Section: 5.3.9 Handling of NAS level mobility management congestion control
Content: The network may detect EMM signalling congestion and perform NAS level mobility management congestion control. NAS level mobility management congestion control consists of general NAS level mobility management congestion control and subscribed APN based congestion control. Under general overload conditions the network may reject mobility management signalling requests from UEs as specified in 3GPP TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [10]. The network should not reject the following requests: - requests for emergency bearer services; - DETACH REQUEST message; - service request or tracking area update request triggered by paging; - requests from UEs that were received via NAS signalling connections established with RRC establishment cause "High priority access AC 11 – 15"; - requests for attach, normal tracking area updating or periodic tracking area updating, when emergency is indicated by lower layers; - requests for CS fallback emergency call or 1xCS fallback emergency call; and - requests for tracking area update when the UE is reporting unavailability information due to enhanced discontinuous coverage. When subscribed APN based mobility management congestion control is active for a particular APN, the network may reject attach requests from UEs with a subscription to this APN. In mobility management the network may detect NAS signalling congestion and 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 UEs with a subscription to a particular APN exceeds or falls below certain thresholds; and/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 T3346 in the reject messages. The UE starts the timer T3346 with the value received in the mobility management reject messages. To avoid that large numbers of UEs simultaneously initiate deferred requests, the network should select the value for the timer T3346 for the rejected UEs 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 UE enters state EMM-DEREGISTERED, the UE remains switched on, and the USIM in the UE remains the same, then timer T3346 is kept running until it expires or it is stopped. If the UE is switched off when the timer T3346 is running, the UE shall behave as follows when the UE is switched on and the USIM in the UE remains the same: - let t1 be the time remaining for T3346 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 UE is not capable of determining t, then the UE 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, TRACKING AREA UPDATE REQUEST, CONTROL PLANE SERVICE REQUEST or EXTENDED 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 UE configured for dual priority shall handle mobility management requests as indicated in clauses 5.5.1.2.6, 5.5.3.2.6 and 5.6.1.6. If the UE enters a new PLMN while timer T3346 is running, and the new PLMN is not equivalent to the PLMN where the UE started timer T3346, the UE shall stop timer T3346 when initiating mobility management procedures in the new PLMN. After change in TAI which is not part of TAI list, if the timer T3346 is running and EMM update status is EU1 UPDATED then UE shall set the EMM update status to EU2 NOT UPDATED and enter state EMM-REGISTERED.ATTEMPTING-TO-UPDATE.