What is SRVCC in LTE?

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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, a data-only network, uses  Internet Multimedia Services (IMS) protocols for setting up a voice call.

In a packet network, 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 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.

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