The 5G User Plane Function (UPF) is a fundamental component of the 3GPP’s New Radio (NR) mobile 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 4G/LTE Evolved Packet Cores (EPCs) by the 3GPP in their Release 14 specifications. CUPS decouples Packet Gateway (PGW) 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. The PGW’s handling signaling traffic (PGW-C) remain in the core, northbound of the Mobility Management Entity (MME).
The primary goal CUPS was to support 5G New Radio implementations enabling early IoT applications and higher data rates. Committing to a complete implementation of control and user plane separation, however, is a complex proposition which only provides a subset of the advantages adopting a 5G User Plane Function affords, such as network slicing. Deployed within a dynamic cloud native compute infrastructure, the User Plane Function delivers the packet processing foundation for Service Based Architectures (SBAs).
Defined in 3GPP technical specification 23.501, the UPF provides:
The User Plane Function has four distinct reference points:
The protocol employed on the N3 and N9 interfaces could be the GPRS Tunnelling Protocol (GTP) with header extensions for 5G, segment routing (SRv6 or NSH) or even an Information Centric Networking (ICN) protocol. The Locator/ID Separation data plane protocol (LISP-DP) over GTP or Identifier Locator Addressing (ILA) could also be utilized. A relay function would be performed by an I-UPF, while the UPF PDU Session Anchor would terminate these protocols.
The UPF identifies user plane traffic flow based on information received from the SMF over the N4 reference point. The N4 interface employs the Packet Forwarding Control Protocol (PFCP), which was originally defined within 3GPP technical specification 29.244 for use on Sx reference points in support of CUPS. PFCP is similar to OpenFlow but can be limited to only the functionality required to support mobile networks. PFCP sessions, 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).
To meet 5G’s fundamental mandate for granular capacity or application-driven, instantiation and up/down-scaling, the UPF must be implemented as a pure cloud native network function using modern microservices methodologies and deployable within a serverless framework. In order to achieve high packet throughputs with complex pipelines and low latencies, such components have historically been realized using dedicated hardware and custom silicon. Delivering a cloud native UPF on shared resources demands solving complex real-time data processing problems and providing a high degree of flexibility in the pipeline to support emerging traffic tunneling, mobility and infrastructure overlays. UPF instantiation must also be highly automated and orchestrated. This means developing or enhancing user-space data plane acceleration, runtime programmability techniques and tightly integrating with numerous cloud orchestration systems, such as Kubernetes.
The User Plane Function is a critical new component for supporting the new generation of service-based architectures. The velocity of innovation in this area, however, should also allow the early majority of perspective CUPS adopters to adopt the UPF over interim PGW/SGW-U solutions, smoothing the migration from 4G to 5G.
To learn about Microsoft’s mobile core offerings for public and private 5G network infrastructures, visit aka.ms/operators
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