For paging purposes, the mobile is paged with the S-TMSI. S-TMSI is a shorter format of GUTI that can be used .
GUTI has two main components as explained below:
one that uniquely identifies the MME which allocated the GUTI (GUMMEI)
one that uniquely identifies the UE within the MME that allocated the GUTI (M-TMSI).
The part of the GUMMEI that identifies the MME node is called the MMEC, and this is included in the S-TMSI.
GUTI (not more than 80 bits) = GUMMEI + M-TMSI
S-TMSI (40 bits) = MMEC + M-TMSI
MMEC- MME code
M-TMSI-The M-TMSI identifies the UE within the MME.
I think to simplify the paging procedure only S-TMSI is used with MMEC+M-TMSI.