A line coded signal must not contain a DC-component (the average signal level should be 0), because transport of a DC-component is impossible in case of long wires. An included DC-component would therefore lead to an offest at the recieving end, and thus to a higher decoding error probability.
Secondly, lineLine coding should make it possible for the receiver to synchronise itself to the [[phase]] of the received signal. If the synchronisation is not ideal, then the signal to be decoded will not have optimal differences (in amplitude) between the various digits or symbols used in the line code. This will increase the error probability in the recieved data.
Thirdly, itIt is also preferred for the line code to have a structure that will enable error detection.
Note that athe line codecoded maysignal differand froma the codesignal generatedproduced at a user [[terminal (telecommunication)|terminal]] may differ, and thus may requirerequiring translation.
A line code will generallytypically reflect technical requirements of the [[transmission medium]], such as [[optical fiber]] or [[shielded twisted pair]]. These requirements are unique for each medium, because each one has different behavior related to interference, distortion, capacitance and loss of amplitude.
