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Power Ratings
WLAN equipment is usually specified in decibels compared to known values.
Transmit Power and Receive Sensitivity are specified in "dBm," where "m" means 1 milliWatt (mW). So, 0 dBm is equal to 1 mW;
3 dBm is equal to 2 mW; 6 dBm is equal to 4 mW, and so on. For example, a Cisco Aironet 350 Series Access Point at 100 mW
transmit power is equal to 20 dBm. dBw is occasionally used for the same purpose, but as a comparison against 1 Watt (1000 mW).
Common mW values to dBm values are shown below in Table 2.
Table 2
Common mW Values to dBm Values
dBm
0 dBm
1 dBm
3 dBm
6 dBm
7 dBm
10 dBm
12 dBm
13 dBm
15 dBm
17 dBm
20 dBm
30 dBm
40 dBm
1. The 1, 2, 5, 20, 50, and 100 mW transmit power settings for the Cisco Aironet 350 Series radios correspond to 0, 3, 7, 13, 17, and 20 dBm.
Outdoor Range
The range of a wireless link is dependent upon the maximum allowable path loss. For outdoor links this is a straightforward
calculation as long as there is clear line of sight between the two antennas with sufficient clearance for the Fresnel zone.
For line of sight, you should be able to visibly see the remote locations antenna from the main site. (Longer distance may require
the use of binoculars). There should be no obstructions between the antennas themselves. This includes trees, buildings, hills, and
so on.
As the distance extends beyond 6 miles, the curve of the earth (commonly called earth bulge) affects installation, requiring antennas
to be placed at higher elevations.
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mW
1
1 mW
1.25 mW
1
2 mW
4 mW
1
5 mW
10 mW
16 mW
1
20 mW
32 mW
1
50 mW
1
100 mW
1000 mW (1 W)
10,000 mW (10 W)
Cisco Systems, Inc.
Page 10 of 29
dBm
0 dBm
-1 dBm
-3 dBm
-6 dBm
-7 dBm
-10 dBm
-12 dBm
-13 dBm
-15 dBm
-17 dBm
-20 dBm
-30 dBm
-40 dBm
mW
1 mW
0.8 mW
0.5 mW
0.25 mW
0.20 mW
0.10 mW
0.06 mW
0.05 mW
0.03 mW
0.02 mw
0.01 mW
0.001 mW
0.0001 mW
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