GUTI Reallocation Procedure

• GUTI Reallocation procedure is used to allocate GUTI and optionally new TAI list to the UE.
• This procedure is always initiated by MME in EMM-REGISTERED state
• This procedure is performed in chipered mode, normally takes place in conjunction with other mobility management procedure.
• If UE lower layers indicate transmission failure of GUTI REALLOCATION COMPLETE message with TAI change and is not part of new TAI list UE should abort current procedure and starts TAU.
• IF lower layer failure detected at NW before T3450 timer expiry,NW should consider both old and new GUTI and old and new TAI list as valid.
• For paging first UE should try with S-TMSI from old GUTI and try in old and new TAI list, if response received from UE network should restart the GUTI reallocation procedure again
• In no response received network should use S-TMSI from new GUTI and page in new TAI list.
• If UE sends attach, detach or tracking area update request while on going GUTI procedure network should abort current GUTI procedure and proceed with specific EMM procedure. 

  

                               

Figure : GUTI Reallocation Procedure:

GUTI:

                          

• GUTI size is 80 bits and is composed of Globally Unique MME Identity (GUMMEI) which is of 48 bits long and M-TMSI (Temporary Mobile Subscriber Identity)which of 32 bits long.
• GUMMEI : used to identify the MME which allocated this GUTI. Composed of PLMN (MMC MNC) code, MME Group Id and MME code

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)