What is MSRN?
The full form of MSRN is mobile subscriber roaming number. This is an E.164 type of number according to telecom standards. Roaming network allocates a roaming for an incoming call to the called subscriber. The allocated number is MSRN. Mobile network operator reserves a range of numbers for roaming subscribers. These are temporary numbers and allocated per call only. Once call completes , number becomes usable again for another call. This enables a subscriber roam across the glob and keep connected. Lets discuss how a mobile terminated (MT) call works in roaming scenario.
Call Flow for MSRN:
When a subscribers dials a mobile number , call reaches to the GMSC of the called number , located in home network. For call setup, protocol message is the IAM message which belongs to the ISUP layer of ss7 protocol. Once GMSC receives the call, first it has to find the status and roaming information (MSRN) of the called number. To get the information GMSC sends the SRI (Send Routing Info) map protocol operation to the HLR. Following describes the actions on HLR for MSRN.
Upon getting SRI, HLR looks for the subscriber provisioning status, if subscriber is provisioned. if no, returns an error to the GMSC and call disconnects. Then checks for the service status of the subscriber. Service status means , if a subscriber is allowed for MT calls. If no, then call disconnects.
Next is the online status check, online status means, if a subscriber is attached to the network. If yes , HLR sends the Provide Roaming Number (PRN) map protocol operation to the visiting MSC/VLR. If all is good , then roaming network returns the MSRN to the HLR. In turns HLR sends the MSRN to the GMSC in SRI response.
GMSC initiates a new IAM towards the MSC using MSRN as called party. Now phone rings and calls gets connected.
Why need MSRN?
As me mentioned earlier that msrn is similar to a normal subscriber number. Then why we need another number? Yes , call can work without MSRN, but only if subscriber is not roaming. Telecom network does the routing based on county code and network code. What is network code and country code you can find in MSISDN tutorial on our blog. If a subscriber is roaming in another mobile operator's network , the combination of country code and network code changes.
With the help of MSRN , mobile network can hide the actual subscriber for security.