What does a wireless client do first when its countdown timer reaches zero and it...

When a wireless client’s countdown timer (also known as a backoff timer) reaches zero during contention-based access periods, and it receives a Transmit Opportunity (TXOP), it has the right to transmit its data frames on the medium. After sending the data frames, it expects an acknowledgment (ACK) from the Access Point (AP) to ensure the frames were received successfully.