T3346 - NAS Level Mobility Management Congestion Control

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)

PLMN selection

At switch on or recovery from loss of coverage UE should select the last registered-PLMN (RPLMN) if exists and start with the PLMN selection operating mode that was used before switch off/loss of coverage
Two different modes of PLMN selection
i)Automatic PLMN Selection Mode
This mode utilizes a list of PLMN's in priority order. The highest priority PLMN which is available and allowable is selected.
Ms selects and attempts registration on other PLMN/access technology combinations, if available and allowable in the following order
1. Either HPLMN  (if EHPLMN list not present/empty on SIM) or the highest EHPLMN that is available (if EHPLMN list present)
2. PLMN/access technology combination in "User Controlled PLMN selector with access technology" data file in SIM.
3. PLMN/access technology combination in "Operator Controlled PLMN selector with access technology" data file in SIM.
4. Other PLMN/access technology combinations with received high quality signal in random order 
5. Other PLMN/access technology combinations in order of decreasing signal quality

ii) Manual PLMN Selection Mode
Here the MS indicates to the user which PLMN's are available, Only when the user makes a manual selection does the MS try to obtain service on the PLMN.
MS indicates the list of PLMN's with all supported access technologies, this list includes PLMN's present in the "Forbidden PLMNs" list, "Forbidden PLMNs for GPRS Service" list
and PLMNs which offers services not supported by the MS.
Plmns meeting above criteria are presented in following order.
1. Either HPLMN  (if EHPLMN list not present/empty on SIM) or the highest EHPLMN that is available, if more EHPLMNs are available display either the highest priority one or all based on the flag on the SIM.
2. PLMN/access technology combination in "User Controlled PLMN selector with access technology" data file in SIM.
3. PLMN/access technology combination in "Operator Controlled PLMN selector with access technology" data file in SIM.
4. Other PLMN/access technology combinations with received high quality signal in random order 
5. Other PLMN/access technology combinations in order of decreasing signal quality

Once the UE has registered on a PLMN selected by the user, the UE shall not automatically register on a different PLMN unless
i) The new PLMN is indicated as an equivqlent PLMN by the registered PLMN or
ii) The user selects automatic mode

If no such PLMN selected or that PLMN is no longer available, the MS shall attempt to camp on any acceptable cell and enter the limited service state.

Roaming:
If the MS is roaming and in registered on a VPLMN, the MS shall periodically attempt to select HPLMN (if EHPLMN list is empty) or one of its EHPLMNs (if list is available on SIM) or a higher priority PLMN/access technology combinations listed
in "User and Operator controlled PLMN's list.
A value T minutes stored in the SIM which can range from 6 minutes to 8 hours in steps of 6 minutes to perform periodic attempts.
If no value is present in SIM a default value of 60 minutes is used for T.
The attempts to access the HPLMN or an EHPLMN or higher priority PLMN shall be as specified below:
a) The periodic attempts shall only be performed in automatic mode when the MS is roaming, and not while the MS is attached for emergency bearer services or has a PDN connection for emergency bearer services;
b) The MS shall make the first attempt after a period of at least 2 minutes and at most T minutes:
c) The MS shall make the following attempts if the MS is on the VPLMN at time T after the last attempt;
d) Periodic attempts shall only be performed by the MS while in idle mode

PLMN - Public Land Mobile Network
HPLMN - Home PLMN
EHPLMN - Equivalent HPLMN
RPLMN - Registered PLMN


Reference : 3GPP TS 23.122