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LANCOM Reference Manual LCOS 3.50 ̈ Chapter 11: Wireless LAN – WLAN
260
Wireless LAN – WLAN
The data transmission rate is set according to the reception power. A
WLAN module has an input sensitivity equivalent to a power level of,
for example, -80dBm. If the received power falls below this level, then
a lower data rate can be switched in that corresponds with an
improved sensitivity with a lower level of power.
ባ Outdoor wireless connections are usually realised with external antennae
and extension cables together with lightning protection for safety. The
power loss from the cable is approx. 1 dB per metre. A cable 4 m long thus
reduces power by 4 dB, the lightning protection and the various plug
connections also lead to the loss of a further 1 dB. Thus the power of the
external antenna is:
17 dBm - 4 dB - 1 db = 12 dBm.
ቤ The power received by the antenna is then amplified. An AirLancer
Extender O-18a (with an emitting angle of 18°) supplies an antenna gain
of 18 dBm. The total power output from the antenna is thus:
12 dBm + 18 dBm = 30 dBm.
This power emission must be within the legal limits of the country
where the antenna is in operation!
ብ Radio transmission through air is subject to power attenuation from the
so-called "free-space loss" x, which is logarhythmically related to the
distance d (in km) between transmitter and receiver.
x = 100 + 20 * log (d) [dB] in the 2.4-GHz band
x = 105 + 20 * log (d) [dB] in the 5-GHz band
A 802.11a transmission over a distance of 4 km results in a free-space loss
x of:
x = 105 dB + 20 * log (4) dB = 105 dB + 12 dB = 117 dB.
ቦ A 10 dB safety margin is added to this attenuation so that the total loss
for this example can be taken as 127 dB.
ቧ This loss between the transmitting and receiving antenna is subtracted
from the output power of the transmitting antenna:
30 dBm - 127 dBm = - 97 dBm.
This determines the reception power at the receiving antenna.