Node-to-node data transfer

In telecommunications, node-to-node data transfer is the movement of data from one node of the network to the next. In the OSI model it is handled by the lowest two layers, the data link layer and the physical layer.

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.

Channel coding

See main article Error correction and detection.

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.

Types of channel coding include:

• Parity checks
• Hamming code
• Reed-Muller code
• Reed-Solomon code
• Turbo coding

Examples of source coding are: (see main article source coding / data compression)

• Huffman coding
• morse code
• Binary coding

Examples of line coding include: (see main article line code)

• B8ZS
• HDB3
• 2B1Q
• AMI
• Gray coding

Modulation

See main article Modulation

Modulation is the process of varying a carrier signal, typically a sinusoidal signal, in order to use that signal to convey information. One of the three key characteristics of a signal are usually modulated: its phase, frequency or amplitude.

In digital modulation, the changes in the signal are chosen from a fixed list (the modulation alphabet) each entry of which conveys a different possible piece of information (a symbol). In analogue modulation, the change is applied continuously in response to the data signal.

Modulation is generally performed to overcome signal transmission issues such as to allow

• Easy (low loss, low dispersion) propagation as electromagnetic waves
• Multiplexing — the transmission of multiple data signals in one frequency band, on different carrier frequencies.
• Smaller, more directional antennas

Carrier signals are usually high frequency electromagentic waves.

Examples of modulation include:

• amplitude modulation
• frequency modulation
• Phase-shift keying