In the paging message send from MME to eNB , which paging ue identity is used ?

Question

PagingUE-Identity ::= CHOICE {
s-TMSI S-TMSI,
imsi IMSI,
...
}

As pagingUE identity should contain any one of these in paging message so how MME decides which one to send, though for security purpose MME prefers t-o send send S-TMSI but in which scenarion MME wil use imsi as paging ue identity.

Answer 1

I think these could be scenarios when MME uses IMSI.
1>Restart MME when UE is in IDLE Mode.
2> MME Relocation (IDLE Mode relocation i.e when S10 is removed)

Answer 2

MME does paging for UEs with IMSI after recovery of a network failure at MME.
For example: UE is attached to a MME and after some time it goes to IDLE mode. The time UE is in IDLE state, MME goes down/restart and it recovers only few data after recovery. Now Its MME responsibility to inform UE. That's why MME does paging with IMSI. Once UE receives paging with IMSI, it deletes all context locally and moves to EMM-DEREGISTERED state. After that UE initiates Attach Procedure towards network.