UPF Architecture
The User Plane Function (UPF) is a fundamental component of a 3GPP 5G core infrastructure system architecture. The UPF represents the data plane evolution of a Control and User Plane Separation (CUPS) strategy, first introduced as an extension to existing Evolved Packet Cores (EPCs) by the 3GPP in Release 14 specifications. The CUPS decoupless Packet Gateway (P-GW) Control and User Plane functions, enabling the data forwarding component (PGW-U) to be decentralized. This allows packet processing and traffic aggregation to be performed closer to the network edge, increasing bandwidth efficiencies while reducing network load. The P-GW handling signaling traffic (PGW-C) remains in the core, northbound of the Mobility Management Entity (MME).
The primary goal of CUPS is to support 5G New Radio (NR) implementations enabling early IoT applications and higher data rates. Committing to a complete implementation of CUPS is a complex proposition as it only provides a subset of advantages to the operator adopting a 5G User Plane Function (5G-UPF), offering network slicing. Deployed as a Virtual Machine (VM), the User Plane Function delivers the packet processing foundation for Service-Based Architectures (SBAs).
The UPF identifies User Plane traffic flow that is based on information received from the SMF over the N4 reference point. The N4 interface employs the Packet Forwarding Control Protocol (PFCP), which is defined in the 3GPP technical specification 29.244 for use on Sx/N4 reference points in support of CUPS. The PFCP is similar to OpenFlow but can be limited to only the functionality that is required to support mobile networks. The PFCP sessions, which are established with the UPF, define how packets are identified (Packet Detection Rule / PDR), forwarded (Forwarding Action Rules / FARs), processed (Buffering Action Rules / BARs), marked (QoS Enforcement Rules / QERs) and reported (Usage Reporting Rules / URRs).