Handover out (GAN to 3GPP)

Description

Handover out is a mobility procedure defined in 3GPP specifications 43.318 and 43.901. It enables a mobile station (MS) to transition an ongoing session from a Generic Access Network (GAN) — typically a Wi-Fi or unlicensed wireless network — to a 3GPP access network such as GERAN, UTRAN, or E-UTRAN. The procedure is part of the Generic Access Network (GAN) architecture, which allows cellular services to be delivered over generic IP access networks. Handover out is the counterpart to handover in, which moves a session from a 3GPP network to GAN.

The GAN architecture introduces a Generic Access Network Controller (GANC) that connects to the core network via standard 3GPP interfaces (e.g., A, Gb, Iu). The MS communicates with the GANC over an Up interface, which is an IP-based tunnel. When the MS detects that the GAN signal is weakening and a 3GPP cell is available, it initiates a handover out. The decision can be based on radio conditions, user preferences, or operator policies. The handover out procedure involves coordination between the MS, the GANC, and the target 3GPP access network. The GANC acts as an anchor point and forwards the session context to the target network.

The handover out procedure typically includes several phases: measurement reporting, handover initiation, resource reservation in the target network, and execution of the handover. The MS sends a handover request to the GANC, which then communicates with the target 3GPP network to prepare resources. Once the target network is ready, the GANC instructs the MS to switch to the 3GPP access. The MS then performs the necessary procedures to attach to the 3GPP network and continue the session. The exact signaling depends on the target access technology (e.g., GERAN, UTRAN, E-UTRAN) and the core network interfaces used.

Handover out is critical for maintaining seamless service continuity as users move between different access technologies. It allows operators to offload traffic to unlicensed networks when available and fall back to licensed spectrum when needed, improving coverage and capacity. The procedure must meet stringent requirements for latency and packet loss to avoid degrading voice or data sessions. 3GPP specifications define the detailed procedures, message flows, and timers to ensure interoperability between different vendors' equipment.

In later releases, the concept of handover out has evolved with the introduction of new access technologies and architectures, such as LTE-WLAN Aggregation (LWA) and 5G access traffic steering, switching, and splitting (ATSSS). However, the fundamental principle remains: enabling a mobile station to move from a non-3GPP access to a 3GPP access while maintaining session continuity. The term is primarily used in the context of GAN, but similar procedures exist in other interworking scenarios.

Purpose & Motivation

Handover out exists to provide seamless mobility between Generic Access Networks (GAN) and 3GPP access networks. Before its introduction, users moving from a Wi-Fi network to a cellular network would experience session drops or interruptions, as there was no standardized way to transfer an ongoing session. This was particularly problematic for voice calls, which require continuous connectivity. The handover out procedure solves this by defining a standardized mechanism for transferring session context and resources from the GAN to the 3GPP network, ensuring that the user experience remains uninterrupted.

The motivation for handover out stems from the growing demand for ubiquitous coverage and the need to leverage unlicensed spectrum for offloading. Operators wanted to extend coverage and capacity by allowing users to connect via Wi-Fi when available, but they also needed a reliable fallback to cellular when the user moved out of Wi-Fi coverage. Handover out provides that fallback, enabling operators to offer a seamless service across both access types. It also allows for better load balancing and resource utilization across networks.

Historically, early attempts at interworking between Wi-Fi and cellular were proprietary and lacked standardization, leading to interoperability issues and poor user experience. 3GPP introduced GAN in Release 6, and handover out was defined as part of that architecture. Over subsequent releases, the procedure has been refined and extended to support new access technologies and core network architectures. The concept has influenced later developments such as LTE-WLAN interworking and 5G ATSSS, which build on the principles established by handover out.