User Plane Interworking Unit

Description

The User Plane Interworking Unit (UP-IWU) is a network function that operates at the user plane level to enable seamless data session continuity and interworking between a 5G Core Network (5GC) and a 4G Evolved Packet Core (EPC). It acts as a protocol translator and data forwarder, sitting on the user plane path between the Radio Access Network (RAN) and the core network. In a typical interworking scenario, such as when a UE connects via a 5G New Radio (NR) but the session is anchored in the 4G Packet Gateway (PGW), the UP-IWU ensures that GTP-U tunnels from the RAN can be properly terminated and mapped to the appropriate core network interfaces and GTP-U versions.

Architecturally, the UP-IWU is often co-located with or integrated into a gateway function like the User Plane Function (UPF) in 5GC or the Serving Gateway (SGW) in EPC. Its primary operation involves adapting between the GTP-U protocol used over the N3 interface (between 5G RAN and UPF) and the GTP-U protocol used over the S5/S8 interface (between SGW and PGW in EPC), or similar combinations. It handles the encapsulation and decapsulation of user data packets, manages tunnel endpoint identifiers (TEIDs), and may perform necessary mapping of QoS parameters between the different QoS models of 5G (QoS Flow) and 4G (EPS Bearer). The UP-IWU ensures that packets are correctly routed between the disparate core network domains without loss of connectivity.

Key components of its functionality include the interworking logic for GTP-U header fields, buffering capabilities during handover procedures, and support for dual-stack operations. Its role is critical in non-standalone (NSA) 5G deployment architectures, like EN-DC, where the 5G NR is used as a secondary cell group for data boost while the control plane remains anchored in LTE and EPC. The UP-IWU enables the aggregation and splitting of user plane traffic between the two RANs and the single core anchor point. It is a pivotal element in ensuring service continuity during network evolution, allowing operators to introduce 5G radio coverage without immediately upgrading the entire core network, thereby facilitating a cost-effective and gradual migration path.

Purpose & Motivation

The UP-IWU was developed to solve the practical challenges of inter-system mobility and coexistence between 4G and 5G networks during the transition period. As 5G standards were finalized, it was clear that widespread deployment of a full 5GC would take time, and operators needed a way to leverage new 5G radio assets immediately using their existing EPC investments. This led to the Non-Standalone (NSA) mode of operation, but it created a technical problem: how to connect a 5G RAN node (gNB) using new protocol stacks to a 4G core network (EPC) designed for LTE RANs. The UP-IWU provides the necessary adaptation layer to bridge this gap.

Its creation was motivated by the need for a standardized, efficient interworking solution that avoids proprietary gateways and ensures multi-vendor interoperability. Prior to its specification, interworking between different network generations often required complex, non-standard gateways or forced all traffic through a single type of core, limiting deployment flexibility. The UP-IWU addresses the specific protocol disparities, such as differences in GTP-U versions, extension headers, and PDU session versus EPS bearer contexts. It solves the problem of user plane path continuity when a UE moves between 4G and 5G coverage areas or operates in a dual-connected mode.

Introduced in Rel-9 for earlier interworking scenarios and significantly enhanced for 4G-5G interworking in later releases, the UP-IWU enables key operator strategies like 'EPC anchoring' for early 5G deployments. It allows for the re-use of existing packet core services, policy control, and charging systems while introducing new radio capabilities. This reduces time-to-market for 5G services and lowers capital expenditure. Ultimately, the UP-IWU is a transitional but essential network function that supports the long-term co-existence and smooth evolution path from EPC to 5GC, ensuring subscribers experience seamless service regardless of the underlying core network generation.