Section: 4.1.1.7 Handling of NAS level mobility management congestion control
Content: The network may detect GMM or MM signalling congestion and perform NAS level mobility management congestion control. PS domain NAS level mobility management congestion control consists of general NAS level mobility management congestion control and subscribed APN based congestion control. CS domain NAS level mobility management congestion control consists of general NAS level mobility management congestion control. Under NAS level mobility management congestion control the network may reject mobility management signalling requests from MSs as specified in 3GPP TS 23.060[ General Packet Radio Service (GPRS); Service description; Stage 2 ]  [74]. The network should not reject requests for emergency bearer services. When subscribed APN based congestion control is active for a particular APN, the network may reject attach request from MSs with subscription to this APN. In mobility management the network may detect NAS signalling congestion. The network may start or stop performing the subscribed APN based congestion control based on mobility management level criteria such as: - rate of mobility management NAS messages from a group of MSs with a subscription to a particular APN exceeds or falls below certain thresholds; or - setting in network management. When the NAS level mobility management congestion control is active, the network may include a value for the mobility management back-off timer T3246 or T3346 in the reject messages. The MS starts the mobility management back-off timer with the value received in the mobility management reject messages. To avoid that large numbers of MSs simultaneously initiate deferred requests, the network should select the value for the mobility management back-off timer for the rejected MSs so that timeouts are not synchronised. For subscribed APN based congestion control the value of timer T3346 for a particular APN may be APN dependent. If the timer T3346 is running when the MS enters state GMM-DEREGISTERED, the MS remains switched on, and the SIM/USIM in the MS remains the same, then timer T3346 is kept running until it expires or it is stopped. If the MS is switched off when the timer T3246 or T3346 is running, the MS shall behave as follows for each running timer when the MS is switched on and the SIM/USIM in the MS remains the same: - let t1 be the time remaining until timeout at switch off and let t be the time elapsed between switch off and switch on. If t1 is greater than t, then the timer shall be restarted with the value t1 – t. If t1 is equal to or less than t, then the timer need not be restarted. If the MS is not capable of determining t, then the MS shall restart the timer with the value t1; and - if prior to switch off, timer T3346 was started due to a NAS request message (ATTACH REQUEST, ROUTING AREA UPDATE REQUEST or SERVICE REQUEST) which contained the low priority indicator set to "MS is configured for NAS signalling low priority", then if timer T3346 is restarted at switch on, the MS configured for low priority shall handle mobility management requests as indicated in subclauses 4.7.3.1.5, 4.7.5.1.5 and 4.7.13.5. If the MS enters a new PLMN while timer T3246 is running, and the new PLMN is not equivalent to the PLMN where the MS started timer T3246, the MS shall stop timer T3246 when initiating mobility management procedures in the new PLMN. If the MS enters a new PLMN while timer T3346 is running, and the new PLMN is not equivalent to the PLMN where the MS started timer T3346, the MS shall stop timer T3346 when initiating mobility management procedures in the new PLMN. At an inter-system change from S1 mode to A/Gb mode or from Iu mode to A/Gb mode within the same RA, if the timer T3346 is running and the TIN indicates "RAT-related TMSI", the MS shall set the GPRS update status to GU2 NOT UPDATED and enter state GMM-REGISTERED.ATTEMPTING-TO-UPDATE. After change in RAI, if the timer T3346 is running and GPRS update status is GU1 UPDATED then MS shall set the GPRS update status to GU2 NOT UPDATED and enter state GMM-REGISTERED.ATTEMPTING-TO-UPDATE. After change in LAI, if the timer T3246 is running and MM update status is U1 UPDATED then MS shall set the MM update status to U2 NOT UPDATED and enter state MM IDLE, ATTEMPTING TO UPDATE.