The available airtime for each Wi-Fi channel determines the capacity for a Wi-Fi cell in a particular location. It is a valuable resource that should not be wasted.

If a battery-operated mobile device with a weak signal roams to an AP with a very strong signal, then there is a mismatch between the strength of the transmit signal from the AP and the strength of the transmit signal from the client device.

If the client device is moving, or the RF environment changes, then there will come a point when the AP can no longer ‘hear’ the client. Whilst the client remains associated with an AP, an AP will retransmit each message 'frame' to a client until it receives an acknowledgement 'frame' from the client to indicate that the message was received. As the client device made the decision to associate with the AP, because of the strong signal from the AP to the client, this may continue repeatedly when there is a poor connection from the AP to the client. 

Some retransmissions are to be expected because of the nature of Wi-Fi, but constant retransmissions waste airtime, because no other device can transmit whilst a device is constantly retransmitting.

To demonstrate, the protocol trace summary below shows the activity from a distant high-powered AP and a weak client device (a phone) that is between the AP and our test position and is moving towards us. The more distant AP has a much stronger signal so it can be detected from our test position, but not the weaker client device even though it is closer. We know that the client device is sending acknowledgements to the AP, even though we cannot see them, because the sequence number of the frames from the AP does step up occasionally (and, although not shown here, we do eventually see all the signals from the client device as it gets closer, and we see it roam to a nearer AP.)

In the meantime, what we see here is the AP repeatedly sending the same frame (identified by their sequence number) to the client. The AP clearly misses most of the acknowledgment frames from the client.

Airtime is not only wasted by retransmissions, but also because a lack of acknowledgements indicates that the frame may not have been received, and so the AP starts sending the frame at lower and lower data rates to ensure that it can be received. As data rates are reduced, so it takes longer and longer for the AP to transmit the same frame, wasting more airtime.

In the protocol trace summary we see a data rate reduction from an already low 24 Mbps drop to a very low 6Mbps, but this doesn't solve the problem, and although not shown here, it persists because the issue is not in the quality of the signal from the AP to the client but in the signal from the client to the AP containing the acknowledgments.

Setting a cell size, for good two-way communication not only prevents constant retransmissions, but it also prevents the AP from having to reduce its transmit rate, and further wasting airtime.