The AMF shall allocate a 5G Globally Unique Temporary Identifier (5G-GUTI) to the UE that is common to both 3GPP and non-3GPP access. It is possible to use the same 5G-GUTI for accessing 3GPP access and non-3GPP access security context within the AMF for the given UE.
An AMF may re-assign a new 5G-GUTI to the UE at any time. The AMF provides a new 5G-GUTI to the UE under the specific conditions.
The 5G-GUTI has two main components:
– one that identifies the AMF(s) which allocated the 5G-GUTI; and
– one that uniquely identifies the UE within the AMF(s) that allocated the 5G-GUTI.
Within the AMF(s), the mobile is identified by the 5G-TMSI. The Globally Unique AMF Identifier (GUAMI) is constructed from the MCC, MNC and AMF Identifier (AMFI).
The AMFI is constructed from an AMF Region ID, an AMF Set ID and an AMF Pointer. The AMF Region ID identifies the region, the AMF Set ID uniquely identifies the AMF Set within the AMF Region, and the AMF Pointer identifies one or more AMFs within the AMF Set.
When the UE is assigned a 5G-GUTI with an AMF Pointer value used by more than one AMF, the AMFs need to ensure that the 5G-TMSI value used within the assigned 5G-GUTI is not already in use within the AMF’s sharing that pointer value.
The 5G-GUTI is constructed from the GUAMI and the 5G-TMSI. For paging purposes, the mobile is paged with the 5G-S-TMSI. The 5G-S-TMSI is constructed from the AMF Set ID, the AMF Pointer and the 5G-TMSI.
The operator shall need to ensure that the combination of the AMF Set ID and AMF Pointer is unique within the AMF Region and, if overlapping AMF Regions are in use, unique within the area of overlapping AMF Regions.
The 5G-GUTI is used to support subscriber identity confidentiality, and, in the shortened 5G-S-TMSI form, to enable more efficient radio signalling procedures (e.g. paging and Service Request).
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