What is the difference between dB and dBm? Provide an example of usage.

dB and dBm are both units of measurement used to express the power of a signal, but they are based on different reference points.

dB is admentional unit used to make a comparison between 2 levels of power, voltage or current  dB=10 log (Pa/Pb).

dBm is a unit of any level of power referred to 1mW, in another word:                       dBm = 10log (P(mW)/1 mW) 

It is used forquantifying the ratio between two values. The best example of this is the signal-to-noise ratio.

0dB= 30dbm

dB is the unit of power in Watt

dBm is the unit of power in mWatt

dB vs. dBm. Decibel (dB) and dB relative to a milliwatt (dBm) represent two different but related concepts.The unit dBm denotes an absolute power level measured in decibels and referenced to 1 milliwatt (mW).

Whenever tests are performed on fiber optic networks, the results are displayed on a meter readout in “dB.” Optical loss is measured in “dB” while optical power is measured in “dBm.” Loss is a negative number (like –3.2 dB) as are most power measurements.

db and dbm r differnt but they r relative.db is the ratio of two values.dbm is the absolute power level in db's in milliwatt

Decibel (dB) and dB relative to a milliwatt (dBm) represent two different but related concepts. A dB is a shorthand way to express the ratio of two values. ... The unit dBm denotes an absolute power level measured in decibels and referenced to 1 milliwatt (mW)

Decibel (dB) and dB relative to a milliwatt (dBm)

dBm value=(dB value+)

because dBm is reference to milli 

dB is a measurement unit to quote the signal strength of a strong signal where dBm is a smaller unit , usually used with the prefix milli