HLR knows in what MSC/VLR Service Area the subscriber is located. In order to provide a temporary number to be used for routing, the HLR requests the current MSC/VLR to allocate and return a Mobile Station Roaming Number (MSRN) for the called subscriber.
At reception of the MSRN, HLR sends it to the GMSC, which can now route the call to the MSC/VLR exchange where the called subscriber is currently registered.
The interrogation call routing function (request for an MSRN) is part of the Mobile Application Part (MAP). All data exchanged between the GMSC - HLR - MSC/VLR for the purpose of interrogation is sent over the No. 7 signalling network.
The Mobile Station Roaming Number (MSRN), according to the GSM recommendations, consists of three parts:
MSRN = CC + NDC + SN
CC = Country Code
NDC = National Destination Code
SN = Subscriber Number
Note: In this case, SN is the address to the serving MSC.
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