GSM Location Update Sequence Diagram

Location Update (GSM Location Update Procedure)Subscribers GSM Network

GSM Mobiles Maryland Location Area MarylandGSM

Equipment

GSMDatabases

VirginiaLocation

Area

Virginia GSMEquipment

Other GSMMobile

GSMMobile

RockvilleCell

BethesdaCell

MarylandMSC VLR

HLR Vienna Cell VirginiaBSC

VirginiaMSC VLR

EventStudio System Designer 4.0

01-Oct-08 20:39 (Page 1)

This sequence diagram was generated with EventStudio System Designer 4.0 (http://www.EventHelix.com/EventStudio). The EventStudiosource files for this document can be downloaded from http://www.eventhelix.com/call-flow/gsm-location-update.zip.

Have you ever wondered how your cellular provider is able to route calls to you virtually anywhere? How does the cellular provider know whereyou are?

The short answer to these questions is that your cell phone keeps the cellular operator informed about your location. In this sequence diagramwe will examine how a mobile phones keep selecting the best cell to service your call and also keeps the cellular provider informed about yourlocation.

We will be tracing the journey of a mobile phone from Rockville, Maryland to Vienna, Virginia. The path taken by the mobile phone is describedin the following article:

http://www.EventHelix.com/RealtimeMantra/Telecom/GSM_network_example.htm

Copyright © 2004-2008 EventHelix.com Inc. All Rights Reserved.

Before we go any further, let us discuss a few important terms.

Location Area (LA)

A GSM network is divided into cells. A group of cells is considered a location area. A mobile phone in motion keeps the network informedabout changes in the location area. If the mobile moves from a cell in one location area to a cell in another location area, the mobile phoneshould perform a location area update to inform the network about the exact location of the mobile phone.

Home Location Register (HLR)

The HLR maintains a database for the mobile subscribers. At any point of time, the HLR knows the address of the MSC VLR that control thecurrent location area of the mobile. The HLR is informed about a location area update only if the location area change has resulted in a changeof the MSC VLR.

Mobile Switching Center - Visitor Location Register (MSC VLR)

The MSC VLR is responsible to switching voice calls and it also keeps track of the exact location area where the mobile user is present. Notethat a typical MSC VLR will service several location areas.

Rockville is the primarycell

The mobile phone is currently inthe Rockville cell.

GSM Mobile reaches cell boundary (both cells in same Location Area)

Monitor the BCCH fromthe current cell and the

neighboring cells

When idle, the GSM mobilephone keeps monitoring thebeacon frequency for its currentcell(Rockville) and its neighbors.The GSM mobile measures thecell strength to see if it shouldchange its primary cell. Thesignal strength of the BroadcastControl CHannel (BCCH) will bemonitored to select the best cell.

BCCHLocation Area = Maryland, Signal Strength =

Good

The BCCH on the primary cell ismonitored for signal strength.

BCCHLocation Area = Maryland, Signal Strength = Great

The BCCH of the neighboringcells is monitored to determineif any of the neighbors have abetter signal strength. In thiscase, the cell has reached theboundary between Rockville andBethesda cells and it finds thatthe signal quality of theBethesda cell is better.

Bethesda is the primarycell

The mobile phone marks theBethesda cell as the primary



Equipment

GSMDatabases

VirginiaLocation

Area


Other GSMMobile

GSMMobile

RockvilleCell

BethesdaCell

MarylandMSC VLR


VirginiaMSC VLR


01-Oct-08 20:39 (Page 2)

cell.Skip Location update asthe location area for the

old cell (Rockville) and thenew cell (Bethesda) is the

same (Maryland)

Whenever the primary cellchanges, the mobile checks ifthe Location area of the old celland the new cell are different. Inthis case, the mobile finds thatthe location areas are same sono location area update isneeded.

GSM Mobile reaches Location Area boundary (old and new cells are in different Location Areas)

BCCHLocation Area = Maryland, Signal Strength =

Good

The BCCH on the beaconfrequencies is monitored.

BCCHLocation Area = Vienna, Signal Strength = Great

Now the Vienna cell is beingreceived with better signalstrength, so cell will be pickedas primary.

Vienna is the primary cell

Location area update isneeded as the locationarea has changed from

Maryland to Virginia

This time the old and newlocation areas are different. Themobile initiates the LocationArea Update procedure.

RR Connection Setup

RR CHANNEL REQUEST The mobile establishes a RRconnection to send the locationupdate to the network.

RR CHANNEL REQUEST

RR IMMEDIATE ASSIGNMENT

RR IMMEDIATE ASSIGNMENT A radio channel has beenassigned to the GSM mobile.

GSM Location Update Procedure

RR SABM + MM LOCATION UPDATING REQUESTMaryland TMSI, Maryland LAI

The mobile tunes to theassigned radio channel andsends the SABM to initiate theradio connection. The locationupdate is also piggybacked onthe message.

SABM + MM LOCATION UPDATING REQUESTMaryland TMSI, Maryland LAI

The BSC receives the locationupdate with the SABM.

MM LOCATION UPDATING REQUESTMaryland TMSI, Maryland LAI

The location updating request isforwarded to the MSC in the"BSSMAP COMPLETE LAYER 3INFORMATION" message.

RR UA The RR connection setup iscompleted by responding withUA for the received SABM.

RR UA

LEG: Inter MSC-VLR locationupdate

Compare the oldlocation area withthe new location

area

The MSC finds that the oldlocation area was handled by adifferent MSC. Thus the MSCneeds to contact the HLR.



Equipment

GSMDatabases

VirginiaLocation

Area


Other GSMMobile

GSMMobile

RockvilleCell

BethesdaCell

MarylandMSC VLR


VirginiaMSC VLR


01-Oct-08 20:39 (Page 3)

MAP/G SEND PARAMETERSMaryland TMSI

The Virginia MSC VLR does notfind the TMSI in its database. Ituses the old Location AreaIndicator (LAI) to obtain theaddress of the old MSC VLR. Arequest is sent to the old MSCVLR, requesting the IMSI(International Mobile SubscriberIdentity) of the subscriber.

MAP/G SEND PARAMETERS RESULTIMSI

The Maryland MSC VLRprovides the IMSIcorresponding to the TMSI. Notethat the IMSI could have beenobtained from the mobile. Thatis not a preferred option as theLocation Updating Request issent in clear so it could be usedto determine the associationbetween the IMSI and TMSI.

MAP/D UPDATE LOCATION The MSC sends an updatelocation message to the MSC.This message is needed for tworeasons: (1) The HLR needs toupdate its record to point to thenew MSC when queried forlocation. (2) The new MSC doesnot have information about thissubscriber.

Update the currentlyserving MSC address to

the new MSC

At this point, the HLR updatesits records to indicate that thesubscriber is now present in alocation area served by theVirginia MSC VLR.

MAP/D INSERT SUBSCRIBER DATAKc, RAND, SRES

Pass information about the newsubscriber to the new MSC. Themessage contains the a 64-bitciphering key used as a SessionKey (Kc), a 128-bit randomchallenge (RAND) and a 32-bitSigned Response (SRES). Theseparameters will be used in theauthentication process.

Create a record forthe new visitingsubscriber andstore the (Kc,RAND, SRES)

tuples

Subscriber information isupdated in the new MSC.

MAP/D INSERT SUBSCRIBER DATA RESULT The new MSC replies back.

MAP/D CANCEL LOCATION Ask the Old MSC to delete therecord for this subscriber.

Delete the record for theold subscriber.

The old subscriber's record isdeleted. The TMSI assigned tothe mobile is also released.

MAP/D CANCEL LOCATION RESULT The Old MSC replies back to theHLR.

MAP/D UPDATE LOCATION RESULT The HLR has updated allrecords, so it replies back to thenew MSC.

Authenticate subscriber



Equipment

GSMDatabases

VirginiaLocation

Area


Other GSMMobile

GSMMobile

RockvilleCell

BethesdaCell

MarylandMSC VLR


VirginiaMSC VLR


01-Oct-08 20:39 (Page 4)

MM AUTHENTICATION REQUESTRAND

The MSC VLR decides toauthenticate the subscriber. TheRAND value received from theHLR is sent to the mobile.

Pass the RAND value tothe SIM and obtain the Kc

and SRES values

The SIM applies secret GSMalgorithms on the RAND and thesecret key Ki to obtain thesession key Kc and SRES.

MM AUTHENTICATION RESPONSESRES

The mobile passes thecomputed SRES value in theresponse.

Compare the SRESreported by themobile with theSRES obtained

from HLR

If the SRES obtained from themobile matches the SRES valueobtained from the HLR, thesubscriber authenticationprocedure completessuccessfully.

Enable Ciphering

BSSMAP CIPHER MODE COMMANDThe MSC initiates ciphering ofthe data being sent on thechannel.

RR CIPHERING MODE COMMANDmode = CLEAR

The BSC sends the CIPHERINGMODE COMMAND to the mobile.

RR CIPHERING MODE COMPLETEmode = CIPHERED

Ciphering has already beenenabled, so this message istransmitted with ciphering.

BSSMAP CIPHER MODE COMPLETEThe BSC replies back to theMSC, indicating that cipheringhas been successfully enabled.

MM LOCATION UPDATING ACCEPTVirginia TMSI

The new MSC replies back to themobile via the Virginia BSC. Themessage also assigns a newTemporary Mobile Subscriber Id(TMSI) to the terminal. Since theTMSI assignment is being sentafter ciphering is enabled, therelationship between TMSI andthe subscriber cannot beobtained by unauthorized users.

Start using the new TMSI

MM TMSI REALLOCATION COMPLETE The GSM mobile replies backindicating that the new TMSIallocation has been completed.

RR Connection Release

BSSMAP CLEAR COMMANDThe RR connection is releasedby the MSC.

RR CHANNEL RELEASE The BSC initiates RR releasewith the mobile.

BSSMAP CLEAR COMPLETEThe BSC informs the MSC thatthe RR connection has beenreleased.

RR DISC The mobile sends a disconnectmessage to release the LAPmconnection.

RR UA The BSC replies with anUnnumbered Acknowledgemessage.

Documents

GSM Location Update Sequence Diagram