Two important factors in ethernet communication(AKA manchester coding):

1) Zero Reference voltage on the data line
2) Necessity to sync the send and recieve clocks

Manchester is what most NICs use to transmit the electrical information down the line. True manchester coded data consists of the following:

1) A bit "cell"
2) Either a HighToLow or LowToHigh transition within EACH bit "cell"
3) A HighToLow pulse in a bit "cell" consists of a change from +.7v to -.7v. This keeps zero reference voltage on the line (no capacitance problems..longer distance). A LowToHigh is the oposite

Because a wave form transition occurs in each cell at a precise time, the transitions can be used for timing purposes. NICs are required to generate timed data when transmitting only!! Receiving manchester data consists of adjusting to the timing of the sending station.

With that said, the speed that your controller will be interfaced too may become a real issue. If your implimenting a recieving or sending station, your controller must be able to keep up to speed with the NIC.

Hope that helps,
John