802.11n is an amendment to the IEEE 802.11-2007 wireless-networking standard. Its purpose is to improve network throughput over the two previous standards—802.11a and 802.11g—with a significant increase in the maximum net data rate from 54 Mbit/s to 600 Mbit/s (slightly higher gross bit rate including for example error-correction codes, and slightly lower maximum throughput) with the use of four spatial streams at a channel width of 40 MHz.
MIMO is a technology that uses multiple antennas to coherently resolve more information than possible using a single antenna. One way it provides this is through Spatial Division Multiplexing (SDM), which spatially multiplexes multiple independent data streams, transferred simultaneously within one spectral channel of bandwidth. MIMO SDM can significantly increase data throughput as the number of resolved spatial data streams is increased.
Channels operating with a width of 40 MHz are another feature incorporated into 802.11n; this doubles the channel width from 20 MHz in previous 802.11 PHYs to transmit data, and provides twice the PHY data rate available over a single 20 MHz channel.
There are 76 channels in 2.4 and 5.8 GHz bands with channel 77-127 being reserved. Each channel has varying encoding to support backwards compatibility. Channels can be 20 MHz or 40 MHz depending on other users in the channel space.
The Wi-Fi Alliance was established in mid 2007 to help make sure that draft n implemented interopérable equipment to prevent problems. The Intel Centrino Wi-Fi uses 802.11a to achieve broadband operation with 802.11n wave 2 access points.
The second wave of 802.11n has improved the channel size to 20 or 40 MHz which made for better bandwidth. Mass produced 150 Mbps and 300 Mbps USB adapters are very inexpensive. 802.11n was replaced in the market rather quickly by 802.11ac which is a refinement that provides more bandwidth in mainstream consumer equipment.
40MHz channels work only with 802.11n on the 5-GHz band—40 MHz channels cannot not be configured on a 2.4-GHz radio without significant interference to existing 802.11b abd 802.11g users. In the 5.8 GHz band 802.11n can achieve increased real bandwidth due to the larger number of available channels.
Originally with 802.11b and 802.11g the 14 channels at 2.4 GHz were adequate for the lower bandwidth speeds. The spectrum analyser shows how, with 802.11n, the whole spectrum is overwhelmed. Now channels 1, 6 and 11 are all that can be used with 20 MHz channel width.
With 802.11n there are only 3 practical channels available. The image shows how some users are setup with channel 3 which overlaps and interferes with channel 1 and channel 6. The users on channel 9 overlap channel 6 and cause interference. The user on on channel 10 interferes with the users on channel 11.
The 5-GHz band is actually four frequency bands: 5.1-GHz, 5.3-GHz, 5.4-GHz, and 5.8-GHz. The 5-GHz band has a total of 24 channels with 20 MHz bandwidth available. The span for 20 MHz channels is vast which is why we strongly recommend abandoning 2.4 Ghz completely. 802.11a, 802.11n and above all support 5 Ghz.
FCC AUTOMATIC CHANNEL MANDATE
The 802.11n standard calls for automatic channel selection which is why the image shows very little overlap in the 5GHz bands. The idea is to use a mesh network topology so that access points cooperate to eliminate interference as much as possible. Most overlap is caused by very distant access points that may not be able to interact.
The FCC made some channels available subject to not interfering with airport Terminal Doppler Weather Radar (TDWR). This eliminates the use of channels 120, 124, and 128. Channels 116 and 132 can be used if they are separated by more than 30 MHz (center-to-center) from a TDWR located within 35 km of the device.