4G-Globally Unique Temporary Identifier

Description

The 4G-GUTI is a critical identifier in 5G System (5GS) architecture that serves UEs that have performed inter-Radio Access Technology (inter-RAT) mobility from 4G Evolved Packet System (EPS) to 5GS. It is constructed by the Access and Mobility Management Function (AMF) when a UE registers with the 5G core network (5GC) using 4G credentials and procedures, specifically during an Attach procedure with EPS to 5GS mobility. The identifier consists of two main components: the Globally Unique AMF Identifier (GUAMI) and the 5G-Temporary Mobile Subscriber Identity (5G-TMSI). The GUAMI portion identifies the specific AMF that allocated the 4G-GUTI, while the 5G-TMSI is a unique identifier within that AMF's context.

When a UE initially accesses the 5GS via E-UTRAN (4G radio), it provides its 4G-based credentials, typically including the 4G GUTI from the EPS. The AMF then allocates a 4G-GUTI to this UE, which serves as its temporary identity within the 5GS. This allocation occurs during the registration procedure and is communicated to the UE in the Registration Accept message. The 4G-GUTI is stored in the UE's non-volatile memory alongside its 5G-GUTI, allowing the UE to use either identifier depending on the access network and registration context.

The primary technical function of the 4G-GUTI is to enable the AMF to uniquely identify UEs that have performed EPS to 5GS mobility without requiring them to obtain a native 5G-GUTI. When such a UE initiates communication in 5GS, it includes the 4G-GUTI in the initial NAS message, allowing the AMF to retrieve the UE's context from its database. The AMF maintains a mapping between the 4G-GUTI and the UE's full subscription data, including security context and mobility parameters. This mechanism is particularly important during handovers between 4G and 5G networks, as it allows for seamless continuity of service without requiring re-authentication or complete re-registration.

From a security perspective, the 4G-GUTI serves the same fundamental purpose as other temporary identifiers in mobile networks: it protects the user's permanent subscription identifier (SUPI) from being transmitted over the air interface, thereby enhancing privacy and security. The 4G-GUTI can be reallocated periodically or during specific events (such as inter-AMF handovers) to further enhance security through identifier freshness. The AMF manages the lifecycle of 4G-GUTIs, including allocation, reallocation, and invalidation, ensuring that each identifier remains unique within its operational context and time window.

Purpose & Motivation

The 4G-GUTI was created to address the specific requirements of inter-RAT mobility between 4G EPS and 5GS, particularly during the transition period when many UEs would initially access 5G networks using 4G-based credentials and procedures. As 5G networks were deployed, there was a need to support UEs that could connect to 5GC via both 4G radio (E-UTRAN) and 5G radio (NG-RAN), requiring a consistent identification mechanism that worked across both access types. The 4G-GUTI enables this by providing a temporary identifier that bridges the 4G and 5G identity systems.

Prior to 5G, temporary identifiers were specific to each generation (e.g., 4G GUTI for EPS, 3G P-TMSI for UMTS). This approach created challenges for inter-generation mobility, as UEs moving between systems often needed to obtain new temporary identifiers through complete registration procedures. The 4G-GUTI solves this by allowing UEs that registered via 4G procedures to maintain a consistent temporary identity when operating in 5GS, reducing signaling overhead and improving mobility performance. This was particularly important for early 5G deployments where many UEs supported 5G core network capabilities but accessed via 4G radio infrastructure.

The creation of 4G-GUTI also addressed privacy and security concerns specific to inter-RAT scenarios. By allowing UEs to use a temporary identifier derived from their 4G registration context, the system prevents the need to transmit 5G-specific permanent identifiers over 4G radio links, maintaining the privacy protections established in both systems. This approach also enables more efficient handovers between 4G and 5G, as the network can quickly identify and authenticate UEs based on their 4G-GUTI without requiring full 5G authentication procedures every time they move between the systems.