Hi everyone,
I found this about one of the enhancements of 802.11n:
Block acknowledgment —Normally, 802.11 requires that each frame of data transmitted must be acknowledged by the recipient. If a frame goes unacknowledged, the transmitter can assume that the frame was lost and needs to be resent. The overhead of having acknowledgment messages interleaved with every transmitted frame is inefficient; it uses up airtime on the shared media.
With 802.11n, data frames can be transmitted in one burst. Only one acknowledgment is expected from the recipient after the burst is complete. More airtime can be spent sending data, increasing the overall throughput.
Therefore I think with the standards prior to 802.11n, the stations sent only one frame when they got the medium. From 802.11n on, this changed and they can transmit more than one frame.
Regards,
Julián