The GUTI is assigned only by the MME during initial attach of a UE to the E-UTRAN. The purpose of the GUTI is to provide an unambiguous identification of the UE that does not reveal the UE or the user's permanent identity in the E-UTRAN. It also allows identification of the MME and network to which the UE attaches. The GUTI can be used by the network to identify each UE unambiguously during signaling connections.
GUTI = MCC (3 bit) + MNC (3 bit) + MME Group ID (16 bit) + MME code (8 bit) + M-TMSI(32 bit)
GUMMEI = MCC (3 bit) + MNC (3 bit) + MME Group ID (16 bit) + MME code (8 bit)
MME Identifier (MMEI) = MME Group ID (16-bit) + MME Code (8-bit)
S-TMSI = MME Code (8 bit) + M-TMSI (32 bit)
For paging purposes, the mobile is paged with the S-TMSI. The S-TMSI is constructed from the MMEC and the M-TMSI.
The operator needs to ensure that the MMEC is unique with the MME Pool area and, if overlapping pool areas are in use, unique within the area of overlapping MME pools.
It is important to understand that on the S1 interface the IMSI is typically not seen, just like the GUTI. Exceptions are initial attach to the network when no old GUTI is stored on the USIM card or the true subscriber's identity is checked using NAS signaling, which regularly happens when roaming subscribers attach.