you know, when a mobile user is called in GSM, his HLR will send PRN message to MSC which the user is roaming for a MSRN, and do you know
when a PSI (Provide subscribe Information) message is needed?
and how does the HLR know that PSI is need to send to VLR?
how does HLR tell the calling MSC the responded information from called MSC, by SRI_ACK?
The HLR sends PSI when receives a ATI
msg (any Time interrogation). These msg it is not a message to route call , it is used to location procedures (The origen can be another externa system not only a MSC/VLR) and the HLR return informatión such as Cell id , State (Busy, idle, det) and time of location .In general the destination VLR doesn,t return a MSRN so PSI procedure can not be used instead PRN procedure