I may be wrong. My test is to check the wireless ACK packets from the Cisco AP 1200 and from the Cisco 871W. These two APs have the same speed settings : speed basic-1.0 basic-2.0 5.5 11.0, i.e the basic rates are only 1 and 2.
For the AP1200, the ACK from the AP to the clients are 2Mbps for any data packets sending to the AP with 11Mbps.
For the 871W, the ACK from the AP to the clients are 11Mbps for any data packets sending to the AP with 11Mbps.
If I understand correctly, the ACK transmit speed should be the same as the speed of the data frame to be acked, if that speed belongs to the basic rates, otherwise it must be sent with the highest speed of the basic rate.
In this case, the highest speed of the basic rates is 2mbps and the871W ACKs with 11Mbps.
Am I wrong somewhere ?
Thanks for all your advice.