LOCALIZATION AND CALLING

One fundamental feature of GSM system is the automatic , worldwide localization of users. GSM perform periodic location updates even if a user does not use the MS.The HLR always contains information about the current location , and VLR currently responsible for the MS informs the HLR about location changes. As soon as an MS moves into the range of a new VLR , the HLR sends all user data needed to new VLR.

Changing position of services is also called roaming.

To locate an MS and to address the MS following number are required………..

Mobile station inter national ISDN number (MSISDN) :---------

The only important number for a user of GSM in is the phone number. Phone no is associated with SIM ,which is personalized for a user . This no consists of country code (CC) ,the national destination code(NDC) , and subscriber number (SN) .

International mobile subscriber identity(IMSI) :-------

GSM uses the IMSI for internal unique identification of a subscriber .it consists of mobile country code (MCC) ,the mobile network code(MNC) , and mobile subscriber identification identity(MSIN).

Temporary mobile subscriber identity(TMSI):

To hide the IMSI ,which would give the exact identity of the user signaling over the air interface ,GSM uses the 4 byte TMSI for local subscriber identification .TMSI is selected by the current VLR and is only valid temporarily and within the location area of VLR.

Mobile station roaming number(MSRN) :---Another temporary address that hides the location of a subscriber is MSRN. MSRN contains the current visitor country code(VCC) ,the visitor national destination code (VNDC)

The steps involve in calling from a fixed network to a MS (MTC) , and from a MS to fixed network(MOC) is given below…..



MOBILE TERMINATED CALL(MTC)

1: calling a GSM subscriber

2: forwarding call to GMSC

3: signal call setup to HLR

4, 5: request MSRN from VLR

6: forward responsible
MSC to GMSC

7: forward call to

current MSC

8, 9: get current status of MS

10, 11: paging of MS

12, 13: MS answers

14, 15: security checks

16, 17: set up connection



MOBILE OREGINATED CALL (MOC)

1, 2: connection request

3, 4: security check

5-8: check resources (free circuit)

9-10: set up call




MESSAGE FLOW FOR MTC AND MOC