Access and Mobility Management

Description

Access and Mobility Management (AM) is a fundamental functional entity within the 3GPP network architecture, specifically part of the Access and Mobility Management Function (AMF) in 5G systems. It serves as the primary termination point for Non-Access Stratum (NAS) signaling from the User Equipment (UE), handling all procedures related to initial registration, connection management, and mobility. The AM function is responsible for establishing and maintaining the UE's registration state with the network, which is the foundation for all subsequent communication services.

Architecturally, AM operates within the control plane of the core network, interacting with other network functions through service-based interfaces. In 5G, the AMF (which hosts the AM functionality) communicates with the Session Management Function (SMF) for PDU session establishment, the Authentication Server Function (AUSF) for security procedures, and the Unified Data Management (UDM) for subscriber data. It also connects to the Radio Access Network (RAN) via the N2 interface, receiving initial access requests and managing handover signaling. The AM function maintains the UE context, which includes the UE's registration status, security context, location information, and subscription data.

How AM works involves several key procedures. During initial access, the UE sends a Registration Request to the AMF via the RAN. The AM function authenticates the UE using credentials stored in the UDM and establishes a security context. It then authorizes the UE's access based on subscription profiles and network policies. For mobility management, AM tracks the UE's location at the granularity of a Registration Area (a group of tracking areas) and manages procedures like Registration Area Updates and handovers between different AMF serving areas. The AM function also handles connection management, transitioning the UE between CM-IDLE and CM-CONNECTED states, which controls whether the UE has a signaling connection with the AMF.

The role of AM in the network is critical for several reasons. First, it ensures that only authorized devices can access network resources, providing the first line of security. Second, it enables seamless mobility by managing location tracking and handovers without interrupting active sessions. Third, it optimizes network resource usage by controlling when UEs establish signaling connections. Finally, AM provides the foundation for other network services by maintaining the UE's registration state and context, which is essential for session management, policy enforcement, and charging.

Purpose & Motivation

The Access and Mobility Management function exists to provide a centralized, efficient mechanism for controlling how user devices connect to and move through mobile networks. Before the formalization of AM in 3GPP standards, access control and mobility management were often handled through proprietary or less integrated solutions that led to interoperability issues, inefficient handovers, and security vulnerabilities. The creation of AM addressed the need for a standardized approach that could scale with growing subscriber numbers and support increasingly complex mobility scenarios.

Historically, as mobile networks evolved from 2G to 3G and beyond, the challenges of managing user mobility became more pronounced. Early systems had limited handover capabilities and basic authentication mechanisms. The introduction of AM in 3GPP Release 4 provided a comprehensive framework that separated access and mobility management from session management, allowing for more flexible network architectures. This separation became particularly important with the transition to all-IP networks and the need to support multiple access technologies (like 2G, 3G, LTE, and non-3GPP access).

The AM function solves several key problems: it provides a single point of control for UE registration and authentication across different access types; it enables efficient tracking of UE location without excessive signaling overhead; it supports seamless mobility between cells and between different radio access technologies; and it provides the security foundation for all subsequent network interactions. By addressing these problems, AM enables the reliable, secure, and efficient operation of modern mobile networks that support billions of connected devices with diverse mobility patterns.