The 802.11n variant of Wi-Fi achieves its high through??put (typically four times that of 802.11g) in two ways. First, it uses MIMO (multiple input, multiple output) antenna technology to transmit more data at a time. Intelligent antennas combine streams of data arriving at different times from multi??path signals bouncing off walls, floors, and ceilings. Entry-level routers typically have two receiving and transmitting antennas; midrange and high-end models have three of each.
Second, draft-n uses channel bonding: Instead of the 20-MHz-wide channels found in previous Wi-Fi standards, 802.11n can use 40-MHz-wide channels, which in theory should double their data-carrying capacity.
Unfortunately, the limited bandwidth of the 2.4-GHz range means that just one 802.11n router using channel bonding will take up virtually the entire 2.4-GHz spectrum, leaving no room for neighboring routers, and causing severe interference. For this reason, draft-2.0's so-called good-neighbor policies require that routers ship in 20-MHz mode, and that, when in 40-MHz mode, they drop to 20-MHz operation if they sense nearby Wi-Fi nets or other 2.4-GHz devices. The top link rate in 20-MHz mode is only 150 mbps (rather than the much-advertised 300 mbps); since many users are likely to be within range of other 2.4-GHz traffic, we ran our 2.4-GHz tests with 20-MHz channels.