My 3GPP TS 23.003 notes
5G Globally Unique Temporary UE Identity (5G-GUTI) § 2.10
Introduction § 2.10.1
The purpose of the 5G-GUTI is to provide an unambiguous identification of the UE that does not reveal the UE or the user's permanent identity in the 5G System (5GS). It also allows the identification of the Access and Mobility Management Function (AMF) and network. It can be used by the network and the UE to establish the UE's identity during signalling between them in the 5GS. See 3GPP TS 23.501 [119].
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 shall be identified by the 5G-TMSI.
The Globally Unique AMF Identifier (GUAMI) shall be constructed from the MCC, MNC and AMF Identifier (AMFI).
The format and size of the 5G-GUTI is therefore the following:
<5G-GUTI> = <GUAMI><5G-TMSI>, where <GUAMI> = <MCC><MNC><AMF Identifier> and <AMF Identifier> = <AMF Region ID><AMF Set ID><AMF Pointer>
Structure of the 5G-S-Temporary Mobile Subscriber Identity (5G-S-TMSI) § 2.11
The 5G-S-TMSI is the shortened form of the 5G-GUTI to enable more efficient radio signalling procedures (e.g. paging and Service Request). For paging purposes, the mobile is paged with the 5G-S-TMSI. The 5G-S-TMSI shall be constructed from the AMF Set ID, the AMF Pointer and the 5G-TMSI:
<5G-S-TMSI> = <AMF Set ID><AMF Pointer><5G-TMSI>