Because a dB has no particular defined reference, the dBx, where x
represents a specific value, is often used instead of the dB. For example, the
dBm is referenced to the milliwatt. Since the dBm has a defined reference, it
can also be converted back to watts, if desired. The power gain or loss in a
signal is determined by comparing it to this fixed reference point, the
milliwatt. The following related terms must be understood in order to properly
design and install WLANs:

-
dB milliWatt (dBm) – This is the unit of measurement for signal
strength or power level. If a person receives a signal at one milliwatt, this
is a loss of zero dBm. However, if a person receives a signal that is 0.001
milliwatt, then a loss of 30 dBm occurs. This loss is represented as -30 dBm.
To reduce interference with others, the 802.11b WLAN power levels are limited
to the following:
- 36 dBm EIRP by the FCC
- 20 dBm EIRP by ETSI
-
dB dipole (dBd) – This refers to the gain an antenna has, as
compared to a dipole antenna at the same frequency. A dipole antenna is the
smallest, least gain practical antenna that can be made.
-
dB isotropic (dBi) – This refers to the gain a given antenna has, as
compared to a theoretical isotropic, or point source, antenna. Unfortunately,
an isotropic antenna cannot exist in the real world, but it is useful for
calculating theoretical coverage and fade areas.
- A dipole antenna has 2.14 dB gain over a 0 dBi isotropic antenna. For
example, a simple dipole antenna has a gain of 2.14 dBi or 0 dBd.
-
Effective Isotropic Radiated Power (EIRP) – EIRP is defined as the
effective power found in the main lobe of a transmitter antenna. It is equal to
the sum of the antenna gain, in dBi, plus the power level, in dBm, into that
antenna.
-
Gain – This refers to the amount of increase in energy that an
antenna appears to add to an RF signal. There are different methods for
measuring this, depending on the chosen reference point. Cisco Aironet wireless
is standardized on dBi to specify gain measurements. Some antennas are rated in
dBd. To convert any number from dBd to dBi, simply add 2.14 to the dBd number.