Black Box LWN602A User Manual page 22

Chapter 2: Preparing for a WAN Deployment
The first concept to understand is signal strength and how it relates to throughput. Radio power is measured in decibels relative
to one milliwatt (dBm) where 0 dBm = 1 milliwatt, but decibels increase using a log10 math function. Rather than dusting off
your old math books and pulling out your calculator, look at the dBm-to-milliwatt converter that appears below. Often in Wi-Fi,
dBm and milliwatts (mW)—and microwatts (µW)—are used interchangeably. The following table converts between the two units
of measurement:
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."
Received Signal
Signal-to-Noise
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.
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Table 2-2. dBm-to-milliwatt conversions.
dBm-to-milliwatt
20 dBm = 100 mW
15 dBm = 32 mW
10 dBm = 10 mW
5 dBm = 3.2 mW
4 dBm = 2.5 mW
3 dBm = 2.0 mW
Ratio
Noise
Figure 2-3. Path loss in an open space.
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dBm-to-milliwatt
2 dBm = 1.6 mW
1 dBm = 1.3 mW
0 dBm = 1.0 mW
-1 dBm = 794 µW
-5 dBm = 316 µW
-10 dBm = 100 µW
Distance