Why do we wait for 5QI-5 voice call flow to establish for the gNB to reject and proceed with fallback procedure?
Why don’t we simply reject when the IMS flow with 5QI-1 is established?
It seems additional signaling takes place with no use…
Actually, data bearer with 5QI-5 is needed to establish the SIP signalling user-plane tunnel. We cannot simply reject this bearer establishment as it is not only used for voice call establishment but for other purposes well.
Once the QoS flow with 5QI-5 is established, the UE can communicate with the IMS for voice call establishment, among other call flows (like SMS, video via IMS etc.). But once there is an indication of voice call establishment with QoS flow of 5QI-1, then it is certain that now there is a voice call session initiation request for the UE (either UE initiated or UE terminated). Hence, now the request must be reject to initiate EPS fallback procedure. (in case VoNR is not supported)