Notice that that those 'speeds' area at PHY layer. At IP layer you get from 40% (~ 22Mbps of 54Mpbs for 802.11a/g) to 60% (~ 6Mbps of 11Mpbs for 802.11b) of PHY rate. More complicated modulation/coding schemes (that provide faster PHY rate) usually lowers protocol effciency (so 300 Mbit PHY layer with efficiency 20% is still below wired FastEthernet). Remember that you use the same frequency band as legacy systems and that they should be able at least to detect each other (TDMA random access distributed systems with legacy comaptibility are not easy to design).
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Notice that that those 'speeds' area at PHY layer. At IP layer you get from 40% (~ 22Mbps of 54Mpbs for 802.11a/g) to 60% (~ 6Mbps of 11Mpbs for 802.11b) of PHY rate. More complicated modulation/coding schemes (that provide faster PHY rate) usually lowers protocol effciency (so 300 Mbit PHY layer with efficiency 20% is still below wired FastEthernet). Remember that you use the same frequency band as legacy systems and that they should be able at least to detect each other (TDMA random access distributed systems with legacy comaptibility are not easy to design).