Packet Data Network Gateway

Description

The Packet Data Network Gateway (PDN-GW or PGW) is a critical network function in the 3GPP Evolved Packet Core (EPC), specified across multiple releases starting with Rel-8 (LTE). It sits at the boundary between the mobile operator's trusted EPC and external Packet Data Networks (PDNs), such as the public Internet or private enterprise networks. For each UE, the PGW is the termination point of the GTP-based S5/S8 interface from the Serving Gateway (SGW) and the point of origin for the SGi interface to the external PDN. It acts as the mobility anchor for inter-3GPP mobility (e.g., between LTE and 2G/3G) and is responsible for managing the UE's IP connectivity session.

Architecturally, the PGW combines several key functions. It is the policy enforcement point, applying rules received from the Policy and Charging Rules Function (PCRF) for gating, bandwidth control, and QoS marking. It performs deep packet inspection (DPI) to identify service data flows for accurate charging and policy application. The PGW allocates the UE's IP address (either IPv4, IPv6, or both), typically using DHCP or from a local pool, and acts as the default router for the UE's traffic. It also handles charging functionality, generating Charging Data Records (CDRs) for offline charging or interacting with the Online Charging System (OCS) for real-time credit control.

In terms of packet processing, the PGW routes uplink packets from the UE (received via GTP tunnel from the SGW) out to the external PDN on the SGi interface, and routes downlink packets from the PDN into the appropriate GTP tunnel towards the SGW and ultimately the UE. It performs key security functions like uplink and downlink packet filtering (acting as a firewall), and may implement Network Address and Port Translation (NAPT) if private IPv4 addresses are used. The PGW also supports Lawful Interception (LI) by duplicating and diverting user traffic to law enforcement agencies as required.

With the introduction of Control and User Plane Separation (CUPS) in Release 14, the monolithic PGW was split into a PGW-Control Plane (PGW-C) and a PGW-User Plane (PGW-U). The PGW-C handles signaling (GTP-C, PFCP), session management, and policy control, while the PGW-U handles the high-throughput packet forwarding under the control of the PGW-C via the PFCP protocol. This allows independent scaling of control and user plane resources. In 5G, the functions of the PGW are distributed between the Session Management Function (SMF, control plane) and the User Plane Function (UPF, user plane), continuing the CUPS paradigm.

Purpose & Motivation

The PDN-GW was created as part of the Evolved Packet System (EPS) in 3GPP Release 8 to provide a high-performance, all-IP anchor gateway for LTE networks. It consolidated and evolved functions from the 3G GGSN and added new capabilities required for advanced LTE services. Prior to EPS, the GGSN provided basic IP connectivity but lacked the integrated policy control and sophisticated charging required for tiered data services and IMS-based voice (VoLTE).

The PGW solved the need for a single, stable mobility anchor in a flat, all-IP architecture. In 3G, the SGSN could be an anchor for some mobility scenarios, leading to complex routing. The EPS design simplified this by making the PGW the sole anchor for all 3GPP access (LTE, 3G, 2G). This design minimized packet loss and latency during handovers. Furthermore, its tight integration with the PCRF via the Gx interface enabled dynamic, service-aware policy and charging control, which was crucial for monetizing different data services (e.g., video streaming vs. background sync) and ensuring quality for real-time applications.

Historically, the PGW enabled the commercial deployment of LTE and the shift to mobile broadband. Its ability to allocate public IP addresses, enforce fair usage policies, and generate detailed charging records formed the operational backbone of 4G services. The later introduction of CUPS (splitting PGW into PGW-C and PGW-U) addressed the limitations of monolithic hardware appliances, allowing operators to leverage cloud-native, scalable architectures and paving the way for the 5G core's service-based architecture. The PGW remains a vital node in current 4G networks and in interworking scenarios with 5G.

Key Features

• Serves as the mobility anchor and IP session anchor for UE in EPS
• Terminates GTP-based S5/S8 interface and connects to PDN via SGi
• Allocates IP address(es) to the UE (IPv4/IPv6)
• Enforces policy and charging rules from PCRF (via Gx interface)
• Performs packet routing, forwarding, filtering, and deep packet inspection
• Generates charging data records and supports lawful interception

Evolution Across Releases

Defining Specifications