Digital Line Coding
Digital Line Coding is a special coding system chosen to allow transmission to take place in a communications system. The chosen code or pattern of voltage used to represent binary digits on a transmission medium is called line encoding.
The types of line encoding are polar, unipolar and bipolar.
The polarity of a code can be positive or negative or in certain cases an idle line can be incorporated into the code.
Line encoding takes place at the physical layer of the OSI model.
Click on this link to review the wikipedia resource on digital line encoding.