Section: 5.3.4.2 Handling of abnormal conditions in UE triggered Service Request
Content: Under certain conditions, the current UE triggered Service Request procedure can cause unnecessary Downlink Packet Notification messages which increase the load of the MME. This can occur when uplink data sent in step 6 causes a response on the downlink which arrives at the Serving GW before the Modify Bearer Request message, step 8. This data cannot be forwarded from the Serving GW to the eNodeB and hence it triggers a Downlink Data Notification message. If the MME receives a Downlink Data Notification after step 2 and before step 9, the MME shall not send S1 interface paging messages. However, across all the UEs on that MME, the MME shall monitor the rate at which these events occur. If the rate becomes significant (as configured by the operator) and the MME's load exceeds an operator configured value, the MME shall indicate "Delay Downlink Packet Notification Request" with parameter D to the Serving Gateway, where D is the requested delay given as an integer multiple of 50 ms, or zero. The Serving GW then uses this delay in between receiving downlink data and sending the Downlink Data Notification message. NOTE 1: A low rate of reception of Downlink Data Notifications between steps 2 and 9 should be considered a normal circumstance, e.g. due to the chance that a UE Terminating call/session is initiated at roughly the same time as the UE triggered Service Request procedure. NOTE 2: It is recommended that this rate is determined over 60 second periods. The MME shall use the step 8 Modify Access Bearers Request or Modify Bearer Request of the UE initiated Service Request procedure to indicate "Delay Downlink Packet Notification Request" to the Serving GW. To determine the amount of delay requested by a given MME, the Serving GW either uses the last Modify Access Bearers Request or Modify Bearer Request message which is part of a Service Request procedure, or, just uses one of the Service Request procedure's Modify Access Bearers Request or Modify Bearer Request messages received within the preceding 30 seconds. The latter mode of operation shall be taken into account when implementing the MME. The MME is responsible for setting the value of D. The exact algorithm for setting the value is implementation dependent, two examples are given below to serve as a guideline: EXAMPLE 1: The MME adaptively increases the value of D when the rate of unnecessary Downlink Data Notifications is too high; and correspondingly it decreases the value when the rate is not too high. EXAMPLE 2: When unnecessary Downlink Data Notifications arrive, the MME measures the average time from the reception of the unnecessary Downlink Data Notification to the reception of the Modify Access Bearers Request or Modify Bearer Response from the Serving GW in the same UE triggered Service Request Procedure. The value of D is calculated from this average, by adding a safety margin. Normally, upon receipt of a downlink data packet for which there is no DL-TEID of the S1 user plane tunnel, the S-GW shall send the Downlink Data Notification message to the MME without delay. If the S-GW determines from the last Modify Access Bearers Request or Modify Bearer Request message which is part of a Service Request procedure that a given MME request delaying of the Downlink Packet Notification by a delay of D, it shall (only for UEs of that MME) buffer the Downlink Data for a period D. If the DL-TEID and eNodeB address for the UE is received before the expiry of the timer, the timer shall be cancelled and the Network triggered Service Request procedure is finished without sending the Downlink Data Notification message to the MME, i.e. DL data are sent to the UE. Otherwise the Downlink Data Notification message is sent to the MME when the timer expires. NOTE 3: The above procedure and indicated time values are intended to ensure that the procedure is "stable"; avoids RAN impacts; and, that the negative impacts of shortening the DRX interval on UE battery life are avoided.