T3346 timer value may be included in Attch Reject , Tracking Area Update (TAU) Reject or Service Request Reject sent by network to UE.
During EMM signaling congestion detected network will perform NAS level mobility management congestion control.
By sending reject message with cause value #22 Congestion
If the reject message from network is not integrity protected UE starts T3346 with random value.
if it is integrity protected UE uses the value present in reject message.
NAS Mobility Management Congestion Control consists of
a) general NAS level mobility management congestion control
b) Subscribed APN based congestion control
During general NAS level congestion control
Network shall not reject
i)attach requests for emergency bearer services.
ii) NAS signaling connections with RRC establishment cause as High priority(Access class 11-15).
iii) Requests for CS fall back emergency services
When APN based congestion control is active the network may reject attach requests from UE's with a subscription to this APN.
While assigning T3346 value network send different values for different UE's so that time outs are not synchronized.
Cases when NAS signalling allowed during T3346 running
- Attach for emergency services can be initiated even if T3346 timer is running.
- Attach can be trigger in a new PLMN which is not part of EQPLMN list and also not part of forbidden PLMN list.
- TAU can be initiated in response to paging if UE in attempting to update state.
- Deatch request can be initiated even T3346 is running
Reasons to stop T3346:
Upon reception of paging, UE shall stop T3346 and send service request or TAU request.
If UE switched off while T3346 is running, on switch on UE should restart the timer with value T3346-switchoff(if the value non zero) time,if UE not able to detect switch off time UE should restart with same T3346 value before switch off.
Though all NAS timers shall be stopped,UE should not stop T3346 in Power Saving Mode(PSM)