PDN Gateway Control plane function

Description

The PGW-C (PDN Gateway Control plane function) is a core network function introduced as part of the Control and User Plane Separation (CUPS) architecture in 3GPP Release 14. It represents the decoupling of the traditional P-GW (Packet Data Network Gateway) into separate control plane (PGW-C) and user plane (PGW-U) entities. This separation allows for independent scaling, deployment, and evolution of the control and user plane functions. The PGW-C is responsible for the intelligence and signaling aspects of the gateway.

Architecturally, the PGW-C interfaces with several other core network functions. It communicates with the Policy and Charging Rules Function (PCRF) via the Gx interface to receive policy and charging control (PCC) rules. It interacts with the Online Charging System (OCS) via the Gy interface for online credit control and with the Offline Charging System (OFCS) via the Gz interface for offline charging data record (CDR) generation. For session management, it connects to the Serving Gateway Control plane function (SGW-C) and the Mobility Management Entity (MME) in the Evolved Packet Core (EPC).

Its primary operational role involves managing Packet Data Network (PDN) connections for User Equipment (UE). When a UE attaches to the network and requests a PDN connection, the PGW-C is responsible for session establishment, modification, and termination. It allocates an IP address to the UE (or delegates this to the PGW-U), enforces QoS policies based on PCC rules from the PCRF, and manages charging based on the user's service data flow. It also handles mobility events, such as handovers between different access technologies (e.g., LTE to Wi-Fi), ensuring session continuity.

The PGW-C works in tandem with the PGW-U via a standardized control protocol, defined in 3GPP specifications such as TS 29.244 (PFCP - Packet Forwarding Control Protocol). The PGW-C uses PFCP to instruct the PGW-U on how to handle the user plane traffic. This includes installing, modifying, or deleting packet detection rules (PDRs), forwarding action rules (FARs), QoS enforcement rules (QERs), and usage reporting rules (URR). This master-slave relationship allows the PGW-C to centrally control multiple, potentially distributed, PGW-U instances, enabling flexible and efficient traffic routing and processing.

Purpose & Motivation

The PGW-C was created to address the limitations of the monolithic P-GW architecture used in earlier 3GPP releases. In traditional EPC, the P-GW was a single network node combining both control and user plane functionalities. This monolithic design posed several challenges, including inefficient scaling (the entire node had to be scaled even if only user plane capacity was needed), limited deployment flexibility (control and user plane functions had to be co-located), and hindered innovation (upgrades to one plane could impact the other).

The primary motivation for its creation was the industry-wide drive towards network function virtualization (NFV) and software-defined networking (SDN), which demand disaggregated, software-based components that can be deployed independently. The Control and User Plane Separation (CUPS) architecture, formalized in Release 14, was the 3GPP's response. By splitting the P-GW into PGW-C and PGW-U, operators gained the ability to scale the control plane (which handles signaling and policy) and the user plane (which handles high-throughput data packets) independently based on traffic patterns. For instance, user plane functions could be deployed at the network edge for low-latency services, while control plane functions could remain centralized for efficient management.

This separation also paved the way for the 5G Core (5GC) network architecture, where the Session Management Function (SMF) and User Plane Function (UPF) are direct conceptual successors to the PGW-C and PGW-U, respectively. The introduction of PGW-C allowed for a smoother evolution from EPC to 5GC, enabling early adoption of cloud-native principles and distributed architectures within the 4G network, thus solving the problems of scalability, flexibility, and cost-efficiency in the face of exponentially growing mobile data traffic.