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SRVCC (single radio voice call continuity)

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SRVCC (Single Radio Voice Call Continuity) is a feature in mobile communication systems that allows a voice call to be seamlessly transferred between different types of radio access technologies, specifically between circuit-switched (CS) networks and packet-switched (PS) networks. It enables a smooth handover of voice calls from legacy 2G or 3G networks to newer 4G or 5G networks without interrupting the call.

To understand SRVCC, let's break it down:

  1. Single Radio: In a typical mobile communication system, voice and data services are carried over different radio access technologies. Voice calls usually use circuit-switched networks, which provide a dedicated channel for the duration of the call, while data services, such as internet browsing or video streaming, are carried over packet-switched networks, which use shared resources and offer higher data rates. SRVCC operates within a single radio interface and facilitates the transition between these two types of networks.
  2. Voice Call Continuity: Continuity refers to the uninterrupted transfer of a voice call from one network to another. When a mobile user moves out of the coverage area of the source network (e.g., 3G) and enters the coverage area of the target network (e.g., 4G or 5G), SRVCC ensures that the ongoing voice call is seamlessly handed over to the target network without any interruption, preserving the call quality and user experience.

The SRVCC procedure involves the following steps:

  1. Handover Trigger: When a mobile user moves from a CS network to a PS network coverage area, the handover process is initiated. This can happen when the user enters an area where the CS network coverage is weak, and the device has to switch to a stronger PS network signal.
  2. Network Coordination: The source network (CS) and the target network (PS) coordinate with each other to ensure a smooth handover of the voice call. The source network transfers the necessary call information, including the session context and media streams, to the target network.
  3. Media Path Transfer: The media path carrying the voice call is switched from the CS network to the PS network. This involves transferring the audio streams from the CS network to the PS network without any interruption. The target network takes over the control and routing of the call.
  4. Voice Call Continuity: Once the media path transfer is complete, the voice call continues uninterrupted on the PS network. The mobile user does not experience any noticeable call drop or degradation in call quality.

It's important to note that SRVCC requires support from both the mobile device (UE - User Equipment) and the network infrastructure (MSC - Mobile Switching Center or MME - Mobility Management Entity). The device must be capable of handling the handover process, and the network must be equipped with the necessary protocols and interfaces to facilitate the transfer.

SRVCC plays a crucial role in ensuring a seamless transition between different generations of mobile networks. It allows operators to gradually upgrade their networks without disrupting voice services for their subscribers, providing a smooth migration path from older technologies to the latest ones.