Why do we have magnetices with RJ45

[gpuguy]

What is the purpose of having magnetics with RJ45. The following question talks about it but not the why part:

https://www.edaboard.com/threads/229510/

I was looking at this board when I noticed this (see bottom right)

 

[betwixt]

In a local network there may be hundreds of connections and because they are physically apart their local ground (Earth) potentials may be different. Joining them together with the network cable could cause power line currents to flow down the network cable and in extreme cases, damage the ports on connected machines. The 'magnetics' are a small isolating transformer, it provides a barrier to the direct flow of power in or out of the RJ45 while still allowing the network data to flow normally. It has other functions too, such as impedance matching and DC separation so the 'connected' indicator can work.

Brian.

 

[gpuguy]

In a local network there may be hundreds of connections and because they are physically apart their local ground (Earth) potentials may be different. Joining them together with the network cable could cause power line currents to flow down the network cable and in extreme cases, damage the ports on connected machines. The 'magnetics' are a small isolating transformer, it provides a barrier to the direct flow of power in or out of the RJ45 while still allowing the network data to flow normally. It has other functions too, such as impedance matching and DC separation so the 'connected' indicator can work.

Brian.

Ok. But the doubt now is. Let's say I is the information (the useful data). And N is the noise because of the offset voltage. How does the transformer distinguishes between the tow.

 

[betwixt]

The signal is differential. Noise 'N' will be on both wires, that's why they are twisted in the cable, it's to ensure that as closely as possible they are exposed to the same noise sources. In the magnetics, the input windings are arranged so that signal common to both wires will cancel out and the differential signal will add. Call the signals A and -A, (A + N) - (-A + N) = 2A.

Brian.