Session mobility

Description

Session mobility refers to the capability to transfer an ongoing communication session from one device to another, under the control of the user. This allows a user to start a session on one device (e.g., a smartphone) and continue it on another (e.g., a tablet or laptop) without interruption. The session state, including media streams and signalling context, is transferred between devices, ensuring continuity.

In 3GPP, session mobility is defined in the context of IP Multimedia Subsystem (IMS) and is specified in TS 22.258 and TS 22.978. It is a key feature for enabling a seamless user experience across multiple devices. The transfer can be initiated by the user, for example, by selecting a 'transfer' option on the device. The network then coordinates the transfer of the session, which may involve re-negotiating media parameters and updating the session signalling.

Session mobility is distinct from terminal mobility, which refers to the ability of a device to maintain a session while moving geographically. Session mobility focuses on moving the session between different devices, which may be in different locations. It requires the network to support the transfer of session context, including authentication, authorization, and QoS parameters.

The implementation typically involves the use of SIP signalling to re-INVITE the session to the new device. The new device must be registered and authorized to receive the session. The network may also need to ensure that the old device releases the session resources. This process can be transparent to the remote party, who may not be aware of the device change.

Session mobility is an important enabler for multi-device scenarios, such as when a user wants to switch from a mobile phone to a video conferencing system. It enhances user convenience and allows for better utilization of device capabilities. The concept has been stable since its introduction in Rel-7, indicating its continued relevance.

Purpose & Motivation

Session mobility was introduced to address the growing need for users to seamlessly continue their communication sessions across multiple devices. In the past, a session was tied to a single device, and switching devices meant terminating and restarting the session, which was disruptive. Session mobility solves this by allowing the session to be transferred, preserving the user's context and reducing interruption.

The motivation came from the proliferation of personal devices and the desire for a unified communication experience. Users wanted to start a call on their phone and move it to a conference room system, or continue a video stream on a larger screen. This required a standardized mechanism to transfer session control and media, which 3GPP addressed in Rel-7 with the definition of session mobility.

The feature also enables new services, such as follow-me services, where the session follows the user across devices. It is a key differentiator for operators offering multi-device services, and it aligns with the trend towards ubiquitous connectivity and the Internet of Things (IoT).