Why MME is sending paging with TMSI why not it is sending with GUTI?

Question

Why MME has to send Paging Drx in S1AP paging messge?
Whether this paging Drx is sent to UE or not?Even though paging drx in sib2 why we need paging drx in s1ap paging message?

Answer 1

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.

Comments

I agree with Mohit. S-TMSI would be shortest unique identifier to calculate paging occasion for an UE within the paging frame. An eNodeB would have connection with all the MMEs belonging to same MME group but eNodeB won't be part of two MME groups. That's why within a MME group, S-TMSI would be shortest unique identifier.