5G IMS

IP Multimedia Subsystem (IMS) plays a crucial role in enabling Voice over New Radio (VoNR) in 5G networks. VoNR is the voice service for 5G, similar to VoLTE (Voice over LTE) in 4G, but designed specifically for the 5G architecture. IMS is a standardized network framework that facilitates the delivery of voice, video, and messaging services over IP (Internet Protocol) networks, and it integrates seamlessly with the 5G Core (5GC) to support voice and multimedia services.

Role of IMS in 5G VoNR

In 5G networks, IMS continues to serve as the key platform for delivering voice and video calls, SMS, and multimedia services. As 5G New Radio (NR) focuses on delivering high-speed data and ultra-low latency, IMS ensures that voice services remain robust and are delivered over the 5G network through VoNR. Without IMS, 5G networks would need to rely on fallback mechanisms like EPS Fallback to 4G networks for voice calls, defeating the purpose of a full 5G experience.

IMS enables the following in a 5G VoNR environment:

1. VoNR Call Setup: IMS is responsible for setting up, managing, and terminating voice calls over 5G, ensuring high-quality voice communication.

2. Quality of Service (QoS): IMS works with the 5G Core to provide voice services with the necessary QoS guarantees, such as low latency and high reliability, which are critical for voice communication.

3. Multimedia Services: IMS supports other services such as video calling, messaging (RCS - Rich Communication Services), and enhanced media applications over 5G.

4. Roaming and Interoperability: IMS ensures that VoNR works seamlessly across different networks, enabling users to roam between 5G networks without losing voice capabilities.

5. Interworking with Legacy Networks: IMS supports interworking between VoNR and other legacy voice services like VoLTE and circuit-switched fallback (CSFB) in cases where 5G coverage is unavailable.

Network Functions of IMS in 5G VoNR

The IMS architecture consists of several network functions that work together to deliver voice services. In a 5G VoNR scenario, the main IMS components include:

1. Call Session Control Functions (CSCF):

   • Proxy-CSCF (P-CSCF): Acts as the first point of contact for the User Equipment (UE) within the IMS network. It forwards SIP messages and enforces policy and security.

   • Serving-CSCF (S-CSCF): The central node in IMS that handles SIP signaling, session control, and service invocation. It manages user registrations and routes calls to the appropriate application servers or media gateways.

   • Interrogating-CSCF (I-CSCF): Helps in determining which S-CSCF to route a call to, and handles inter-domain signaling.

2. Media Resource Function (MRF): Provides media processing capabilities, such as mixing, transcoding, and playing announcements during calls.

3. Application Servers (AS): Hosts various services such as voicemail, conference calls, and Rich Communication Services (RCS). These services are invoked by the S-CSCF as required during a voice or multimedia session.

4. Home Subscriber Server (HSS) / Unified Data Management (UDM): Maintains user profiles, authentication information, and service subscriptions. In a 5G context, the UDM performs these roles, providing user data to the IMS network.

5. Telephony Application Server (TAS): Provides call control features for voice calls, such as call forwarding, call waiting, and voicemail.

6. Session Border Controller (SBC): Ensures security, performs NAT traversal, and manages the signaling and media traffic between the IMS core and external networks.

7. Breakout Gateway Control Function (BGCF): Determines the route for outgoing calls that need to leave the IMS network, such as calls to the Public Switched Telephone Network (PSTN).

Integration of IMS with 5G Core (5GC)

The IMS architecture integrates with the 5G Core (5GC) to provide end-to-end voice and multimedia services over the 5G network. The key integration points include:

1. Interfaces Between IMS and 5GC:

   • N5 Interface (between IMS and SMF): IMS interacts with the 5G Session Management Function (SMF) through the N5 interface to manage IP session setup for voice services. The SMF coordinates with the User Plane Function (UPF) to ensure the correct QoS is applied to VoNR traffic.

   • N6 Interface (between UPF and IMS Media Gateways): Media traffic (voice and video) is routed from the 5G UPF to the IMS Media Resource Function (MRF) over the N6 interface. This ensures that media packets are routed correctly through the user plane.

2. 5G QoS and Policy Control:

   • The Policy Control Function (PCF) in 5GC works in tandem with the IMS to enforce QoS policies for voice and multimedia services. IMS requests specific QoS for voice calls, which the PCF enforces across the 5G network.

   • The Access and Mobility Management Function (AMF) handles mobility management and interacts with the IMS core to ensure seamless voice service continuity as the user moves across different 5G cells or when falling back to 4G.

3. User Plane Integration:

   • In a 5G VoNR call, the media stream (voice or video) flows directly between the UE and the IMS media gateway via the 5G Core’s user plane. The UPF ensures that voice packets are prioritized according to the QoS policies defined by the IMS and 5GC.

4. Registration and Authentication:

   • When a UE connects to a 5G network and registers for VoNR, the IMS handles registration via the SIP (Session Initiation Protocol) messages exchanged between the UE and the IMS network. The 5G Core’s AMF forwards the SIP registration to the IMS P-CSCF, which then communicates with the S-CSCF to authenticate and register the user in the IMS domain.

   • IMS leverages the Authentication and Key Agreement (AKA) procedure in coordination with the 5G UDM/Authentication Server Function (AUSF) to securely authenticate users.

5. Voice Continuity and EPS Fallback:

   • In areas where 5G coverage is weak, the IMS and 5GC work together to provide voice continuity by falling back to VoLTE (over 4G LTE) or EPS (Evolved Packet System) Fallback, ensuring that the user does not lose voice service.

   • EPS Fallback allows the 5G network to redirect voice traffic to a 4G LTE network when a VoNR call cannot be maintained, ensuring seamless service continuity.

Network Diagram of IMS and 5G Core Integration for VoNR:

Here’s a simplified diagram showing how IMS integrates with the 5G Core for VoNR:

Conclusion

IMS is fundamental in enabling voice services over 5G through VoNR. It provides the infrastructure for delivering high-quality voice and multimedia services, while seamlessly integrating with the 5G Core to ensure optimal performance, low latency, and efficient resource allocation. The combination of IMS and 5G Core allows operators to offer advanced voice services while future-proofing their networks for next-generation applications.