Control and User Plane Separation of EPC Nodes

Description

CUPS (Control and User Plane Separation) of EPC nodes is a Rel-19 feature that decouples the control plane (CP) and user plane (UP) functions within the Evolved Packet Core (EPC). This separation allows each plane to be scaled, upgraded, and deployed independently, improving network flexibility and efficiency. CUPS is specified in 3GPP TS 28.708 (management and orchestration), TS 33.107 (lawful interception architecture), TS 33.127 (lawful interception for 5G), and TS 33.128 (security assurance).

In a traditional EPC, nodes such as the Serving Gateway (SGW), Packet Data Network Gateway (PGW), and Traffic Detection Function (TDF) combine both control and user plane functionalities. CUPS splits these into separate entities: the SGW-C, SGW-U, PGW-C, PGW-U, TDF-C, and TDF-U. The control plane entities handle signaling, session management, and policy enforcement, while the user plane entities handle packet forwarding, QoS enforcement, and traffic detection. The interface between CP and UP is standardized (e.g., Sxb, Sxc, Sxa) to allow multi-vendor interoperability.

CUPS architecture introduces new reference points: Sxa between SGW-C and SGW-U, Sxb between PGW-C and PGW-U, and Sxc between TDF-C and TDF-U. These interfaces use PFCP (Packet Forwarding Control Protocol) for session management and rule provisioning. The control plane can dynamically instruct the user plane to apply specific forwarding, QoS, and traffic handling rules. This enables centralized control with distributed user plane, which is ideal for edge computing and low-latency applications.

Key components include the CP function (CPF) and UP function (UPF). The CPF manages session contexts, selects UPF, and enforces policies. The UPF executes packet processing, including GTP-U encapsulation/decapsulation, QoS marking, and traffic detection. CUPS also supports lawful interception (LI) by separating LI control and user plane functions, as detailed in TS 33.107 and TS 33.127. Security aspects, such as secure communication between CP and UP, are covered in TS 33.128.

CUPS is a precursor to the 5G Service-Based Architecture (SBA) and CUPS in 5G Core (5GC), where the UPF is fully separated from SMF. In EPC, CUPS enables flexible deployment of user plane functions at the network edge, supporting MEC and low-latency services. It also allows independent scaling of control and user plane, reducing costs and improving resource utilization. Management aspects, including lifecycle management and fault supervision, are specified in TS 28.708.

Purpose & Motivation

CUPS was created to overcome the limitations of monolithic EPC nodes, which combined control and user plane functions in a single entity. This monolithic design made it difficult to scale user plane capacity independently from control plane capacity, leading to inefficient resource usage. For example, a surge in data traffic required scaling the entire node, including control plane components that might not be the bottleneck. CUPS solves this by separating the planes, allowing operators to deploy and scale user plane functions based on traffic demand, while keeping control plane functions centralized.

The motivation also includes the need for flexible deployment of user plane functions at the network edge to support low-latency services like MEC. By separating the user plane, operators can place UPFs close to the radio access network, reducing latency and backhaul costs. CUPS also simplifies the introduction of new features, as control plane and user plane can be upgraded independently. Additionally, CUPS aligns with the broader industry trend toward SDN and NFV, where control and user plane separation is a fundamental principle.

Historically, CUPS was first introduced in 3GPP Rel-14 for EPC, but Rel-19 brings enhancements in management, security, and lawful interception. The Rel-19 work focuses on integrating CUPS with 5G core principles, ensuring consistent LI and security frameworks. CUPS in EPC serves as a stepping stone to the full CUPS architecture in 5GC, where the UPF is a standalone network function. The Rel-19 specifications (28.708, 33.107, 33.127, 33.128) provide updated management and security requirements for CUPS deployment in multi-vendor environments.