This document defines radio frequency (RF) power levels and the most
common measure, the decibel (dB). This information can be very useful when you
troubleshoot intermittent connectivity.
Cisco recommends that you have knowledge of basic mathematics, such as
logarithms and how to use them.
This document is not restricted to specific software and hardware
versions.
Refer to
Cisco
Technical Tips Conventions for more information on document
conventions.
The dB measures the power of a signal as a function of its ratio to
another standardized value. The abbreviation dB is often combined with other
abbreviations in order to represent the values that are compared. Here are two
examples:
You can calculate the power in dBs from this formula:
Power (in dB) = 10 * log10 (Signal/Reference)
This list defines the terms in the formula:

log10 is logarithm base
10.

Signal is the power of the signal (for
example, 50 mW).

Reference is the reference power (for
example, 1 mW).
Here is an example. If you want to calculate the power in dB of 50 mW,
apply the formula in order to get:
Power (in dB) = 10 * log10 (50/1) = 10 * log10 (50) = 10 * 1.7 = 17 dBm
Because decibels are ratios that compare two power levels, you can use
simple math in order to manipulate the ratios for the design and assembly of
networks. For example, you can apply this basic rule in order to calculate
logarithms of large numbers:
log10 (A*B) = log10(A) + log10(B)
If you use the formula above, you can calculate the power of 50 mW in
dBs in this way:
Power (in dB) = 10 * log10 (50) = 10 * log10 (5 * 10) = (10 * log10 (5)) +
(10 * log10(10)) = 7 + 10 = 17 dBm
These are commonly used general rules:
An Increase of:

A Decrease of:

Produces: 
3 dB 

Double transmit power 

3 dB 
Half transmit power 
10 dB 

10 times the transmit power 

10 dB 
Divides transmit power by 10 
30 dB 

1000 times the transmit power 

30 dB 
Decreases transmit power 1000 times 
This table provides approximate dBm to mW values:
dBm

mW

0 
1 
1 
1.25 
2 
1.56 
3 
2 
4 
2.5 
5 
3.12 
6 
4 
7 
5 
8 
6.25 
9 
8 
10 
10 
11 
12.5 
12 
16 
13 
20 
14 
25 
15 
32 
16 
40 
17 
50 
18 
64 
19 
80 
20 
100 
21 
128 
22 
160 
23 
200 
24 
256 
25 
320 
26 
400 
27 
512 
28 
640 
29 
800 
30 
1000 or 1 W 
Here is an example:

If 0 dB = 1 mW, then 14 dB = 25 mW.

If 0 dB = 1 mW, then 10 dB = 10 mW, and 20 dB = 100 mW.

Subtract 3 dB from 100 mW in order to drop the power by half (17 dB =
50 mW). Then, subtract 3 dB again in order to drop the power by 50 percent
again (14 dB = 25 mW).
Note: You can find all values with a little addition
or subtraction if you use the basic rules of algorithms.
You can also use the dB abbreviation in order to describe the power
level rating of antennas:

dBi—For use with isotropic antennas.
Note: Isotropic antennas are theoretical antennas that transmit equal
power density in all directions. They are used only as theoretical
(mathematical) references. They do not exist in the real world.

dBd—With reference to dipole
antennas.
Isotropic antenna power is the ideal measurement to which antennas are
compared. All FCC calculations use this measurement (dBi). Dipole antennas are
more realworld antennas. While some antennas are rated in dBd, the majority
use dBi.
The power rating difference between dBd and dBi is approximately
2.2—that is, 0 dBd = 2.2 dBi. Therefore, an antenna that is rated at 3 dBd is
rated by the FCC (and Cisco) as 5.2 dBi.
The radiated (transmitted) power is rated in either dBm or W. Power
that comes off an antenna is measured as effective isotropic radiated power
(EIRP). EIRP is the value that regulatory agencies, such as the FCC or European
Telecommunications Standards Institute (ETSI), use to determine and measure
power limits in applications such as 2.4GHz or 5GHz wireless equipment. In
order to calculate EIRP, add the transmitter power (in dBm) to the antenna gain
(in dBi) and subtract any cable losses (in dB).
Part

Cisco Part Number

Power

A Cisco Aironet Bridge 
AIRBR350AK9 
20 dBm 
That uses a 50 foot antenna cable 
AIRCAB050LLR 
3.35 dB loss 
And a solid dish antenna 
AIRANT3338 
21 dBi gain 
Has an EIRP of 

37.65 dBm 
The distance that a signal can be transmitted depends on several
factors. The primary hardware factors that are involved are:

Transmitter power

Cable losses between the transmitter and its
antenna

Antenna gain of the transmitter

Localization of the two antennas
This refers to how far apart the antennas are and if there are
obstacles between them. Antennas that can see each other without any obstacles
between them are in line of sight.

Receiving antenna gain

Cable losses between the receiver and its antenna

Receiver sensitivity
Receiver sensitivity is defined as the minimum signal power level (in
dBm or mW) that is necessary for the receiver to accurately decode a given
signal. Because dBm is compared to 0 mW, 0 dBm is a relative point, much like 0
degrees is in temperature measurement. This table shows example values of
receiver sensitivity:
dBm

mW

10 
10 
3 
2 
0 
1 
3 
0.5 
10 
0.1 
20 
0.01 
30 
0.001 
40 
0.0001 
50 
0.00001 
60 
0.000001 
70 
0.0000001 
The receiver sensitivity of the radios in Aironet products is
84 dBm or 0.000000004 mW.
Cisco has an
Outdoor
Bridge Range Calculation Utility to help determine what to expect from
an outdoor wireless link. Because the outputs of the calculation utility are
theoretical, it is helpful to have some guidelines on how to help counteract
outside factors.

For every increase of 6 dB, the coverage distance
doubles.

For every decrease of 6 dB, the coverage distance is cut in
half.
In order to make these adjustments, choose antennas with higher (or
lower) gain. Or use longer (or shorter) antenna cables.
Given that a pair of Aironet 350 Bridges (with 50 feet of cable that
connects to a dish antenna) can span 18 miles, you can modify the theoretical
performance of that installation:

If you change to 100foot cables instead of 50foot (which adds 3 dB
of loss on each end), the range drops to 9 miles.

If you change the antenna to 13.5dBi yagis instead of the dishes
(which reduces gain by 14 dBi overall), the range drops to less than 4 miles.
There is no antenna calculation utility for indoor links. Indoor RF
propagation is different than outdoor propagation. However, there are some
quick calculations that you can do in order to estimate performance.

For every increase of 9 dB, the coverage area doubles.

For every decrease of 9 dB, the coverage area is cut in half.
Consider the typical installation of an Aironet 340 Access Point (AP)
with the rubber ducky 2.2dBi dipole antenna. The radio is approximately 15
dBm. If you upgrade to a 350 AP and replace the rubber duckies with a highgain
omnidirectional antenna that is rated at 5.2 dBi, the range nearly doubles.
The increase in power from a 340 AP to a 350 AP is +5 dBi. And the antenna
upgrade is +3 dBi, for a total of +8 dBi. This is close to the +9 dBi that are
required to double the distance.