What is MSRN in GSM?

The full form of MSRN is a mobile subscriber roaming number.  This is a 3GPP-specified E.164 type of number. A roaming network provides the called subscriber with a temporary identification, referred to as MSRN, for each incoming call.

A mobile network operator reserves a range of numbers for roaming subscribers from the range of MSISDNs. These are temporary numbers and the network allocates one number per call only. Once the call completes, the number becomes usable again for another call.

This enables a subscriber to roam across the globe and keep connected. Let’s discuss how a mobile terminated (MT) call works in the roaming scenario.

MT Call Flow with MSRN:

Call flow for msrn
The call flow for MSRN
  • When a subscriber dials a mobile number, the call reaches the GMSC of the called subscriber.
  • For the call setup, the caller’s MSC starts ISUP signaling by initiating an IAM message.
  • Once GMSC receives the call (or IAM), first it finds the status and roaming information (MSRN) of the called subscriber.
  • To get the information GMSC sends the  SRI (Send Routing Info) map protocol operation to the HLR.

The following describes the actions on HLR for MSRN.

  • Upon getting SRI, HLR looks for the subscriber provisioning status. If a subscriber is not provisioned, HLR returns an error to the GMSC, and eventually the call disconnects.
  • Once an entry is found it checks for the service status of the subscriber. Service status means if a subscriber is allowed for MT calls. If not, then the 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 the roaming network returns the MSRN to the HLR.  In turn, HLR sends the MSRN to the GMSC in SRI response.
  • GMSC initiates a new IAM towards the MSC using MSRN as called party. Now the phone rings and calls get connected.

Why need an MSRN (a temporary number)?

As we mentioned earlier that MSRN is similar to a normal subscriber number. Then why do we need another number? Yes, the call can work without MSRN, but only if the subscriber is not roaming. The Telecom network does the routing based on county code and network code. What are the network code and country codes you can find in the 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, the mobile network can hide the actual subscriber for security reasons.