Phase Encoding of Signals

To be able to detect collisions on LANs, we need to understand how digital signals are encoded on the physical medium.

A digital signal is a sequence of discrete, discontinuous pulses, or signal elements.

If all signal elements have the same (voltage) sign, they are termed unipolar.

The modulation rate of a LAN is the number of signal elements (transitions) per second, or the baud rate.

To correctly interpret a signal, the receiver must know the length (time) of each signal element and the expected voltage levels of the bits 0 and 1.

The two simplest encoding schemes are the:

• Non-return to Zero Level (NRZ-L) and
• Non-return to Zero Inverted (NRZ-I).

NRZ-I is an example of differential encoding in which the signal transition, rather than the signal level, indicates the value of each bit.

See also: Sound of the dialup modem explained.