Line code

In telecommunication, a line code is a code chosen for use within a communications system for transmitting a digital signal down a line. Line coding is often used for digital data transport. Some line codes are digital baseband modulation or digital baseband transmission methods, and these are baseband line codes that are used when the line can carry DC components.

Line coding consists of representing the digital signal to be transported, by a waveform that is appropriate for the specific properties of the physical channel (and of the receiving equipment). The pattern of voltage, current or photons used to represent the digital data on a transmission link is called line encoding. The common types of line encoding are unipolar, polar, bipolar, and Manchester encoding.

For reliable clock recovery at the receiver, one usually imposes a maximum run length constraint on the generated channel sequence, i.e., the maximum number of consecutive ones or zeros is bounded to a reasonable number. A clock period is recovered by observing transitions in the received sequence, so that a maximum run length guarantees such clock recovery, while sequences without such a constraint could seriously hamper the detection quality.

After line coding, the signal is put through a "physical channel", either a "transmission medium" or "data storage medium". Sometimes the characteristics of two very different-seeming channels are similar enough that the same line code is used for them. The most common physical channels are:

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