Section: 4.3.17.3 Optimising periodic TAU Signalling
Content: To reduce network load from periodic TAU signalling and to increase the time until the UE detects a potential need for changing the RAT or PLMN (e.g. due to network problems) the longer values of the periodic TAU timer and Mobile Reachable timer shall be supported. A long periodic RAU/TAU timer value may be locally configured at MME or may be stored as part of the subscription data in HSS. During Attach and TAU procedures the MME allocates the periodic RAU/TAU timer value as periodic TAU timer to the UE based on VPLMN operator policy, low access priority indication from the UE, periodic RAU/TAU timer value requested by UE, subscription information received from the HSS and Start of Unavailability Period and/or Unavailability Period Duration if using a RAN that provides discontinuous coverage (see clause 4.13.8.2). If MME receives a subscribed periodic RAU/TAU timer value from the HSS it allocates the subscribed value to the UE as periodic TAU timer. A visited PLMN MME may use subscribed periodic RAU/TAU timer value, if available, as an indication to decide for allocating a locally configured periodic RAU/TAU timer value to the UE. If no subscribed periodic RAU/TAU timer value is received from the HSS, the MME should: - if the periodic RAU/TAU timer value requested by UE is within the boundaries of the VPLMN operator policy, allocate the value requested by the UE; - if the periodic RAU/TAU timer value requested by UE is higher than allowed per the VPLMN operator policy, allocate the highest allowed value per the VPLMN operator policy; - if the periodic RAU/TAU timer value requested by UE is lower than allowed per the VPLMN operator policy, allocate the lowest allowed value per the VPLMN operator policy.