LTE: What is extended service request in lte NAS and when it is required ?

Question

I was looking into NAS header. First four bits are used for security header type. Few values I understood but one value I could not understand which was 1100 saying "security header for service request message". I could not understand why such thing is defined only for service request message not for the other messages ?

Answer 1

In LTE....

When UE finds it's identity in the message it triggers the Random Access Procedure. followed by establishing the RRC Connection.

If PAGING is for PS domain then UE NAS layer triggers SERVICE REQUEST otherwise, if the PAGING is for CSFB then UE triggers Extended SERVICE REQUEST.