Extreme Networks Summit WM User Manual page 64

Configuring the Altitude AP
streams of the same packet to achieve the increased throughput.
The Diversity feature is meant to offset the liability of RF corruption, arising out of multipath, whereas MIMO
converts the liability of multipath to its advantage.
Because the 802.11n AP operates with multiple-antennas, it is capable of picking up even the weakest
signals from the client devices.
Channel bonding
In addition to MIMO technology, the 802.11n AP makes a number of additional changes to the radio to
increase the effective throughput of the Wireless LAN. The radios of regular Altitude APs use radio
channels that are 20 MHz wide. This means that the channels must be spaced at 20 MHz to avoid
interference. The radios of 802.11n AP can use two channels at the same time to create a 40 MHz wide
channel. By using the two 20 MHz channels in this manner, the 802.11n AP achieves more than double
throughput. The 40-MHz channels in 802.11n are two adjacent 20-MHz channels, bonded together. This
technique of using two channels at the same time is called channel bonding.
Shortened guard interval
The purpose of the guard interval is to introduce immunity to propagation delays, echoes and
reflections of symbols in orthogonal frequency division multiplexing (OFDM) - a method by which
information is transmitted via a radio signal in Altitude APs.
In OFDM method, the beginning of each symbol is preceded by a guard interval. As long as the echoes
fall within this interval, they will not affect the safe decoding of the actual data, as data is only
interpreted outside the guard interval. Longer guard periods reduce the channel efficiency. The 802.11n
AP provides reduced guard periods, thereby increasing the throughput.
MAC enhancements
The 802.11n AP also has an improved MAC layer protocol that reduces the overheads (in the MAC layer
protocol) and the contention losses. This results in increased throughput.
The 802.11n AP is available in the following two models:
Altitude 450 - Six internal antennas
Altitude 451 - Three external antennas
NOTE
The 802.11n AP cannot be deployed in an outdoor environment.
Altitude 802.11n AP's radios
The Altitude 802.11n AP is equipped with two radios - radio a/n and radio b/g/n. The following is a
block diagram of the Altitude 802.11n AP equipped with external antennas.
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Summit WM User Guide, Software Version 5.3