The sequence of operations for registration is presented schematically in Figure 8.1.
After a subscriber has requested registration at his or her current location by sending a location update request with his or her IMSI and the current location area (LAI), first the MSC instructs the VLR with a MAP message update location area to register the MS with its current LAI.
The difference between the location update procedure and the location registration procedure is that in the first case the mobile station has already been assigned a TMSI.
Figure 8.2 illustrates the standard case of a location update.
The MS has entered a new LA, or the timer for periodic location updating has expired, and the MS requests to update its location information.
the VLR must continually update its supply of security parameters (authentication parameter request).
After successful authentication (see Section 6.3.2), the subscriber is assigned a new MSRN, which is stored with the LAI in the HLR, and a new TMSI is also reserved for this subscriber; this is TMSI Reallocation (see Figure 7.25).