Node-to-node data transfer
In digital telecommunications, channel coding is a pre-transmission mapping applied to a digital signal or data file, usually designed to make error-correction (or at least error detection) possible.
Error correction is implemented by using more digits (bits in case of a binary channel) than the number strictly necessary for the samples, and having the receiver compute the most likely valid message that could have resulted in the received one.
Channel coding should not be confused with source coding, which is the elimination of redundancy in order to make efficient use of storage space and/or transmission channels.
Channel coding should also not be confused with line coding, which is the coding performed in order to adapt the transmitted signal to the (electrical) characteristics of a transmission channel.
In most communication systems, the transmitting point applies source coding, followed by channel coding, and lastly, line coding. This produces the baseband signal. Some systems then use modulation to multiplex many baseband signals into a broadband signal. (The reciever un-does these transformations in reverse order: demodulation, trellis decoding, error detection and correction, decompression). Some communication systems omit one or more of these steps, or use techniques that combine several of these steps together. For example, a Morse code transmitter combines source coding, channel coding, and line coding into one step, typically followed by an amplitude modulation step. Barcodes, on the other hand, add a checksum digit during channel coding, then translate each digit into a barcode symbol during line coding, omitting modulation.
Types of channel coding include:
Examples of source coding are: (see main article source coding / data compression)
Examples of line coding include: (see main article line code)
Examples of modulation include: (see main article modulation)