Purpose of STN-SR: According to TS 23.003, STN-SR follows the E.164 telecommunications number format, and is used by the MSC server for session transfer of the media path from the PS domain to the CS Domain.

VoLTE is a key application in LTE. A UE should have continuous and seamless voice service as they move across different access technologies. The IMS network provides the call anchor which enables such seamless service continuity or what is known as Single Radio Voice Call Continuity (SRVCC) as one of the standard methods for accomplishing Voice Call Continuity (VCC).  The entity in the IMS network which manages SRVCC is the Service Centralization and Continuity Application Server (SCC AS).

What role does the STN-SR play in SRVCC?

Below is a list of items that elaborates on the usage of the STN-SR in SRVCC:

- According to TS 23.237, the STN-SR is configured for an SRVCC subscriber at the time of service provisioning.

- According to section 5.3.7 of TS 23.216, the HSS provides the STN-SR to the MME that is provisioned for a UE during its attach procedure.

- If the user has an active context at the MME, the HSS also updates the MME if the STN-SR has been modified for that UE.

- According to section 6.2.2.1 of TS 23.216,

    • The MME sends the STN-SR in the SRVCC PS to CS request to the MSC over the Sv interface.
    • The MSC initiates a request for an access transfer of the active session from the IMS network and includes the STN-SR in the request.

-          According to section 6.3.2.1.4 of 23.237

    • The SCC AS eventually receives an INVITE with the STN-SR which indicates the need for an access transfer of an active session.

The SCC AS is the entity in the IMS domain that handles management of the access transfer of an active session according to section 6.3.2.1.4 of TS 23.237. The SCC AS coordinates the proper media and signaling exchange to the access network of where the UE is present.  The STN-SR is the number that is used to facilitate that coordination.