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Networking Fundamentals · Intermediate

Decibel (dB)

A logarithmic unit expressing the ratio between two power levels; dBm expresses absolute power relative to 1 milliwatt.

Definition

A 3 dB loss halves power and a 10 dB loss reduces it to one tenth. Optical transceivers specify transmit and receive power in dBm, for example -3 dBm transmit and -14 dBm minimum receive.

Example

An optical power meter reading of -20 dBm at the receiver indicates very low light.

Exam tip

Every 3 dB is a factor of 2 in power; every 10 dB is a factor of 10.

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