End-user mobility

Description

End-user mobility is a concept in 3GPP that defines the capability for a subscriber to use any device of their choice to access communication services. Unlike terminal mobility, which focuses on the movement of a device across network coverage areas, end-user mobility emphasizes the user's freedom to select and switch between devices while maintaining service continuity and a consistent user experience. This concept is particularly relevant in the context of IMS (IP Multimedia Subsystem) and fixed-mobile convergence, where a user may have multiple devices (e.g., smartphone, tablet, laptop) and expects to access the same services, contacts, and preferences seamlessly.

The technical realization of end-user mobility involves several network functions and procedures. In IMS, the user is identified by IMPI (IP Multimedia Private Identity) and IMPU (IP Multimedia Public Identity). The IMPU can be shared across multiple devices, allowing the user to register multiple contact addresses (e.g., IP addresses of different devices) under the same public identity. When a session is established to the IMPU, the network can fork the request to all registered devices, enabling the user to answer on any device. This is known as forking and is a key enabler of end-user mobility. Additionally, the user's service profile and preferences are stored in the HSS (Home Subscriber Server) and can be accessed from any device, ensuring a consistent experience.

End-user mobility also encompasses the ability to transfer an ongoing session from one device to another, a feature known as session transfer or session continuity. This is achieved through mechanisms like IMS Service Continuity (ISC) and Single Radio Voice Call Continuity (SRVCC), which allow a voice call to be moved between devices or between access networks (e.g., from LTE to Wi-Fi) without interruption. In 5G, end-user mobility is further enhanced with the concept of User Equipment (UE) differentiation and the support for multiple simultaneous connections, enabling seamless switching between devices and networks.

From a network architecture perspective, end-user mobility requires coordination between the access network, core network, and service layer. The access network must support multiple devices and provide sufficient bandwidth and low latency. The core network must manage user identities, authentication, and session control across devices. The service layer must handle service invocation, presence, and context sharing. Standards such as 3GPP TS 22.258 and TS 22.978 define the requirements and architecture for end-user mobility, ensuring interoperability across vendors and operators.

The importance of end-user mobility has grown with the proliferation of personal devices and the demand for ubiquitous connectivity. Users expect to start a conversation on their phone and continue it on their tablet or laptop without interruption. They also expect their services and preferences to follow them across devices. End-user mobility addresses these expectations by decoupling the user from the device, making the user the central entity in service delivery. This shift from device-centric to user-centric networking is a fundamental trend in modern telecommunications.

Purpose & Motivation

End-user mobility was created to address the limitations of traditional device-centric mobility. In early mobile networks, services were tied to a specific device and SIM card. If a user wanted to use a different device, they had to transfer the SIM card or use a different subscription. This was inconvenient and did not support the growing trend of multi-device ownership. End-user mobility solves this problem by allowing the user to access services from any device of their choice, using the same identity and subscription.

The motivation for end-user mobility also comes from fixed-mobile convergence. As users increasingly use both fixed and mobile networks, they expect a seamless experience across both. End-user mobility enables this by providing a unified service layer that abstracts the underlying access technology. For example, a user can start a video call on their smartphone over Wi-Fi and continue it on their smart TV over a fixed connection. This flexibility is essential for operators to compete with over-the-top (OTT) service providers who already offer multi-device support.

Furthermore, end-user mobility supports regulatory requirements such as emergency services and lawful interception. By maintaining a clear association between the user and their devices, the network can ensure that emergency calls are routed correctly and that lawful interception can be performed across all devices. The concept has evolved from Rel-7 to Rel-19, with each release adding new capabilities and refinements, but the core idea remains the same: putting the user, not the device, at the center of the communication experience.