Question: In a PDCP (Packet Data Convergence Protocol) entity for DRBs (Dedicated Radio Bearers), what triggers the start of a new `discardTimer` on the transmitting side?
Option 1: Expiration of the `t-Reordering` timer
Option 2: Receipt of an SDU (Service Data Unit) from upper layers
Option 3: Configuration update from Radio Resource Control (RRC) layers
Option 4: Successful transmission of a PDCP PDU (Protocol Data Unit)
Explanation: The `discardTimer` is started upon reception of an SDU from upper layers on the transmitting side of each PDCP entity for DRBs. This ensures that if an SDU is not successfully transmitted within a certain timeframe, it can be discarded to prevent buffer overflow and maintain network efficiency. This highlights the importance of timely data transmission in telecom networks, particularly in managing the flow of service data units between layers.