When power is measured in watts or milliwatts, it is considered to be an absolute power
measurement. In other words, something has to measure exactly how much energy is
present in the RF signal. This is fairly straightforward when the measurement is taken at
the output of a transmitter because the transmit power level is usually known ahead of
time.
Sometimes you might need to compare the power level between two different transmitters.
For example, suppose that device T1 is transmitting at 1 mW, while T2 is transmitting
at 10 mW, as shown in Figure 1-13 . Simple subtraction tells you that T2 is 9 mW
stronger than T1. You might also notice that T2 is 10 times stronger than T1.