It’s reported that when customer is doing a UE triggered service request procedure, sometimes he/she sees downlink data notification after service request (as shown in picture below).
How could it happen?
Let’s have a look at UE triggered service request procedure(Section 18.104.22.168 of TS 23.401), from the procedure below, UE will sent out uplink data after Radio Bearer establishment.
Figure 22.214.171.124-1: UE triggered Service Request procedure
According to Section 126.96.36.199 of TS 23.401
“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”