Question: When is a Downlink Data Notification message sent by the SGW to the MME/S4-SGSN on the S11/S4 interface?
Option 1: Only when the SGW has received an Error Indication from eNodeB/RNC/MME across S1-U/S12/S11-U interface.
Option 2: Only as part of the network triggered service request procedure or Mobile Terminated Data Transport in Control Plane CIoT EPS optimisation with P-GW connectivity.
Option 3: Only when the SGW needs to re-establish all previous released bearer(s) for a UE upon receipt of downlink data for a UE in connected mode but without corresponding downlink bearer available.
Option 4: In multiple scenarios, including network triggered service request procedure, Paging with no established user plane on S4, re-establishing bearers, receiving Error Indication from eNodeB/RNC/MME or S4-SGSN, and as part of the Mobile Terminated Data Transport in Control Plane CIoT EPS optimisation with P-GW connectivity.
Explanation: The correct answer is Option 4 because a Downlink Data Notification message can be sent by the SGW to the MME/S4-SGSN on the S11/S4 interface in various scenarios, including but not limited to network triggered service request procedure, Paging with no established user plane on S4, re-establishing bearers, receiving Error Indication from eNodeB/RNC/MME or S4-SGSN, and as part of the Mobile Terminated Data Transport in Control Plane CIoT EPS optimisation with P-GW connectivity.