UE behavior when VoNR unsupported, network attempts dedicated bearer

5G NR
1

UE supports VoLTE and SA, but not VoNR. UE capability Info to the n/w clearly indicates that the device does not support VoNR. When a call is triggered in 5G SA, and if the network configures the VoNR bearer (ignoring UE capability) what should the UE do here? Any specification reference would be helpful

2
  • The UE should reject the VoNR bearer establishment attempt with an appropriate cause code.
  • According to 3GPP TS 24.501 (5G NAS protocol), section 6.4.1.3 “UE requested PDU session establishment procedure accepted by the network,” when the network attempts to establish a PDU session with parameters the UE cannot support, the UE should:
  • Send a PDU SESSION ESTABLISHMENT REJECT message
  • Include cause #43 “Invalid PDU session identity” or the appropriate failure cause
  • Additionally, 3GPP TS 38.331 (5G RRC protocol) specifies that when receiving RRC configuration for capabilities the UE doesn’t support, the UE should:
  • Reject the configuration
  • Send an RRC reconfiguration failure message
  • For fallback procedures, 3GPP TS 23.502 section 4.13 details the “EPS Fallback for IMS voice” procedure. The UE should trigger EPS fallback to utilize its VoLTE capability instead.
3

This is a very good question and there are several ways we can do it.During the initial registration IMS default bearer will get established via 5G SA is the most common method.In this case.UE is in 5G SA up until the PDN connectivty connection request for the dedicated voice bearer 5QI 1 comes.there are several ways can achieve this.First method is AMF asks UE to do EPS fallback(Extendard Service request)or TAU request.then all the bearers will re-arrange. In this case initial invite carries a PANI with access type 5GSA or 5G NR.Other method is PCSCF can send a 380 asking the UE to do a fresh registration when the initial invite reaches PCSCF.This way entire call including signalling will be on 4G.First method is easier when you are close to impliment VoNR in the network but really troublesome for billing.Later one eliminate all charging issues but VoNR will become more work for IMS.