At the physical layer, the hardware is actually turning data into signals. The receiving computer must understand how long a bit lasts. This might seem silly, but if the computer doesn't know how long one bit is, it can't determine how many bits have gone by as it's reading the wire. If a computer reads the wire too long, it gets a pattern it can't understand, or misinterprets.
Return to Zero is a bit more elegant. Protocols are often called "return to zero" (RZ) or "non-return to zero" (NRZ).
Return to Zero protocols usually use a "high" signal, a "low" signal and a "zero state" to signal data, eliminating the need for separate signal circuits and extra logic in the chipset. When a company manufactures a computer product, the simpler a system is, the cheaper it is to make.
In an electrical system, a Return to Zero signalling pattern would look something like this if you graphed it over time. Notice that in the graph, every time a zero or a one is signalled on the wire, the voltage changes from zero volts to another state, either positive or negative voltage. This return to zero is used to mark off the timing for each bit so that a computer on one side can transmit 111 and the computer on the other side won't think it received 11 or 1111.
RETURN TO ZERO
When computers communicate, they recieve by "listening" to the wire. Let us say that -3 volts is used to signal a binary "1" and +3 volts is used to signal a binary "0". As shown above, the voltage state returns to zero voltage between each bit.
When the computer reads the wire at time points 0 - 7, it sees the following voltages: +3, 0, -3, 0, -3, 0,+3, 0.
Out of eight readings of the wire, we only managed to pass four bits! This method clearly halves the data rate by returning the electrical signal to zero after every bit. As you will see later, there are more efficient ways of doing things.
- RZ control logic chips are simpler
- RZ control logic chips are cheaper (because of #1)
- The transmitting computer needs only one electrical circuit to transmit the data (one pair of wires).
RZ systems are less data-efficient than NRZ signalling methods. Signal bandwidth is cut in half because half of the bandwidth is used to return the signal to zero volts, during which no data is transmitted.