|A modem is a machine that translate digitalsignals to analog signals and vica versa. Modem stands forMOdulator/DEModulator.This signal must have a certain look so the modem in the other end knows what in the signal that is a 0 and what is a 1! So clever engineers have come up with diffrent ways to encode the signal so the modems knows the diffrence.
The easiest / simplest one look like this:
Where the +12v signals is 1 and the -12v is 0. Easy isnt it? This encoding system is called NRZ (No Return to Zero) and it use the point of 0v as a reference point.
The biggest problem with the NRZ-encoding is the synchronization, especially at high rates and long bit streams. Imagine a stream with only 0s! You will soon loose track of how many 0s that have beentransfered. And so will the computer (you will problably loose track way sooner than the computer =) ). NRZ have many aftercommers, and many encoding systems have used NRZ asa base for futher developing. Example of this is NRZI and BPRZ.
An other signal is the Manchester Encoding System. This signal systemuse voltage changes to mark 1s or 0s. If there is a change frompositive to negative; its 0 and if there is a voltage change fromnegativ to positive, is 1. Ok, you may ask, if the voltage is already positive and the nextbit is 1? If you add more voltage for every 1... it can go verywrong! Yes, but every bite has a certain time to be transfered. This time units have a start, middle and a stop. Just like an hour: The hour between 1 and 2 starts exactly at 1:00 and stops right before 2:00 andthe middle is 1:30. But I dont need to explain that for you! Thevoltage change that marks 0 or 1 occurs in the middle of this timeunit. The beginning of a time unit is used to correct the voltage so it is possible to have two (or more) 1s in a row without the modems and cables going nuts.
A Manchester code can look like this: (or at least something like this)
That wasnt to hard, was it?
This is two examples of signal enconding. There is many...many more!
This article was originally written by barnseyboy