of the Mobile. If entire PLMN area is termed as one location area then the MSC has to page for every mobile in every BSC which increases Paging load and as a result signaling load and hence wastage of resources.

HWhat is Location UpdateMSC should know the Location of the Mobile for paging. Mobiles are continuously changing Location Area.Mobile when changes Location Area informs the MSC/VLR-HLR about its new L.A

Process of informing MSC about new Location Area is Location Update

Location Update

H

H IMSI ATTACH MESSAGE FLOW

Location Area is the area covered by one or more BTS' s where a mobile can move freely without updating the system.One Location Area can be covered by more than one BSC,but only by one MSC.

HNORMAL LOCATION UPDATEMobile turns on power.

Reads the new LAI.

If new LAI different from strored LAI, MS performs a NORMAL Location Update.

MS moves from one location area to another.

Once a mobile turns on power ,it camps to the BCCH .On the BCCH is broadcasted the Location area identity (LAI) of the cell to in which the mobile is.Mobile reads this LAI and compares this LAI with the LAI which it has store d when it switched off.If this new Lai is different form the one stored,it does a normal location update.

HLOCATION UPDATE Messages

HPERIODIC LOCATION UPDATEMobile remains in IDLE mode.

Mobile has to inform MSC after a set period. This period is defined by timer value 3212.

Mobile performs periodic location update.

The Mobile many a times may enter a non-coverage zone for a very long period.The MSC has no information about it .The MSC goes on sending Paging messages for the Mobile. In order to avoid this a period is set after which the Mobile has to inform MSC that it is still in the attach state.Now,if the mobile remains in the non-coverage zone for this period,it will not be able to send any message,.As a result of this after the expiry of this period the MSC marks this mobile as detached and rejects all incoming calls to this mobile. This type of update which is done everytime after an expiry of a fixed period is known as Periodic Location Update. Period set ranges from 0 to 255 decihours

H

HIMSI DETACH MESSAGE FLOW

HLocation Update SummaryMobile Changes Location Area

Reads the new Location Area from BCCH

Sends a RACH ( request for channel )

Gets a SDCCH on AGCH

Sends its TMSI (for IMSI attach/Deatch/Period LU)

and new & old LAI in a NORMAL Location Update Request to MSC on SDCCH

MSC starts Authentication.

Reallocates TMSI (depends on MSC parameter settings).

If successful, Updates the new Location area for the Mobile in the VLR

Sends a confirmation to the Mobile

Mobiles leaves SDCCH , and comes to idle mode

As a convention and for simplicity in representation, Cells in Cellular system are shown as Hexagons. The actual cell covered by a Base station takes a very different shape depending upon the terrain, obstuctions and transmitting antenna charecteristics. The cells are of different sizes and shapes. To cover a densly populated areas, smaller cells are used. Where as large cells cover a low subscriber density areas. On the highways and main roads, cell size is optimized to cover larger length of the cell along the road. This is made possible by using a highly directive antenna. BTS : Base Transceiver StationBSC : Base Station ControllerMS : Mobile StationTRAU : Transcoder Rate Adaptation UnitMSC : Mobile Switching CenterHLR : Home Location RegisterVLR : Visiting Location RegisterAUC : Authentication CentreEIR : Equipment Identity RegisterSMSC : Short Message Service CenterVMSC : Voice Mail Service CenterBC : Billing CenterOMC : Operation SIM card as defined by GSM is a removable module which can be inserted inside the Mobile Phone when the user wants to use the MS. It is available in two sizes the Credit Card size ; known as ISO SIM or the Stamp size ; known as Pug in SIM. SIM card has a unique information on it which is its identity the IMSI ( International Mobile Subscriber Identity ). This identity is unique to each SIM used in any of the GSM Networks. this identity is used by the Network to accomplish Signaling with the MS and also to d signaling with other GSM networks for information transfer related to particular connection. The IMSI is a 15 digit identity which consists of MCC ( Mobile Country Code ) which is unique to each GSM Operations country, the MNC( Mobile Network Code) which discriminates the networks within a MCC , further the MSIN ( Mobile Subscriber Identity Number ) which discriminates individual subscribers in network.SIM also has a two level protection to prevent misuse. SIM can be optionally by the user be set for a PIN code . Whenever the Mobile is turned ON or the SIM is inserted , the Mobile will ask the user to enter the PIN code. If three false entries of SIM card are entered then the SIM is blocked . The the user needs to enter a Pin Unblocking Key ( PUK) . Ten false entries of PUK will permanently disable the SIM card permanently.Apart from the IMSI, SIM also contains some other essential information which is required to establish successful communication with the network like the subscriber key and algorithms are used for authentication and Ciphering. SIM also contains a list of preferred networks which the Mobile hunts for when it turns on. Ther are several other features and contents on the SIM which are personalized by the Service Provider which activates the SIM card.MS also stores some information on the SIM card EEPROM like its location information and some timer values.When a subcriber takes service from an operator, it is issued with a SIM card, this SIM card as we discussed earlier has an IMSI corresponding to which is the MSISDN ( 98xxx.... ) which is known to the subscriber. Now IMSI is an identity which is used by the network to communicate with the mobile. So it can be easily be captured and a bogus or fraud SIM can be manufactured easily. TO avoid this the process of Authentication is used, which is the verification of SIM before a service is alloted to a subscriber. This is done with the help of a Secret Key ( code ) which is avialble on the SIM. This Key associated with every SIM which means with every IMSI and is different for each SIM. This pair of IMSI and Key is also available at the HLR -AUC. The AUC associated with the HLR does this job of verification of SIM. The Mobile is asked to send its secret key by performing some operations on it by using some number send by AUC. The secret key is then send in this coded form to the MSC , where it is verified.EIR is the records of the valid IMEI's . Valid IMEI's mean those mobile equipments which are type approved. MSC may ask a mobile to send its IMEI, which it checks with the EIR, to find its validity. In the EIR, the IMEI's are classifiefd into three categories.White List : This contians the IMEI of type approved mobiles.Black List : List of IMEI's which should be barred because either they are stolen or are not functioning properlyGrey List : List of IMEI's who are to evaluated before they are put in black list.

Once a mobile turns on power ,it camps to the BCCH .On the BCCH is broadcasted the Location area identity (LAI) of the cell to in which the mobile is.Mobile reads this LAI and compares this LAI with the LAI which it has store d when it switched off.If this new Lai is different form the one stored,it does a normal location update.

Location Area is the area covered by one or more BTS' s where a mobile can move freely without updating the system.One Location Area can be covered by more than one BSC,but only by one MSC.By dividing a PLMN area into various location areas,paging load reduce since whenever there is page for a mobile it is send by the MSC to those BSC's which have cells with location areas same as that of the current location area of the Mobile. If entire PLMN area is termed as one location area then the MSC has to page for every mobile in every BSC which increases Paging load and as a result signaling load and hence wastage of resources.Location Area is the area covered by one or more BTS' s where a mobile can move freely without updating the system.One Location Area can be covered by more than one BSC,but only by one MSC.Once a mobile turns on power ,it camps to the BCCH .On the BCCH is broadcasted the Location area identity (LAI) of the cell to in which the mobile is.Mobile reads this LAI and compares this LAI with the LAI which it has store d when it switched off.If this new Lai is different form the one stored,it does a normal location update.

The Mobile many a times may enter a non-coverage zone for a very long period.The MSC has no information about it .The MSC goes on sending Paging messages for the Mobile. In order to avoid this a period is set after which the Mobile has to inform MSC that it is still in the attach state.Now,if the mobile remains in the non-coverage zone for this period,it will not be able to send any message,.As a result of this after the expiry of this period the MSC marks this mobile as detached and rejects all incoming calls to this mobile. This type of update which is done everytime after an expiry of a fixed period is known as Periodic Location Update. Period set ranges from 0 to 255 decihours