As we saw in the Return-To-Zero (RZ) page, you can indicate clocking at the bit level on a transmission medium by returning the signal to a zero state between each bit. If we take out the return to zero requirement, then we can get twice as much data through, right?

Take a look at the graph below. At first, it looks like we actually get eight bits through the sytem in eight reads of the wire! But think again. How does the remote computer know where one bit leaves off and the other begins? Easy enough while the bits are flipping between one and zero, but what about when the signal transmits a series of zeroes or ones? The transmitting station knows exactly what it sent, but the reciever hasn't got a clue! Here's an example of what this would look like to the receiving station:

+3 Volts ___ ___ ________ | | | | 0 Volts | | | | |___| |___________| -3 Volts ________________________________________________ TIME: | | | | | | | | 0 1 2 3 4 5 6 7ooking at the graph above, can YOU tell what is zero, and what is one? How many zeroes and ones are getting transmitted? Tough one, isn't it?

To fix this problem we alter the data pattern that is transmitted and make sure both sides understand the pattern that is going to be used ahead of time when we build the logic chips.

Let's say that instead of always using +3 volts for a binary "1" and -3 volts for a binary "0", we could alternate the voltage when the data pattern doesn't change, and leave the voltage the same when the data pattern IS changing. By sending a specific pattern of data we could indicate what the bits are.

Have a look at this graph of how this voltage alternating pattern would work.

0 1 1 1 1 0 0 1 +3 Volts _______ ___ _______ | | | | | | | | -3 Volts |___| |_______| ________________________________________________ TIME: | | | | | | | | 0 1 2 3 4 5 6 7

First, we are able to send eight bits where we were only able to send four
using RZ. Second,
by alternating the voltage when the bits *don't* change, we are able
to indicate where one bit stops and where another bit begins. If we send the
same pattern of ones and zeroes over the line every time we transmit (let's
call this a "preamble") then the remote station should be able to
figure out the timing on the line from the data stream without wasting bandwidth
by returning the signal to zero between bits. This increases data transmission
efficiency by utilizing almost twice as many cycles for transmitting data over
the Return To Zero method.

Of course, this also means that the communications logic chips have to be more complicated and thus, more expensive, but they don't require a second control channel as do some of the other electrical communication systems.

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