802.11n Wireless Technology
802.11n provides enhanced performance and range over prior 802.11 technologies. In its most basic form, 802.11n operates in the
2.4 GHz space while maintaining backward compatibility with 802.11g. Optionally, 802.11n devices can also operate in the 5 GHz
space; such 802.11n devices are described as dual band.
Additionally, 802.11n adds two significant new technologies:
MIMO (multiple input, multiple output – pronounced MyMoh) and 40 MHz wide channels. MIMO uses multiple
antennae to transmit and receive data over multiple data
streams, increasing performance and range significantly. The
40 MHz channel width allows for twice the usable radio
spectrum to transmit data, essentially doubling performance
compared to a normal 20 MHz channel width (often called channel bonding)802.11n Performance (MIMO 2x2, 3x3, etc): MIMO allows for multiple antenna paths to be used to combine multiple signals for increased range and performance. The amount of antenna paths per device can be complicated to understand, but it has serious implications for performance. Many Wi-Fi equipment manufacturers choose to use terminology like 2x2 (two-by-two) to indicate what MIMO technology is in the product.
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2x2 – 2 transmit antennas/paths and 2 receive antennas/paths
3x3 – 3 transmit antennas/paths and 3 receive antennas/paths |
Explaining spatial multiplexing and the actual scientific benefit of MIMO is beyond the scope of this document, so the benefit of MIMO is simplified here into two categories: range and speed. Having more spatial streams does increase the range, but this document will mainly focus on speed impact.
Each 802.11n stream provides a maximum signaling rate of 150 Mbps (compared to the 802.11g single stream of 54 Mbps). It’s crucial to note that to achieve 150 Mbps, the stream must operate at 40 MHz wide. Since 2.4 GHz spectrum is highly congested, running 40 MHz mode in 2.4 GHz is often difficult if not impossible to do (more on this in the next section). Thus a 2x2 product is capable of two 150 Mbps streams and would be considered a 300 Mbps wireless device and a 3x3 product a 450 Mbps device.
Finally, there are some 1x1 wireless devices on the market. These products utilize a single 802.11n transmit antenna and a single receive antenna. 1x1 devices are commonly used in portable electronics such as PDAs, mobile phones, etc. due to their limited size; installing multiple antennas is a challenge. The Wi-Fi Alliance allows certification for 1x1 client devices, but it does not allow certification of 1x1 access points.
Therefore, if a 2x2 device connects to a 3x3 device, the wireless communication between devices would occur at 300 Mbps. The following table shows the maximum link rate for various configurations using both channel widths; please note that manufacturers always market products based on a 40 MHz maximum speed rate.
40 MHz Limitations: A great source of confusion for users of 802.11n equipment is the inability to achieve maximum speed with their devices. This is particularly so on the 2.4 GHz band. To avoid excessive interference, the Wi-Fi Alliance mandates for certification that Wi-Fi CERTIFIED® access point devices must ship by default in 20 MHz mode in the 2.4 GHz channel (40 MHz width is approved in 5 GHz devices). Vertical higher gain antennas have increased x-axis coverage at the cost of y-axis coverage. Additionally, many manufacturers recommend not using 40 MHz wide channels in 2.4 GHz due to interference.
Various technologies exist in 802.11n to prevent operation of 40 MHz if neighboring networks stand to be too negatively affected. This is sometimes called “Safe Neighbor” technology. Generally, it’s good practice to only enable 40 MHz mode in 2.4 GHz if you are in an isolated area where neighboring networks won’t be negatively affected.
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