Description
The HRPD Serving Gateway (HS-GW) is a functional entity defined in 3GPP specifications for non-3GPP access interworking, specifically for connectivity with 3GPP2 CDMA2000 1xEV-DO (Evolution-Data Optimized), also known as High Rate Packet Data (HRPD) networks. It is a critical component in the Evolved Packet Core (EPC) architecture, enabling seamless mobility and service continuity for User Equipment (UE) moving between LTE/UMTS/GSM and CDMA HRPD networks. The HS-GW resides in the visited or home PLMN and interfaces with the HRPD Access Network (AN) on one side and the 3GPP EPC on the other. Its primary roles are to serve as a local mobility anchor point for the UE during handovers between the two access types and to manage the user plane tunneling for data traffic.
From an architectural perspective, the HS-GW implements functionalities analogous to both the Serving Gateway (SGW) and the Packet Data Network Gateway (PGW) defined for 3GPP accesses, but tailored for the HRPD interface. It terminates the HRPD-specific Generic Routing Encapsulation (GRE) tunnels from the HRPD AN. For the 3GPP EPC side, it establishes GTP-based S5/S8 tunnels towards a PGW (if it is not integrated with PGW functionality) or acts as the PGW itself (if configured as an HSGW/PGW combo). The HS-GW also interacts with the 3GPP AAA server for authentication, authorization, and accounting procedures for the UE accessing via HRPD. It allocates the UE's IP address or delegates this function to the PGW.
During an inter-RAT handover from E-UTRAN to HRPD (or vice versa), the HS-GW is a pivotal node. For a handover from LTE to HRPD (optimized handover), the target HS-GW is prepared via signaling between the MME (Mobility Management Entity) and the HS-GW over the S101 reference point. The HS-GW establishes the necessary resources with the HRPD AN. When the UE attaches via HRPD in idle mode, the HS-GW performs the initial attachment procedures, contacting the 3GPP AAA and HSS to authenticate the subscriber and retrieve their profile. It then establishes a PDN connection to a PGW, creating an end-to-end bearer for the UE's data traffic. The HS-GW also performs policy enforcement, applying QoS policies and charging rules received from the Policy and Charging Rules Function (PCRF) via the Gxa interface.
Purpose & Motivation
The HS-GW was introduced in 3GPP Release 8 alongside the initial LTE/EPC specifications to address a critical real-world deployment requirement: seamless interworking between the new 3GPP LTE networks and widely deployed legacy 3GPP2 CDMA2000 networks, particularly in markets like North America, Japan, and South Korea where CDMA was predominant. Operators deploying LTE needed a standardized, efficient path to leverage their existing CDMA HRPD networks for coverage and fallback, especially in the early stages of LTE rollout when coverage was sparse. Without a gateway like the HS-GW, mobility between these technologies would be limited to simple, break-before-make redirection, causing significant service interruption.
The HS-GW solves the problem of heterogeneous network mobility by providing a unified anchor point within the EPC for both 3GPP and HRPD accesses. It abstracts the specifics of the HRPD access network, presenting it to the core as another IP-based access technology. This enables key EPC features like seamless handover (via optimized handover procedures), consistent policy enforcement, and unified charging to work across the technological boundary. Its creation was motivated by the need for a non-3GPP interworking architecture that was more tightly integrated than generic I-WLAN access, offering lower latency and better performance for handovers, which was essential for voice and real-time services as networks evolved towards VoLTE and SRVCC.
Key Features
- Mobility anchor point for handovers between 3GPP (LTE/UMTS) and 3GPP2 HRPD access
- Termination point for HRPD-specific GRE tunnels from the HRPD Access Network
- Interfaces with 3GPP AAA server for subscriber authentication over HRPD
- Establishes GTP tunnels (S5/S8) towards a PGW or integrates PGW functionality
- Supports optimized handover procedures via the S101 (signaling) and S103 (data forwarding) interfaces
- Enforces QoS and charging policies received from the PCRF via the Gxa interface
Evolution Across Releases
Initial introduction of the HS-GW as part of the EPC architecture for interworking with CDMA2000 HRPD networks. It defined the basic functions: serving as a mobility anchor, terminating GRE tunnels from HRPD AN, interfacing with 3GPP AAA, and connecting to PGW via S2a (PMIPv6) or S2b (PMIPv6/GTP). The S101 (MME-HSGW for signaling) and S103 (SGW-HSGW for data forwarding during handover) interfaces were defined for optimized handover.
The role of the HS-GW became stable as LTE coverage expanded, reducing the reliance on HRPD interworking. Subsequent releases focused on maintenance, optimizations for power saving, and ensuring compatibility with new EPC features like Dual Connectivity and network slicing, though its core architecture remained largely unchanged.
Defining Specifications
| Specification | Title |
|---|---|
| TS 33.402 | 3GPP TR 33.402 |