IMSI is sent in GSM when :
1) U switch on ur mobile and the handshake between MS and BS takes place. At that time ur IMSI is sent as TMSI (so that IMSI is protected). This is the 1st scenario.
2) When 'location updating' takes place when the MS moves out of the home MSC and goes into a cell controlled by different MSC, the IMSI of the MS is updated in the VLR of the new MSC (ie. roaming scenario). The new VLR informs your HLR about this update and sends your IMSI to the HLR. So that the HLR knows about your current location.
thanx alot i ask only at air interface
and i have confustion on the paging when layer 2 3 setup (RR SABM+ RR PAGE RESPONSE why mobile B send timsi and imsi in this massge)
and what u meen by in timsi form
yes waleed, infact from what i have studied... IMSI is never directly sent over air-interface. Its always protected by sending TMSI instead of IMSI.
In this case, let us regard TMSI is sent whenever IMSI is said to be sent.
Is that what u meant to say?