Certain 802.11 implementations use a fixed ACK timeout value, with a default val

Question

Certain 802.11 implementations use a fixed ACK timeout value, with a default value that is set for indoor (for example, less than 100 meters) communication. The timeout value determines the amount of time that the 802.11 sender will wait for an ACK frame after sending a DATA frame. If the ACK does not arrive within this amount of time, the DATA frame is assumed to be lost, and the DATA frame will be retransmitted.

Suppose we want to use 802.11 with an outdoor, long-distance directional antenna that can transmit over a distance of say, 5 miles. What do you think will happen if the ACK timeout value remains set for the 100 m indoor case?

Explanation / Answer

Clearly if the ACK timeout is shorter than the time it takes for the end of the last data packet to propagate to the receiver + the start of the ACK for that packet to propagate back to the sender, then the sending MAC will assume that the packet has been lost and will unnecessarily retransmit the data packet. The retransmitted packet will end up colliding with the ACK that is on its way back, inducing the back-off part of the protocol thus reducing throughput. If, conversely, the ACK timeout is set too long, the transmitter waits unncesessarily long before retransmitting any lost frames and thus reduces the throughput of the link (more important as the bit error of the environment increases.)

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