HRPD Serving Gateway

Description

The HRPD Serving Gateway (HSGW) is a critical functional entity defined within the 3GPP Evolved Packet Core (EPC) architecture to enable seamless mobility and service continuity between 3GPP LTE networks and 3GPP2 CDMA2000 High Rate Packet Data (HRPD) networks. This interworking solution is standardized under the term evolved High Rate Packet Data (eHRPD). The HSGW resides in the EPC and serves as the gateway between the eHRPD radio access network (RAN) and the core Packet Data Network Gateway (PGW). Its primary role is to emulate the behavior of a Serving Gateway (SGW) and, to some extent, a Mobility Management Entity (MME) from the perspective of the HRPD access, allowing it to connect to the common EPC anchors.

Architecturally, the HSGW interfaces with several network elements. On the access side, it connects to the HRPD Radio Network Controller (RNC) or eNodeB-equivalent via the S101 reference point for control signaling and the S103 reference point for user plane tunneling of downlink data during handovers. On the core network side, it connects to the PGW via the S2a reference point based on the Proxy Mobile IPv6 (PMIPv6) protocol, which is the standard interface for trusted non-3GPP accesses. The HSGW also interfaces with the 3GPP AAA server (via STa) for authentication, authorization, and accounting, and with the Home Subscriber Server (HSS) for retrieving subscriber profiles.

The HSGW performs several key functions. It acts as the local mobility anchor for the UE while it is attached via the eHRPD network, managing the mobility tunnel (GTP or PMIPv6) to the PGW. It participates in the authentication and session establishment procedures, interacting with the AAA infrastructure. During active handovers between LTE and eHRPD (optimized handovers), the HSGW plays a central role in the S101-based signaling, facilitating the transfer of the UE's context and buffering downlink data. It also enforces Quality of Service (QoS) policies received from the Policy and Charging Rules Function (PCRF) via the PGW for sessions established over eHRPD.

From a protocol perspective, the HSGW implements PMIPv6 as the mobility protocol towards the PGW, unlike the GTP protocol used between the SGW and PGW in native 3GPP accesses. This design choice aligns with the IETF-based mobility management for non-3GPP accesses within EPC. The HSGW also handles the translation between HRPD-specific signaling (e.g., in A11 interface messages) and the 3GPP EPC signaling procedures, making the HRPD network appear as a trusted non-3GPP access network to the EPC. This abstraction is crucial for providing a unified service experience across heterogeneous technologies.

Purpose & Motivation

The HSGW was created to solve a critical market and technological problem: enabling smooth migration from existing 3GPP2 CDMA2000 networks (primarily HRPD/EV-DO) to 3GPP LTE networks, particularly in regions like North America, Japan, and South Korea where CDMA operators held significant market share. Without such an interworking function, operators would face a 'forklift upgrade,' requiring a complete and simultaneous replacement of radio and core networks to deploy LTE, which is economically and operationally infeasible.

The primary problem the HSGW addresses is service continuity and seamless mobility. It allows an operator to deploy an LTE network gradually while leveraging their existing HRPD network for coverage, especially in initial LTE deployment phases where coverage is limited to hotspots or urban areas. A subscriber with a multi-mode device (LTE/HRPD) can move between the two technologies without dropping ongoing data sessions (like a VoIP call or video stream). This 'optimized handover' capability was essential for maintaining a high-quality user experience and making LTE devices attractive from day one.

Historically, prior to EPC and the HSGW, interworking between different technology families was complex and often involved circuit-switched fallback for voice and break-before-make data handovers. The EPC architecture, with its access-agnostic core defined in Release 8, provided the framework. The HSGW is the specific instantiation of the Trusted Non-3GPP Access Gateway function for eHRPD. It solved the limitations of previous loose-coupling interworking solutions by providing tight integration at the core network level, allowing for policy consistency, unified charging, and common service enablers across both accesses, which was a fundamental requirement for operators managing convergent networks.

Key Features

• Acts as the mobility anchor and gateway for UEs attached via eHRPD access
• Supports Proxy Mobile IPv6 (PMIPv6) protocol on the S2a interface towards the PGW
• Enables optimized handover (seamless mobility) between LTE and eHRPD networks using S101 interface
• Interworks with 3GPP AAA server for authentication and authorization of eHRPD users
• Enforces QoS policies for data sessions established over the eHRPD network
• Provides data path support for uplink and downlink traffic during eHRPD attachment

Evolution Across Releases

Defining Specifications