I have few questions about transformer operation.
One of the main points of the transformer is ground isolation. 

Let's say if have a transformer in an ethernet transmitter, and my circuit in the primary of the transformer runs at 5 volts. 

*First quesiton:* What is going to be the voltage on the secondary terminals of the transformer?

*Second question:* Will we be also transmitting a ground reference to the receiving end? Or will it will just be a differential signal?

*Third question:* Do we always need a reference voltage when we send a differential signal? If not, then how does the receiving end know the reference voltage?