 Set of procedures to:  track a mobile user  find the mobile user to deliver it calls  Current location of MS maintained by 2-level hierarchical strategy with HLRs and VLRs. 1

1. Obtaining at location update – MSRN for the MS is assigned at the time of each location update, and is stored in the HLR. This way the HLR is in a position to immediately supply the routing info (MSRN) needed to switch a call through to the local MSC. 2. Obtaining on a per call basis – This case requires that the HLR has at least an identification for the currently responsible VLR. When routing info is requested from the HLR, it first has to obtain the MSRN from the VLR. This MSRN is assigned on a per call basis, i.e. each call involves a new MSRN assignment 2

 If MSRN is allocated to each subscriber visiting at an MSC, then the number of MSRNs required is large. If instead, an MSRN is allocated only when a call is to be established, then the number of MSRNs is roughly equal to number of circuits at MSC – a much smaller number – hence MSRNs typically allocated per call by VLR/MSC 3 MSISDN GMSC HLR MSISDN  MSC/VLR  MSRN  IMSI MSRN  MSISDN  IMSI, VLR number MSRN

4 GMSC BSC EIR HLR AUC VLR MSC BTS LA 1 LA 2 ISDN 1 MS 1 MSISDN 6 TMSI 4 MSRN 3 MSRN 2 MSISDN 7 TMSI 7 TMSI 7 TMSI 8 TMSI 5 MSRN MSC

5 PSTN GMSC HLR VLR Target MSC Originating Switch GMSC HLR VLR Target MSC 1. ISUP IAM 2. MAP_SEND_ROUTING_INFO 3. MAP_PROVIDE_ROAMING_NUMBER 4. MAP_PROVIDE_ROAMING_NUMBER_ack 5. MAP_SEND_ROUTING_INFO_ack 6. ISUP IAM

 ISDN switch recognizes from the MSISDN that the call subscriber is a mobile subscriber. Therefore, forward the call to the GMSC of the home PLMN (Public Land Mobile Network)  GMSC requests the current routing address (MSRN) from the HLR using MAP  By way of MSRN the call is forwarded to the local MSC  Local MSC determines the TMSI of the MS (by querying VLR) and initiates the paging procedure in the relevant LA  After MS responds to the page the connection can be switched through. 6