PDN Gateway

Description

The PDN Gateway (PGW) is a central node in the 3GPP Evolved Packet Core (EPC) architecture, introduced with LTE in Release 8. It resides at the boundary between the mobile operator's trusted network and external Packet Data Networks (PDNs), such as the public internet, an IMS network, or a corporate intranet. For each User Equipment (UE), at least one PGW is assigned to handle its data sessions, known as PDN connections. The PGW establishes a GTP (GPRS Tunnelling Protocol) tunnel with the Serving Gateway (SGW) for user plane traffic and interacts with control plane entities like the MME and PCRF.

Architecturally, the PGW performs several vital roles. It is the point of IP address allocation for the UE, typically using DHCP or acting as a DHCP server. It enforces policy and charging control rules received from the Policy and Charging Rules Function (PCRF), which dictate how traffic should be treated (e.g., bandwidth limits, QoS marking) and how it should be accounted for billing. The PGW performs deep packet inspection, packet filtering, and lawful interception. It also acts as the mobility anchor for the user plane when the UE moves between different SGWs, ensuring session continuity. For non-3GPP accesses (like Wi-Fi via S2a/b/c interfaces), the PGW serves as a common anchor, enabling seamless mobility between 3GPP and trusted/untrusted non-3GPP networks.

In operation, when a UE attaches to the network, the MME selects a PGW based on the Access Point Name (APN) requested by the UE or subscribed in the HSS. The PGW then establishes a default bearer for that PDN connection, assigning an IP address and applying default QoS and charging characteristics. As applications on the UE generate traffic, the PGW applies the appropriate Traffic Flow Templates (TFTs) to route packets to the correct bearer, enforces QoS policies (setting DSCP markings), and generates charging data records (CDRs) for offline or online charging systems. It is the ultimate router for UE-originated packets heading to the internet and the first point of entry for packets destined for the UE.

Purpose & Motivation

The PGW was created as part of the 'System Architecture Evolution' (SAE) to address limitations of the pre-LTE GPRS core network. In 2G/3G GPRS, the gateway functions were split between the SGSN (control) and the GGSN (gateway). The GGSN was often a bottleneck and complex to scale. The EPC architecture aimed for a flatter, all-IP network with reduced latency and higher throughput to support LTE's advanced radio capabilities. The PGW consolidated and enhanced the gateway functions, separating the control and user planes more clearly and enabling more flexible policy enforcement.

It solves several key problems. First, it provides a stable anchor point for mobility, hiding the UE's movement within the radio access network from the external PDN. The UE's IP address, assigned by the PGW, remains constant even as it changes cells or SGWs. Second, it enables sophisticated, real-time policy control. By integrating with the PCRF, operators can implement service-aware charging and QoS (e.g., prioritizing VoIP traffic, throttling peer-to-peer traffic), which was more cumbersome in earlier architectures. Third, it simplifies integration of multiple access technologies (LTE, 3G, Wi-Fi) by providing a single, consistent IP anchor point. Its creation was motivated by the need for a high-performance, scalable, and policy-rich core network to unlock the potential of mobile broadband and enable new revenue-generating services.