Access Gateway

Description

The Access Gateway (AGW) is a standardized network function within the 3GPP architecture, specifically defined in the context of network-based IP flow mobility (NB-IFOM) and access network discovery and selection function (ANDSF) enhancements. Architecturally, it resides at the boundary between the user equipment's point of attachment (e.g., 3GPP access like LTE or 5G NR, or non-3GPP access like Wi-Fi) and the packet core network (evolved packet core or 5G core). Its primary operational mechanism involves establishing and managing IP connectivity sessions for user equipment, acting as the first IP hop from the device's perspective within the operator's network.

Functionally, the AGW performs several critical roles. It serves as an anchor point for IP flows, enabling seamless mobility and session continuity as users move between different access networks. The gateway implements policy enforcement received from the policy and charging rules function (PCRF) or policy control function (PCF), applying quality of service (QoS) markings, bandwidth limitations, and charging rules to user data traffic. It also handles IP address allocation and management for connected user equipment, often working in conjunction with a DHCP server or using network-based mobility protocols.

From a protocol perspective, the AGW interfaces with user equipment over various access-specific protocols (e.g., GTP-U over S1-U for LTE, N3 interface for 5G) and connects to the core network over standardized interfaces. In 3GPP specifications, the AGW's behavior is detailed in relation to how it supports selective IP traffic offload (SIPTO) and local IP access (LIPA), allowing certain traffic flows to be routed locally without traversing the entire core network. This local breakout capability reduces latency and core network load for appropriate traffic types.

The AGW's implementation varies depending on the access technology and network architecture. In integrated deployments, it may be collocated with other network functions like the serving gateway (SGW) in EPC or the user plane function (UPF) in 5GC. As a logical entity, its key components include the data forwarding engine for packet routing, the policy enforcement module for applying QoS and charging rules, the mobility management module for handling handovers between access points, and the control interface for communication with network controllers. Its distributed nature allows operators to deploy AGWs closer to network edges for improved performance.

Purpose & Motivation

The Access Gateway was introduced to address the growing complexity of heterogeneous network environments where user equipment can connect through multiple access technologies simultaneously. Prior to its standardization, networks struggled with efficient traffic steering and mobility management across 3GPP and non-3GPP accesses, often requiring separate gateways for each access type with limited coordination between them. This led to suboptimal resource utilization, increased handover latency, and difficulty implementing consistent policies across different access networks.

The AGW concept emerged specifically to support network-based IP flow mobility (NB-IFOM), which allows operators to dynamically route individual IP flows through the most appropriate access network based on policies, network conditions, and application requirements. This was a significant advancement over earlier approaches where all traffic from a device had to use the same access network or where mobility solutions operated at the application layer rather than the network layer. By providing a common gateway function across accesses, the AGW enables more granular traffic management while maintaining session continuity.

Furthermore, the AGW facilitates the implementation of advanced traffic offload mechanisms like selective IP traffic offload (SIPTO) at the local network, which became increasingly important as mobile data traffic volumes exploded. Without a standardized AGW function, operators faced interoperability challenges when trying to implement multi-access solutions from different vendors. The 3GPP standardization of AGW ensured consistent behavior across implementations, enabling seamless integration with existing core network functions and supporting the evolution toward more flexible, access-agnostic network architectures.

Key Features

• Multi-access connectivity support for 3GPP and non-3GPP networks
• Network-based IP flow mobility (NB-IFOM) enforcement
• Policy and charging rule enforcement from PCRF/PCF
• Local IP access (LIPA) and selective IP traffic offload (SIPTO) support
• IP address management and allocation for user equipment
• Session continuity during inter-access handovers

Evolution Across Releases

Defining Specifications