NG User Plane Interface

Description

The NG User Plane interface (NG-U) is a logical interface in the 5G System (5GS) that connects the Next Generation Radio Access Network (NG-RAN) to the User Plane Function (UPF) in the 5G Core Network (5GC). It is responsible for the transport of user data packets (e.g., IP packets, Ethernet frames) between the UE and the Data Network (DN). NG-U is part of the overall NG interface, which also includes the control plane interface NG-C between the NG-RAN and the Access and Mobility Management Function (AMF). NG-U was introduced in 3GPP Release 19, although earlier releases defined the NG interface in general; Release 19 brought specific enhancements and clarifications for the user plane, as documented in TS 38.410 (NG-RAN overall description) and TS 38.912 (Study on New Radio access technology).

Architecturally, NG-U is established between a gNB (or ng-eNB) and a UPF. It is based on the GPRS Tunnelling Protocol for the User Plane (GTP-U), which encapsulates user data packets within GTP-U tunnels. Each PDU session or QoS flow is associated with a GTP-U tunnel identified by a Tunnel Endpoint Identifier (TEID). The NG-U interface supports multiple QoS flows per PDU session, allowing differentiated treatment of traffic. The interface is also used for the transport of Ethernet PDU sessions, enabling industrial IoT and other Ethernet-based services.

The operation of NG-U involves the establishment, modification, and release of GTP-U tunnels, which are controlled by the Session Management Function (SMF) via the N4 interface to the UPF, and by the AMF via NG-C to the NG-RAN. When a UE requests a PDU session, the SMF selects a UPF and allocates TEIDs for the uplink and downlink directions. These TEIDs are communicated to the NG-RAN via NG-C, and to the UPF via N4. The NG-RAN then uses the uplink TEID to send user data to the UPF, and the UPF uses the downlink TEID to send data to the NG-RAN. The NG-U interface also supports features like reflective QoS, where the UE marks packets based on downlink QoS flow information.

In Release 19, NG-U enhancements focus on supporting new services and improving efficiency. These include support for Multi-Path QUIC proxying (as per MPQUIC-UDP), which may involve the UPF acting as a proxy and using NG-U to transport the encapsulated traffic. Additionally, Release 19 may include optimizations for time-sensitive networking (TSN), reduced capability (RedCap) devices, and network slicing, ensuring that NG-U can handle diverse traffic types with appropriate QoS. The interface also supports the transport of small data transmissions for IoT devices, reducing signaling overhead.

Overall, NG-U is a fundamental interface in the 5G architecture, enabling the separation of control and user planes (CUPS) and allowing flexible deployment of UPFs. It ensures that user data can be efficiently routed from the RAN to the core and beyond, with the necessary QoS and security. Its evolution in Release 19 reflects the ongoing need to support new use cases and technologies in 5G-Advanced and beyond.

Purpose & Motivation

The NG-U interface exists to provide a standardized, efficient, and scalable means of transporting user data between the 5G radio access network and the 5G core network. In earlier generations, the user plane interface (e.g., S1-U in 4G) was tightly coupled with the control plane and had limited flexibility. With 5G, the architecture adopted a service-based, cloud-native approach with separation of control and user planes (CUPS). NG-U was defined to allow the UPF to be deployed flexibly, potentially at the edge or in a centralized data center, and to support diverse QoS requirements through multiple QoS flows per PDU session.

The motivation for NG-U also includes the need to support new services such as ultra-reliable low-latency communication (URLLC), enhanced mobile broadband (eMBB), and massive machine-type communication (mMTC). These services have varying requirements for latency, throughput, and reliability, which NG-U addresses by enabling per-flow QoS handling and efficient tunneling. The use of GTP-U provides a proven, interoperable tunneling mechanism that can be extended with features like Ethernet support and reflective QoS.

Release 19 enhancements to NG-U were driven by the emergence of new use cases like multi-path communication, time-sensitive networking, and the proliferation of IoT devices. The interface needed to evolve to support these without disrupting existing deployments. By specifying NG-U in TS 38.410 and TS 38.912, 3GPP ensures that vendors and operators have a clear reference for implementing and deploying the interface, fostering a competitive ecosystem.