dBm, dBi and dBd
. . . .

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dBm

dBm (dB) - Unit of comparison (Absolute)

• Decibles milliwatt (dBm) provides a comparison of a signal to 1 milliwatt of power.
• 0 dBm is equal to 1 milliwatt. 1 milliwatt is the reference point.
• dBm is a measurement that is compared to a known value, 1 milliwatt, it is actually a measure of absolute power.
• Decibels (relative) are referenced to 1 milliwatt (absolute), think of a dBm as an absolute assessment that measures change of power referenced to 1 milliwatt.
• Any absolute power measurement of –dBm indicates amplitude less than 1 mW.
• Most 802.11 radios usually ranges from 1 mW to 100 mW.
• A transmission amplitude of 100 mW is equal to + 20 dBm.
• Because of FSPL, received signals will always measure below 1 mW. A very strong received signal is –40 dBm, which is the equivalent of 0.0001 mW (1/ 10,000th of 1 milliwatt).

dBi

dBi (dB) - Unit of comparison (Relative)

• Decibels isotropic (dBi) is the comparison of gain or increase in power of antenna when compared to an isotropic antenna.
• dBi measures the antenna gain. The dBi value is measured at the strongest point, or the focus point, of the antenna signal.

dBd

dBd (dB) - Unit of comparison (Relative)

• Decibels dipole (dBd) is the comparison of gain or increase in power of antenna when compared to an dipole antenna.
• A common antenna used on access points is the half-wave dipole antenna.
• The half-wave dipole antenna is a small, typically rubber-encased, general-purpose omnidirectional antenna. A 2.4 GHz half-wave dipole antenna has a dBi value of 2.14.

Comparing dBi and dBd Antennas

• On the rare occasion that you do run into an antenna measured with dBd, just add 2.14 to the dBd value and you will know the antenna’s dBi value.
• A standard dipole antenna has a dBi value of 2.14.
  dBd -> dBi
• If an antenna has a value of 3 dBd, this means that it is 3 dB greater than a dipole antenna.
• To calculate the value of a dBb antenna to dBi antenna, all you need to do is add 3 to 2.14.
• A 3 dBd antenna is equal to a 5.14 dBi antenna.
  dBi -> dBd
• If an antenna has a value of 6 dBi, this means that it is 6 dB greater than a isotropic antenna.
• To calculate the value of a dBi antenna to dBd antenna, all you need to do is subtract 2.14 from 6.
• A 6 dBi antenna is equal to a 3.86 dBd antenna.

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Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.