Terminal Mobility

Description

Terminal mobility refers to the capability of a terminal (user equipment) to access telecommunication services while moving or when used at different locations, and the network's ability to identify and locate that terminal. This concept is fundamental to mobile networks, ensuring that users can maintain ongoing sessions (e.g., voice calls, data transfers) as they move within the coverage area or between different access technologies. It encompasses both intra-system mobility (e.g., handovers within the same radio access technology) and inter-system mobility (e.g., handover between 3GPP and non-3GPP access).

In 3GPP architecture, terminal mobility is managed through a combination of radio access network (RAN) and core network (CN) procedures. The RAN handles radio-level mobility, such as handovers between cells (intra-RAT) or between different RATs (inter-RAT), based on signal strength measurements and network policies. The CN manages higher-level mobility, including tracking area updates, paging, and session continuity. Key entities involved include the Mobility Management Entity (MME) in EPC, the Access and Mobility Management Function (AMF) in 5GC, and the various gateways (SGW, PGW, UPF) that anchor user plane traffic.

Terminal mobility is achieved through several mechanisms. In idle mode, the terminal performs cell reselection and informs the network of its location via tracking area updates (TAU) or registration area updates. In connected mode, the network controls handovers based on measurement reports from the terminal. For seamless handover, the network may use techniques like make-before-break or dual connectivity to minimize interruption. Mobility across different access systems (e.g., from LTE to 5G NR) is supported via inter-RAT handover procedures, which may involve network interfaces like N26 (between AMF and MME) for context transfer.

The concept also includes the network's ability to locate the terminal for incoming services (e.g., mobile-terminated calls). This is done via paging, where the network broadcasts a paging message in the tracking area where the terminal is registered. The terminal responds, allowing the network to establish a connection. For location-based services, more precise positioning methods (e.g., OTDOA, A-GNSS) may be used.

Terminal mobility is distinct from session mobility (e.g., IP flow mobility) and service mobility (e.g., continuity of a service across devices). However, these concepts are interrelated, and 3GPP has developed frameworks like Access Traffic Steering, Switching, and Splitting (ATSSS) to manage mobility across multiple access networks in 5G. The evolution of terminal mobility has moved from simple cell reselection in 2G to complex, multi-RAT, multi-connectivity scenarios in 5G and beyond, supporting diverse use cases like high-speed trains, vehicular communication, and seamless IoT roaming.

Purpose & Motivation

Terminal mobility exists to enable users to stay connected while on the move, which is the core value proposition of mobile communication. Without it, users would have to manually re-establish connections when changing location or access technology, leading to service interruptions and poor user experience. The primary problem it solves is maintaining session continuity and reachability as the terminal's point of attachment changes.

Historically, early mobile systems (1G) provided limited mobility within a single cell or required manual handover. 2G introduced automatic handovers between cells and roaming between networks, but data services were limited. 3G and later generations expanded mobility to support high-speed data and seamless inter-RAT handovers. The introduction of IP-based core networks (EPC) in LTE brought always-on connectivity and flattened architecture, improving mobility performance. 5G further enhanced mobility with beam-based access, dual connectivity, and support for high-speed scenarios (up to 500 km/h).

The definition of terminal mobility evolved from focusing on handovers within a single system (Rel-7) to a broader concept encompassing any scenario where the same terminal equipment moves or is used at different locations (Rel-12). This change reflected the growing importance of multi-access and heterogeneous networks, where users may switch between Wi-Fi, 4G, and 5G seamlessly. It also aligned with the need to support diverse devices, including IoT, which may be stationary but still require location management. Thus, terminal mobility remains a cornerstone of 3GPP systems, continuously adapting to new technologies and use cases.