The purpose of the GUTI is to provide an unambiguous identification of UE that doesn't reveal the UE or the user's permanent identity in the EPS. It also allows the identification of MME and network. It can be used by the network and the UE to establish the UE's identity during the signaling between them in the EPS.
The GUTI has two main components
- one that uniquely identifies the MME which allocated the GUTI; and
- one that uniquely identifies the UE within the MME that allocated GUTI.
within the MME, the mobile shall be identified by M-TMSI.
The Global Unique MME Identifier (GUMMEI) shall be constructed from MCC, MNC and MME Identifier (MMEI)
The MMEI shall be constructed from MMEGI and MMEC.
The GUTI shall be constructed from GUMMEI and M-TMSI.
For paging purposes, the mobile is paged with the S-TMSI. The S-TMSI shall be constructed from MMEC and M-TMSI
The operator shall need to ensure that the MMEC is unique within the MME pool area and , if overlapping pool areas are in use, unique within the are of overlapping MME pools.
The GUTI shall be used to support subscriber identity confidentiality, and ,in shortened S-TMSI form to enable more efficient radio signaling and service request
The format and size of GUTI is therefore the fallowing,
GUTI = GUMMEI + M-TMSI
GUMMEI = MCC + MNC + MMEI
MMEI = MMEGI + MMEC
M-TMSI := 32 bit length
MMEGI := 16 bit length
MMEC := 8 bit length
Structure of S-Temporary Mobile Subscriber Identity (S-TMSI)
The S-TMSI is the shortened form of the GUTI to enable more efficient radio signaling procedure (e.g. Paging and Service Request). The S-TMSI shall be cosntructed from the MMEC and M-TMSI
S-TMSI = MMEC + M-TMSI
Abbrevations
MMEC = MME Code
MMEGI = MME Group ID
MMEI = MME Identifier
GUMMEI = Global Unique MME Identifier
