Handover Path Switching

Description

Handover Path Switching (HPS) is a fundamental process within the 3GPP mobility management framework, responsible for re-establishing the user plane data path when a User Equipment (UE) transitions from one cell or Radio Access Technology (RAT) to another. Architecturally, it involves coordination between the Radio Access Network (RAN), the core network control plane (e.g., MME in 4G, AMF in 5G), and the user plane gateways (e.g., S-GW/P-GW in 4G, UPF in 5G). The procedure works by the source network node initiating a handover request, triggering the target node and core network to set up new bearer contexts or PDU sessions and modify the existing ones to route data through the new path.

Key components involved include the source and target base stations (eNB/gNB), the mobility management entity, and the serving and packet data network gateways. The process typically involves several steps: handover preparation (resource allocation in the target), execution (the UE connects to the target), and completion (path switch and release of old resources). A critical action is the 'Path Switch Request' sent by the target RAN node to the core network, which instructs the user plane gateways to switch the downlink data path from the source to the target node. This updates the GTP tunnels' endpoints, ensuring data packets are delivered to the UE's new location without session interruption.

HPS's role is to minimize packet loss and latency during mobility events. It supports various handover types, including intra-RAT, inter-RAT, and inter-system handovers. The procedure is tightly integrated with other mobility mechanisms like Tracking Area Updates and is optimized in later releases for features like dual connectivity, where multiple data paths can be maintained simultaneously. Efficient HPS is crucial for the quality of experience in real-time services like voice and video, as it directly impacts handover interruption time.

Purpose & Motivation

HPS was developed to solve the fundamental challenge of maintaining active data sessions during user mobility in cellular networks. Early cellular systems had simpler, break-before-make handovers that caused noticeable service interruption. As networks evolved to support packet-switched data and real-time IP services like VoIP, the requirement for seamless, make-before-break handovers became critical. HPS provides the standardized core network procedure to reroute data flows efficiently, which is the backbone of seamless mobility.

Its creation was motivated by the need for a unified, access-agnostic method to manage the user plane path during mobility events across increasingly heterogeneous networks (2G/3G/4G/5G, non-3GPP). Prior to its formalization, path management could be vendor-specific and less efficient. HPS standardizes the signaling between RAN and core network to coordinate the switch, ensuring interoperability and optimizing network resource usage by promptly releasing the old path. It addresses the problems of session continuity, reduced data loss, and efficient utilization of backhaul and core network resources during the critical handover phase, which is essential for supporting mobile broadband and low-latency services.