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rSRVCC Reverse SRVCC

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rSRVCC, or Reverse Single Radio Voice Call Continuity, is a feature in telecommunications networks that enables seamless handover of voice calls from an LTE (Long-Term Evolution) network to a legacy circuit-switched network such as GSM (Global System for Mobile Communications) or UMTS (Universal Mobile Telecommunications System).

To understand rSRVCC, let's first discuss the need for it. With the advent of LTE, voice calls began to be transmitted over IP (Internet Protocol) using a technology called Voice over LTE (VoLTE). VoLTE offers advantages such as superior call quality, faster call setup time, and the ability to simultaneously use voice and data services. However, there are situations where a user may move out of an LTE coverage area while on a voice call and enter an area covered only by a legacy circuit-switched network. In such cases, without a mechanism like rSRVCC, the call would be dropped, causing inconvenience to the user.

Reverse SRVCC addresses this issue by allowing the seamless transfer of an ongoing VoLTE call from the LTE network to the legacy circuit-switched network without interrupting the call. Here's how the process works:

  1. Initial Call Setup (LTE Network): When a user initiates a voice call while connected to an LTE network, the call setup process follows the VoLTE procedures. The call is established over the LTE network using IP technology.
  2. Handover Trigger: As the user moves out of the LTE coverage area and enters an area covered by a legacy network, a handover trigger is generated. This can occur due to factors like a weak LTE signal or a deliberate network policy to offload the user to a legacy network.
  3. Handover Decision: The LTE network's mobility management entity (MME) determines whether rSRVCC is supported by the target legacy network. If rSRVCC is supported, the handover decision is made to transfer the call.
  4. Transfer of Control (To LTE): The LTE network informs the user equipment (UE) to transfer the control of the call to the LTE network for the handover process. The UE acknowledges the transfer request.
  5. Pre-Handover Procedures (LTE Network): The LTE network performs various preparations for the handover, such as preparing the necessary signaling messages and updating call-related information.
  6. Handover Execution: The LTE network initiates the handover procedure by sending a Handover Required message to the target legacy network, indicating the call transfer.
  7. Preparation for Circuit Switched Voice Call (Legacy Network): Upon receiving the Handover Required message, the target legacy network (e.g., GSM/UMTS) begins preparing for the circuit-switched voice call. It reserves the necessary resources and sets up the required call context.
  8. Handover Completion (LTE Network): The LTE network waits for a Handover Command message from the target legacy network, indicating the readiness for call transfer. Once received, it releases the resources associated with the call and acknowledges the handover completion.
  9. Handover Completion (Legacy Network): The target legacy network sends the Handover Command message to the UE, instructing it to switch to the circuit-switched voice call. It also releases the resources associated with the LTE portion of the call.
  10. Circuit-Switched Voice Call (Legacy Network): The UE establishes a circuit-switched voice call with the legacy network, ensuring a seamless transition for the user. The voice call continues without interruption, allowing the user to converse as usual.

rSRVCC is designed to provide a smooth transition from the advanced IP-based voice services of an LTE network to the legacy circuit-switched voice services of GSM or UMTS networks. It ensures that voice calls remain uninterrupted even when moving between different network technologies, thereby enhancing the user experience and network coverage.