db and dbm

What is the difference between db and dbm?
 dB is used for  expressing the ratio between two power values. 
dbm is used for  expressing an absolute value of power.
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examples:
If P1=1,000,000,000,000 and P2=1 then
P1/P2=1,000,000,000,000 /1
This result looks very long. In short this can be written as : 10 * log(1,000,000,000,000 /1) = 10* 9=90db
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dbm:
dBm denotes  absolute power level measured in decibels and referenced to 1 milliwatt (mW). 
To convert from absolute power "P" (in watts) to dBm, the formula  is dBm = 10*log (P/1 mW). 


0dbm = 1 milliwatt
20 dbm=100 milli watt
-20dbm=0.01 milli watt
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