What is SRVCC?

Single Radio Voice Call Continuity (SRVCC) is the full name of this acronym. With the SRVCC capability, an ongoing voice call over a packet-based  LTE network (4G) can be switched to a circuit-based SS7 (3G) network, without dropping any communication. An LTE which is a data-only network uses  Internet Multimedia Services (IMS) protocols for setting up a voice call.

In a packet network, there is no dedicated voice channel is set up before starting any voice communication. While in a circuit-based network (e.g., SS7), E1 or T1 channels are reserved for media transfer. During the course of a call, a subscriber may be transferred to a network where only a 3g or 2g network is accessible.

This needs an additional capability.  SRVCC provides uninterrupted call experience from VoLTE to 3G or 2G networks.

While registration to the home network during attach, the MME sends the SRVCC capability indication in the Update Location Request (ULR) message to the HSS.