Signal Encoding in Ethernet / 802.3
If two circuits on a single circuit board were exchanging bits they could simply raise and lower a voltage to represent ‘1’s and ‘0’s. They would know the exact moment to raise or lower the voltage because the two circuits would share a common clock pulse from a trace on the circuit board. At the pulse of the clock one circuit would raise the voltage and the other circuit would know to expect the voltage raise at the same moment.
When two circuits are sending bits to each other across a network they need a mechanism to synchronize their clocks. In this way they both know when a bit starts and when it stops.
The process of sending a clock pulse along with a data signal is referred to as a ‘Manchester’ signal encoding scheme. Ethernet and IEEE 802.3 networks represent their data differently than do Token-Ring networks but they both use the same clocking scheme.
The clock signal is a constant pulse. On an Ethernet or 802.3 network the signal is present only when a frame is being transmitted. Otherwise the cable is electrically ‘0’. On a Token-Ring / 802.5 network there is a constant clock signal generated by a station called the ‘Active Monitor’.
A receiving station synchronizes its own clock with the clock signal in the received signal. In this way it knows when a bit starts and when it ends. The signal that is occurring at this ‘start’ of a bit determines whether the bit will be a ‘1’ or a ‘0’.
Ethernet and 802.3 describe a ‘1’ as a LOW VOLTAGE at the beginning of a bit time and a ‘0’ as a HIGH VOLTAGE at the beginning of a bit time. Consequently, a ‘1’ transitions from LOW to HIGH in the middle of the bit-time and a ‘0’ transitions from HIGH to LOW in the middle of the bit-time to create the clock pulse.
Comparison to Token-Ring Signaling
Token-Ring / 802.5 uses the same clock signal transition in the middle of the bit-time but the encoding of the data is done by including, or not including
another transition at the beginning of the bit-time. The spec describes a ‘1’ as the ABSENCE OF A SIGNAL TRANSITION at the beginning of a bit time and a ‘0’ as
a SIGNAL TRANSITION occurring at the beginning of a bit time. The transition can be from HIGH to LOW or from LOW to HIGH, it doesn't matter. (The direction of
the clock transition in the middle of the bit-time also doesn't matter.)