>Local paging uses TMSI (This is local to a paging Location area, hence oly BTS’s in say LA=1 are commanded by to issue a paging command)
>Global paging uses IMSI(This will issue a paging command to all BTS’s belonging to a certain MSC(hence high paging load on the MSC, and BSC processor load). This is not good because it can cause congestion on the CP(MSC/BSC).U will experience low location updates, mobiles will be giving unavailable message yet they are on.
MSC/VLR send the paging to the whole LA which MS is in.(IMSI or TMSI),better by TMSI cuz it has small size (4TMSI in one paging block)
IF no responce, MSC/VLR send 2th paging to the whole MSC area. better with IMSI (2IMSI in paging block). cuz MS may just crossing to new LA & its TMSI on VLR not change yet, or my be it powered on or connected to network in diff area & its TMSI not changed correctely, so cuz of all this the 2th paging must be with IMSI.
furthermore there is two stage timer in MSC,one to trigger the 2th page after defined time from the 1st page, and other stage is expire timer to drop the call after its time from 2th page.
regards.