Editing My 3GPP TS 23.003 notes (section)

=== 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.

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 AMFI shall be 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.
:NOTE:	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 shall be constructed from the GUAMI and the 5G-TMSI.

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.

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 shall be 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).

The format and size of the 5G-GUTI is therefore the following:

<code><5G-GUTI> = <GUAMI><5G-TMSI></code>, where <code><GUAMI> = <MCC><MNC><AMF Identifier></code> and <code><AMF Identifier> = <AMF Region ID><AMF Set ID><AMF Pointer></code>

MCC and MNC shall have the same field size as in earlier 3GPP systems. See 3GPP TS 24.008 § 10.5.1.3 for details of MCC (12bits) and MNC (12 bits) size and formatting.

5G-TMSI shall be of 32 bits length.

AMF Region ID shall be of 8 bits length.

AMF Set ID shall be of 10 bits length.

AMF Pointer shall be of 6 bits length.

Diagram:

[[File:5G-GUTI struct.png|5G-GUTI struct]]