Листовка для Cisco Aironet 2702i AIR-CAP2702I-E-K9

 

 

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Greater uplink reliability. Due to multipath, an AP with four antennas receives four copies of a clien

t’s 

signal. Each copy is distorted (constructively or destructively) in four very different ways, so the likelihood 

that all copies are destructively faded all at the same time is very low. Thus the MIMO equalizer within 

the receiver can gather all these copies, cleverly combine them, and achieve greater reliability, delivering 

more predictable data rates and fewer retries. Of course, 

an AP with fewer antennas can’t do as well, 

particularly when the number of spatial streams climbs up toward the number of receive antennas. 

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Greater downlink reliability (maybe). Here 802.11n offers beamforming (with outstanding benefits), space 

time block coding (useful benefits but inferior to beamforming), and cyclic delay diversity (with very 

modest benefits). Yet 802.11n offers many incompatible flavors of beamforming, each involving client 

assistance, and the industry has never put its weight behind any one of them. Thus, beamforming is 

practically available only 

from techniques that don’t expect assistance from the client such as Cisco 

ClientLink. Beamforming is particularly valuable due to the vulnerability of low-antenna-count devices to 

destructive fading. 

This is all described in much greater detail in a 

. 

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Channel bonding. By doubling the channel bandwidth from 20 to 40 MHz, a single transmission can carry 

twice as much data in the same time. Actually, the gain is slightly more than double, since the guard band 

between the two traditional 20-MHz channels can be used as well. 

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Aggregation. If the PHY is like the engine of a car that generates great power, the MAC is like the car 

transmission, which is responsible for efficiently delivering the power to the wheels. 

In 802.11a, each data frame comes with various overheads, such as the preamble for the frame, often an 

acknowledgment frame, and any time gaps between and around these transmissions. When the data size 

gets smaller than this overhead, 

speeding up the data payload doesn’t increase the effective speed by that 

much. The MAC is throwing away the power.