Introduction to Data Communications
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9. Line Encoding

The previous chapter looked at the types of physical media that are used to transport the data. In transmitting data, there must be a method of representing the digital logic levels using the physical attributes associated with the media.

As the length of the media increases and the transfer rate (line speed) increases, new problems of corruption of data appear at the receiving end. This is due to the physical characteristics and limitations of the media itself. Several methods of representing the data on the media have been developed to address these problems - some more successful than others. It is not a trivial solution as it has taken years of development to reach the current state.

Line encoding is the method used to represent the digital information on the media. A pattern, that uses either voltage or current, is used to represent the 1s and 0s of the digital signal on the transmission link. This chapter discusses the methods that are used most often in data communications

Common types of line encoding methods used in data communications are:

• Unipolar line encoding
• Polar line encoding
• Bipolar line encoding
• Manchester line encoding.

9a. Unipolar Encoding

Unipolar line encoding works well for inside machines where the signal path is short but is unsuitable for long distances due to the presence of stray capacitance in the transmission medium. On long transmission paths, the constant level shift from 0 volts to 5 volts causes the stray capacitance to charge up. There will be a "stray" capacitor effect between any two conductors that are in close proximity to each other. Parallel running cables or wires are very suspectible to stray capacitance.

If there is sufficient capacitance on the line and a sufficient stream of 1s, a DC voltage component will be added to the data stream. Instead of returning to 0 volts, it would only return to 2 or 3 volts! The receiving station may not recognize a digital low at voltage of 2 volts!

Unipolar line encoding can have synchronization problems between the transmitter and receiver's clock oscillator. The receiver's clock oscillator locks on to the transmitted signal's level shifts (logic changes from 0 to 1). If there is a long series of logical 1s or 0s in a row. There is no level shift for the receive oscillator to lock to. The receive oscillator's frequency may drift and become unsynchronized. It could lose track of where the receiver is supposed to sample the transmitted data!

Receive oscillator may drift during the period of all 1s

Introduction to Data Communications
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