Watchguard Firebox SOHO 6 Wireless, Руководство пользователя, Страница: 31 / 208

User Guide 9
SOHO 6 Wireless hardware description
- How much directional antenna gain there is at the
transmitter and receiver
- The signal attenuation (path-loss) between the transmitter
and receiver.
Path Loss:
The path-loss is directly proportional to line-of-site
distance between transmitter and receiver, and inversely
proportional to the wavelength of the transmitted signal.
The equation for Signal Loss is:
Loss = 20xLog10(4xpi x(Distance/Wavelength)).
- Wavelength = (speed-of-light/ frequency). This means
that the higher the frequency the shorter the wavelength
and the greater the path-loss will be for a given frequency.
- For an average office environment, a rule-of-thumb is that
line-of-sight signal loss will only pertain to about the first
20 feet and will then increase by about 30 dB per 100 feet,
due the effect of walls, and cubicles and widows, etc.
Second, the signals can arrive by different paths depending
on how many surfaces reflect the signal. This is called
multi-path. Many surfaces will reflect a signal at 2.4 GHz.
The problem is that some combinations of reflected signals
will result in cancellation at a given point, thus by moving a
receiver by as little as ½ wavelength, the signal could vary
by as much as 30dB. The effect is called fading due to
multi-path reflections.
The signal fading effect is highly dependent on antenna
position, so the SOHO 6 Wireless uses antenna receiver
diversity (2 antennas spaced more than ½ wavelength
apart) to reduce the effect of multi-path fading. On the
SOHO 6 Wireless the antenna receiving the stronger signal
is selected automatically.