Serving Gateway / Packet Data Network Gateway

Description

The Serving Gateway (SGW) and Packet Data Network Gateway (PGW) are fundamental components of the 4G LTE Evolved Packet Core (EPC) architecture, defined from 3GPP Release 8 onwards. They are often co-located in network implementations, referred to as the S/P-GW. The SGW is the user-plane anchor point for intra-LTE mobility (e.g., handovers between eNodeBs) and inter-3GPP mobility (e.g., handovers to 2G/3G networks). It manages and stores UE contexts, including parameters of the IP bearer service and network internal routing information. All user IP packets are routed through the SGW. It interfaces with the eNodeB via the S1-U interface and with other SGWs via the S5/S8 interface for handovers. The SGW also generates charging data per UE and PDN connection for use by the charging function.

The Packet Data Network Gateway (PGW) is the point of exit and entry for UE traffic to external Packet Data Networks (PDNs), such as the internet or an IMS network. It is the anchor for mobility between 3GPP and non-3GPP technologies (e.g., Wi-Fi). Key functions include IP address allocation for the UE (via DHCP or from an external AAA server), policy enforcement and flow-based charging as instructed by the Policy and Charging Rules Function (PCRF), packet filtering, and lawful interception. The PGW performs deep packet inspection to apply policy and charging rules. It connects to the SGW via the S5 (in a non-roaming scenario) or S8 (in a roaming scenario) interface and to external networks via the SGi interface.

Together, the SGW and PGW handle the entire user data path. The SGW focuses on mobility management and routing within the 3GPP network, while the PGW manages external connectivity, policy, and charging. In the control plane, they interact with the Mobility Management Entity (MME) via the S11 interface for bearer management and with the PCRF via the Gx interface for policy and charging control. Their combined role is critical for providing seamless mobility, quality of service, and secure, billable internet access to mobile subscribers.

Purpose & Motivation

The SGW and PGW were introduced with the Evolved Packet System (EPS) in 3GPP Release 8 to create a simplified, all-IP core network for LTE. They replaced the complex suite of 2G and 3G core network nodes (like SGSN and GGSN) with a flatter, more efficient architecture optimized for high-speed packet data. The primary motivation was to reduce latency, increase data throughput, and support always-on connectivity, which were limitations of previous 3GPP architectures that were originally designed for circuit-switched voice.

The separation of the SGW and PGW functions allows for flexible network deployment. The SGW can be distributed closer to the radio network to optimize the user data path and reduce latency for local traffic. The PGW can be centralized to efficiently manage policies, charging, and connectivity to multiple external networks. This architecture solved the problem of inefficient data routing in earlier networks and provided a clear anchor point for mobility and policy enforcement, which was essential for enabling new services like VoIP over LTE (VoLTE) and high-quality mobile broadband.

Key Features

• User-plane anchor for intra-LTE and inter-3GPP mobility (SGW)
• Gateway to external Packet Data Networks like the internet (PGW)
• Policy enforcement and flow-based charging control (PGW)
• IP address allocation for User Equipment (PGW)
• Lawful interception support
• Bearer management based on QoS requirements

Evolution Across Releases

Defining Specifications