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The main function of SRVCC is to engage the capable UE in a voice call which determines to be moving away from LTE coverage, and it notifies the LTE network. The LTE network determines that the voice call needs to be moved to the legacy circuit domain. It notifies the MSC server of the need to switch the voice call from the packet to the circuit domain and initiates a handover of the LTE voice bearer to the circuit network. The MSC server establishes a bearer path for the mobile in the legacy network and notifies the IMS core that the mobile’s call leg is moving from the packet to the circuit domain. The circuit-packet function in the IMS core then performs the necessary inter-working functions. When the mobile arrives on-channel in the legacy network, it switches its internal voice processing from VoIP to legacy-circuit voice, and the call continues.
To minimize the voice interruption delay during a voice call transition, Access Transfer Control Function (ATCF) and Access Transfer Gateway (ATGW) are deployed.
SBC sends policy requests to PSX to fetch information on Access Transfer Profile (ATP) and on receipt of PSX policy response, SBC evaluates the profile and associated inclusion policy to determine whether the co-located ATCF/EATF functionality needs to be executed or not.
SBC sends policy requests to PSX and as part of policy response PSX returns the following information:
- Access Transfer Profile—SBC evaluates the profile (received from PSX) and associated inclusion policy to determine if the co-located ATCF/EATF functionality needs to be executed or not.
- UE Roaming—PSX informs about the UEs roaming status to SBC.
- SRVCC Support—For a roaming user, PSX informs SBC if the UE home operator supports eSRVCC in its SCC AS.
Access Transfer Control Function (ATCF)
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Overview
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