3GPP 23.501 v20.2.0 — the document's own text

5.9.4 5G Globally Unique Temporary Identifier

Taught in 7. Every name the system uses (The 5G system architecture, in depth), 4. Knowing who you are and where you are (The 5G system architecture, overview).

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 shall be 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 conditions specified in clause 6.12.3 of TS 33.501 [29]. When the UE is in CM-IDLE, the AMF may delay providing the UE with a new 5G-GUTI until the next NAS transaction.

The 5G-GUTI shall be structured as:

<5G-GUTI> := <GUAMI> <5G-TMSI>

where GUAMI identifies one or more AMF(s).

When the GUAMI identifies only one AMF, the 5G-TMSI identifies the UE uniquely within the AMF. However, when AMF assigns a 5G-GUTI to the UE with a GUAMI value used by more than one AMF, the AMF shall ensure that the 5G-TMSI value used within the assigned 5G-GUTI is not already in use by the other AMF(s) sharing that GUAMI value.

The Globally Unique AMF ID (GUAMI) shall be structured as:

where AMF Region ID identifies the region, AMF Set ID uniquely identifies the AMF Set within the AMF Region and AMF Pointer identifies one or more AMFs within the AMF Set.

NOTE 1: The AMF Region ID addresses the case that there are more AMFs in the network than the number of AMFs that can be supported by AMF Set ID and AMF Pointer by enabling operators to re-use the same AMF Set IDs and AMF Pointers in different regions.

NOTE 2: In the case of SNPNs, the PLMN IDs may be shared among SNPNs such that the constructed GUAMIs are not globally unique. However, PLMN ID and NID are provided together, separate from the GUAMI, to uniquely identify selected or supported SNPN in RRC and N2.

NOTE 3: See TS 23.003 [19] for details on the structure of the fields of GUAMI.

The 5G-S-TMSI is the shortened form of the GUTI to enable more efficient radio signalling procedures (e.g. during Paging and Service Request) and is defined as:

<5G-S-TMSI> := <AMF Set ID> <AMF Pointer> <5G-TMSI>

As specified in TS 38.304 [50] and TS 36.304 [52] for 3GPP access, the NG-RAN uses the 10 Least Significant Bits of the 5G-TMSI in the determination of the time at which different UEs are paged. Hence, the AMF shall ensure that the 10 Least Significant Bits of the 5G-TMSI are evenly distributed.

As specified in TS 38.331 [28] and TS 36.331 [51] for 3GPP access, the NG-RAN's RRC Connection Establishment's contention resolution process assumes that there is a low probability of the same 5G-TMSI being allocated by different AMFs to different UEs. The AMFs' process for allocating the 5G-TMSI should take this account.

NOTE 4: To achieve this, the AMF could, for example, use a random seed number for any process it uses when choosing the UE's 5G-TMSI.