Packet Switched Handover

Description

Packet Switched (PS) handover is a fundamental mobility procedure in 3GPP GERAN (GSM/EDGE Radio Access Network) and GAN (Generic Access Network) that allows a mobile station (MS) to transition from a source cell to a target cell while actively engaged in packet data transfer. Unlike circuit-switched handover, which handles voice calls, PS handover is designed for packet-switched services such as web browsing, email, and streaming. The procedure is network-controlled: the network decides when a handover is necessary based on measurement reports and resource availability, then commands the MS to move.

The core mechanism relies on the PS HANDOVER COMMAND message, which contains the target cell's radio resource configuration, including Temporary Block Flow (TBF) allocations. A TBF is a unidirectional physical connection used for packet data transfer in GERAN. When the MS receives this command, it releases its current TBFs in the source cell and establishes new ones in the target cell using the provided parameters. This ensures that ongoing packet services continue without interruption, maintaining session continuity and minimizing data loss.

For handover to a GAN cell (e.g., via Wi-Fi), the procedure differs slightly: the MS receives the assignment of target cell radio resources before the PS HANDOVER COMMAND message. This pre-assignment allows the MS to prepare the new radio link in advance, reducing handover latency. GAN, defined in 3GPP TS 43.318, extends GSM/GPRS services over IP-based access networks, and PS handover to GAN enables seamless mobility between cellular and Wi-Fi access.

PS handover is specified in 3GPP TS 43.318 (GERAN overall description) and TS 44.060 (RLC/MAC protocol). It supports both intra-SGSN and inter-SGSN handovers, as well as handovers between GERAN and other access technologies like UTRAN and E-UTRAN, though inter-RAT handover involves additional core network signaling. The feature was introduced in Release 8 and has been maintained through Release 19, reflecting its continued importance in legacy networks.

Key components involved include the MS, the source and target Base Station Subsystems (BSS), the Serving GPRS Support Node (SGSN), and optionally the Gateway GPRS Support Node (GGSN). The handover decision is typically made by the source BSS based on measurement reports from the MS. The BSS then coordinates with the target BSS and the core network to reserve resources and initiate the handover. The PS HANDOVER COMMAND is sent via the source cell's downlink, and the MS acknowledges the handover on the target cell.

PS handover is essential for maintaining quality of service (QoS) during mobility, as it avoids the need to re-establish packet data sessions from scratch. It also supports load balancing and network optimization by allowing the network to steer traffic to less congested cells. Despite the evolution towards LTE and 5G, PS handover remains relevant for GSM/EDGE networks, especially in regions where 2G/3G still provide coverage or fallback.

Purpose & Motivation

PS handover was developed to address the limitations of earlier mobility mechanisms in packet-switched networks. Before its introduction, when a mobile station moved between cells during an active data session, the session would often be interrupted, requiring re-establishment and causing delays, data loss, and degraded user experience. Circuit-switched handover existed for voice, but packet data required a different approach due to its bursty, connectionless nature and the need to manage Temporary Block Flows (TBFs).

The primary motivation was to enable seamless continuity of packet-switched services, such as web browsing, email, and streaming, as users moved across cells. This was particularly important for GPRS and EDGE networks, which were the primary data bearers before the advent of UMTS and LTE. PS handover also facilitated interworking with other access technologies, including GAN, allowing operators to offload traffic to Wi-Fi while maintaining service continuity.

Furthermore, PS handover supports network efficiency by allowing the network to manage radio resources dynamically. It enables load balancing across cells and ensures that users are served by the most appropriate cell, improving overall network capacity and performance. The feature was standardized in Release 8 as part of the ongoing evolution of GERAN to support higher data rates and better mobility, and it has been maintained in subsequent releases to ensure backward compatibility and continued operation in legacy networks.