Hi Experts,
5G SA question: which NAS timer rules when the UE should requests a PDU session establishment to AMF after attach complete?
There shouldn’t be any such timer as SM can initiate request as soon as MM Reg Req accepted.
Nor there seems to be any timer between pdu session requests.
Usually I see 3 seconds between attach complete and PDU session estab request.
I don’t think there’s any NAS timer less than 6sec in 24.501.
This may be UE implementation.
As per my understanding, in 4G after attach UE has default bearer.
But in 5G SA, after attach UE still not have default bearer, must be after pdu session establishment, right?
Yes, right 
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Actually it is different. You can have a session PDU and request a new one:
See the 2 highlighted messages.
Agreed, but unlike LTE the Attach/Registration need not be default PDU.
From UE perspective, there are no timers for starting the PDU sessions. As soon as SM module is aware that reg is accepted in SA, it can start the PDU establishment. its just a question of how the UE implementation is. In a multi-PDU sceanrio like data+ims pdu sessions, any of two PDU requests can start independently based on which module(data/ims) asks for connectivity first. Both sessions can go on parallely too.