Probir Sarkar, ARM
EETimes (8/18/2010 12:05 PM EDT)
Overview of MAC Improvements
The primary method used to improve the MAC performance is to amortize the high cost of medium access over a larger number of data frames. First, 802.11n incorporates the mechanisms introduced in 802.11e, a prior amendment to the standard. Though these mechanisms were devised to provide differentiated QoS to MAC users, they also help amortize some of the MAC overheads. It introduced the concept of a Transmit Opportunity (TxOP), whereby a station that acquires the medium, does so for a bounded time period (as opposed to a single frame-ack sequence in the original DCF.) Thus the DIFS wait and backoff countdown steps are required only once in every TxOP duration. Another scheme introduced is the Block Acknowledgement (BA.) Instead of each frame being individually acknowledged, a set of frames may be acknowledged using a BA response. This amortizes the response overhead, over a larger number of data frames. These improvements are shown in the first two rows of Figure (3).