I want to ask: how PD (Powered device) which run on POE Mode A separate Data and Power from pairs? How can it read data signal when power is on same pairs?
Twisted pair Ethernet is transformer isolated. To allow the signals to pass through the transformers (which block DC and low frequencies). the signal is transmitted at high frequency and coding schemes are used to avoid DC and near DC components in the signal. This applies regardless of whether POE is in use.
POE with power on the signal pairs (POE mode A for 10/100, any POE mode for 1G and higher) seperates the power from the signals in two ways.
Firstly the signal is transmitted as a voltage difference between the two wires in a pair while the power is transmitted as a voltage difference between two pairs.
Secondly the power is DC, which will be blocked by the transformers.
The power is applied and extracted by using transformers with a centre tap on the cable side (the transformers may or may not also have centre taps on the device side for unrelated reasons) and a sufficiently beefy core that the DC currents do not cause magnetic saturation.
Isolated DC to DC converters are used to pass power to and from the Ethernet cable while maintaining isolation (this applies equally to modes A and B).