Globally Unique AMF Identifier

Description

The Globally Unique AMF Identifier (GUAMI) is a structured identifier defined in 3GPP for the 5G System (5GS) that provides a globally unambiguous way to identify a specific instance of an Access and Mobility Management Function (AMF). An AMF is a core network function responsible for termination of NAS signaling, registration management, connection management, reachability management, and mobility management for User Equipment (UE). The GUAMI is constructed from three components: the Public Land Mobile Network (PLMN) Identifier (MCC and MNC), the AMF Region ID, the AMF Set ID, and the AMF Pointer. This hierarchical structure allows for efficient routing and scalability within and across PLMNs.

Architecturally, the GUAMI is assigned to each AMF instance by the network operator and is configured within the AMF's operational parameters. It is advertised by the AMF to other network functions, such as the Radio Access Network (RAN) nodes (gNBs) via the N2 interface and to other core functions like the Network Slice Selection Function (NSSF) via service-based interfaces. When a UE initiates registration to the 5G network, the RAN node uses the GUAMI, often derived from the Temporary Mobile Subscriber Identity (5G-S-TMSI) or provided in the initial NAS message, to select an appropriate AMF for the UE's request. The GUAMI enables the RAN to route the initial UE message to the correct AMF instance, even in deployments with multiple AMFs for load balancing and redundancy.

Operationally, the GUAMI plays a key role in several procedures. During registration, if the UE provides a GUAMI from a previous registration (within the 5G-GUTI), the network can use it to efficiently retrieve the UE's context. In mobility scenarios like handovers, the source AMF includes its GUAMI in the handover request to the target, ensuring the target system can identify and communicate with the correct AMF. For paging, the RAN uses the GUAMI to determine which AMF to contact when the network needs to reach an idle UE. The hierarchical nature of the GUAMI (Region ID, Set ID, Pointer) allows operators to organize AMFs into logical groups (Sets) within geographical regions, facilitating flexible deployment, disaster recovery, and efficient routing. The GUAMI is a fundamental element that underpins the stateless and service-based architecture of 5GC, enabling dynamic discovery and selection of AMF instances.

Purpose & Motivation

GUAMI was created to address the scalability, flexibility, and routing challenges inherent in the 5G Core's service-based architecture, which decouples software instances from hardware and allows for dynamic scaling of network functions. In previous generations like 4G EPS, the MME was identified by an MME Code and Group ID within a PLMN, but this lacked a globally unique scope and the granularity needed for cloud-native, distributed deployments. The motivation for GUAMI stemmed from the need to uniquely identify potentially thousands of AMF instances deployed across multiple data centers and geographical regions, supporting network slicing, edge computing, and high-availability requirements.

Historically, identifiers in mobile networks evolved from simple codes to more structured forms to support increasing network complexity. The introduction of 5G and its requirement for network slicing—where a single physical network hosts multiple logical networks—necessitated an identifier that could not only uniquely point to an AMF instance but also convey logical grouping information for efficient slice-aware routing. GUAMI solves this by incorporating PLMN ID, Region ID, Set ID, and Pointer, allowing operators to organize AMFs in a way that reflects their deployment topology and slice associations.

The technology solves critical problems in 5G operation: it enables the RAN to perform efficient AMF selection without requiring a central directory for every UE request, reducing signaling latency. It supports stateless operation, where an AMF instance can fail and another can take over, as the GUAMI provides a consistent handle for locating UE context. Furthermore, it facilitates inter-PLMN mobility and roaming by ensuring AMFs are globally addressable. By providing a structured, globally unique identifier, GUAMI is foundational to achieving the 5G vision of a flexible, scalable, and resilient core network that can support diverse services from massive IoT to ultra-reliable low-latency communications.