Black Box LWN600CM-1 Podręcznik Użytkownika Pobierz pdf (Strona 31)

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Chapter 2: Preparing for a WAN Deployment

In RF, there is also a relative measurement that you can use to compare two numbers. This measurement is simply dB (without the

“m”). To see how this concept is applied, consider how radio signal propagation changes over a distance and how it can be

affected. Figure 2-3 shows signal strength over distance as a curve that has the best signal strength closer to the access point. It

also shows noise. In general, noise is considered to be low-level background RF signals that can interfere with a WLAN. This noise

tends to be the garbled background RF that comes from everything from the sun and stars to man-made interfering devices like

Bluetooth

headsets. It is impossible to block out noise, and it should not be attempted. This low level of background noise is

called the “noise floor.”

Signal-to-Noise

Ratio

Distance

Noise

Received Signal

Figure 2-10. Path loss in an open space.

When clients send a packet, the ratio of the signal-to-noise (SNR) level defines the quality of the link, which is directly related to

the performance of the network. Based on the SNR, the client and AP negotiate a data rate in which to send the packet, so the

higher the SNR the better. For good performance, the SNR should be greater than 20 dB, and for optimal performance it should

be at least 25 dB.

Signal strength not only diminishes over distance, but it can also be affected by objects in the way (see Figure 2-4). This can be a

wall, a tree, or even a person. There is a fairly predictable dB drop through most objects that also decreases the SNR, thus

decreasing the data rate. Although this appears to be a bad thing, clever Wi-Fi installers use it to their advantage. It enables them

to place more access points in a tighter spot by using pre-existing walls and other impediments to Wi-Fi propagation to keep

them from interfering with each other.

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Black Box LWN600CM-1 Podręcznik Użytkownika Strona 31