In the Internet of Things scenarios, it is crucially important to provide low energy consumption of client devices. To address this challenge, new Wi-Fi standards introduce the Target Wake Time (TWT) mechanism. With TWT, devices transmit their data according to a schedule and move to the doze state afterwards. The main problem of this mechanism is the clock drift phenomenon, because of which the devices cease to strictly comply with the schedule. As a result, they can miss the scheduled transmission time, which increases active time and thus power consumption. The paper investigates uplink transmission with two different TWT operation modes. With the first mode, a sensor transmits a packet to the access point (AP) after waking up, using the random channel access. With the second mode, the AP polls stations and they can transmit a packet only after receiving a trigger frame from the AP. For both modes, the paper studies how the average transmission time, the packet loss rate and the average energy consumption depend on the different TWT parameters. It is shown that when configured to guarantee the given packet loss rate, the first mode provides lower transmission time, while the second mode provides lower energy consumption.
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