Voice communication is an essential part of the service providers offers. When 5G Core is deployed, voice is leveraged using the 5G Core functions and IMS (IP Multimedia Subsystem), while the 5G use cases are delivered by the NR(New Radio) and the 5G Core. voice services will depend on the presence of an IMS Core network regardless of the deployment path. 5G will enable a wide range of truly novel applications where voice will play an important role. Voice is an important key component of the 5G services.
5G comes with unique characteristics to deliver responses at a minimal time i.e less than 1msec, and quick exchange of data signals which results in improved signal and picture quality. VoNR 5G is a calling service that utilizes 5G SA (Stand-Alone) Architecture for voice processing whereas ViNR 5G is a video service built on 5G SA to enable customers with high-resolution video quality. Globally, operators are using SA Architecture that is considered as the primary network for future 5G networks. Efforts are being built and designed to configure and upgrade VoNR and ViNR video qualities.
VoNR is the IMS-based calling service that uses the 5G network for its source of IP (Internet Protocol) voice processing.
Types of voice services available over 5G mobile networks:
- Carrier-Grade Voice service: it has strict QoS support and does not belong to the public Internet Voice over New Radio (VoNR) or Vo5G.
- OTT voice services: it will continue to exist in 5G networks and they will continue to be provided through mobile internet access.
Each operator has a unique set of the allocated spectrum and its own deployment strategy. VoNR features can be different to meet the operator’s call quality, voice coverage, service continuity, mobility, call setup delay, call success rates, etc. VoLTE and VoNR use the same IMS network for voice calls. Technological improvements in New Radio, 5GC, and 5G devices are expected to provide a superior user experience in VoNR compared to VoLTE. EPS Fallback (E-UTRAN connected to EPC) and RAT(E-UTRAN connected to 5GC) are two deployment scenarios that support redirection or handover to E-UTRAN during the call set-up phase. The EPS FB mechanism will use the existing IMS Core and EPS for delivering voice or video services. 3GPP also supports several interworking options with the Evolved Packet Core. Network Operators should consider the impact to voice services while finalizing the interworking mode with EPC.
- When a UE initiates a call, an RRC connection is set up between the UE and its serving gNB.
- The 5G Core Network (5GC) sets up a QoS (Quality of service) flow with a 5G QoS Identifier and gNB sets up the corresponding data radio bearer (DRB).
- An RRC connection is set up between the UE and its serving gNB.
- The 5GC sets up a QoS flow to carry SIP signaling and gNB sets up the corresponding DRB.
- The calling and called UE’s and the IMS perform SIP negotiation on the codec scheme, IP address, port number, information of both calling and called UE’s and other voice service information.
- After SIP negotiation is successful, the 5GC sets up QoS flow with a 5QI of 1 for both the calling and called UEs to carry Real-time transport protocol (RTP) and Real-time transport control protocol (RTCP) data flows, and the gNBs set up the corresponding DRBs.
- After the call ends, the calling and called UEs to release their QoS flows.
5G IMS Support:
The IMS (IP Multimedia Subsystem) represents connection management for voice services in 5G. A network offering voice services in 5GS needs to support IMS with the following functionality:
- Indication towards the UE if an IMS voice-over PS session is supported.
- Capability to transport the proxy call session control function (P-CSCF) addresses to a UE.
- Paging policy differentiation for IMS
- IMS emergency service
- Domain selection for UE originating sessions
- Terminating domain selection for IMS voice
- Support for the P-CSCF restoration procedure
- Network repository function based P-CSCF discovery
- NRF or SCP based HSS discovery
- SIP Registration
- Call establishment and termination
- SIP precondition considerations
- Early media and announcements
- Signaling compression
- SIP timer operation
- RCS feature capability discovery
EPS fallback call flow
UE resides on the NG-RAN of the 5G system and initiates the establishment of the MO/MT IMS voice session. The NG-RAN receives a network-initiated PDU session modification request that establishes a QoS stream for voice services.
For the inter-RAT handover case, it is easy to detect from signaling information that an EPS fallback was triggered. In the source-eNodeB-to-target-eNodeB-transparent-container sent by the gNB to the eNB. If a re-direction from voice EPS Fallback is possible or not is indicated in the NGAP Initial Context Setup Request, handover request, and path switch request acknowledge messages, all sent by the AMF to the gNB.