Can MME send paging message without TMSI ID?

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

MME can initiate paging procedure either with S-TMSI or IMSI. As per descriptions present in specifications, MME sends paging with IMSI in abnormal cases. Network failure is one of the examples of abnormal case. After recovery from the network failure, MME might have lost UE tracking area list and other information. In this case, MME initiates paging with IMSI value towards all the eNodeB comes under the tracking area being served by this MME.

Answer 2

I think S-TMSI is needed in paging.UE location is known to MME on a per TA basis. Therefore, MME has to page all eNB's within a group of TA.S-TMSI contains an S-Temporary Mobile Subscriber Identity, a temporary UE identity provided by the EPC which uniquely identifies the UE within the tracking area.