Line code
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In telecommunication, a line code is a code chosen for use within a communications system for transmission purposes.
For digital data transport line coding is often used. Line coding consists of representing the digital signal to be transported, by an amplitude- and time-discrete signal, that is optimally tuned for the specific properties of the physical channel (and of the receiving equipment). The waveform pattern of voltage or current used to represent the 1s and 0s of a digital signal on a transmission link is called line encoding. After line coding, the signal can directly be put on a transmission line, in the form of variations of the current. The common types of line encoding are unipolar, polar, bipolar and Manchester encoding.
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 receiving end, and thus to a higher decoding error probability.
Line 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 received data.
It is also preferred for the line code to have a structure that will enable error detection.
Note that the line coded signal and a signal produced at a terminal may differ, thus requiring translation.
A line code will typically 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.
Common line codes
- AMI
- 2B1Q
- 4B5B
- 4B3T
- B8ZS
- Carrier Amplitude and Phase
- HDB3
- NRZ - Non-return-to-zero
- NRZI - Non-return-to-zero, inverted
- Manchester code
- MLT-3 Encoding
- Modified AMI
- RZ - Return-to-zero
- Surround by complement (SBC)
- TC-PAM
- Trellis code