Examples of such channels are copper wires, optical fibers, wireless communication using radio spectrum, storage media and computer buses.
[7] Data (mainly but not exclusively informational) has been sent via non-electronic (e.g. optical, acoustic, mechanical) means since the advent of communication.
However, the first data electromagnetic transmission applications in modern time were electrical telegraphy (1809) and teletypewriters (1906), which are both digital signals.
The fundamental theoretical work in data transmission and information theory by Harry Nyquist, Ralph Hartley, Claude Shannon and others during the early 20th century, was done with these applications in mind.
[8][9] Donald Davies invented and implemented modern data communication during 1965-7, including packet switching, high-speed routers, communication protocols, hierarchical computer networks and the essence of the end-to-end principle.
[10][11][12][13] Baran's work did not include routers with software switches and communication protocols, nor the idea that users, rather than the network itself, would provide the reliability.
The principles of data transmission are also utilized in storage media for error detection and correction since 1951.
Since the end of the 1990s, broadband access techniques such as ADSL, Cable modems, fiber-to-the-building (FTTB) and fiber-to-the-home (FTTH) have become widespread to small offices and homes.
The current tendency is to replace traditional telecommunication services with packet mode communication such as IP telephony and IPTV.
The data are represented as an electromagnetic signal, such as an electrical voltage, radio wave, microwave, or infrared light.
Digital serial transmissions are bits sent over a single wire, frequency or optical path sequentially.
Because it requires less signal processing and less chances for error than parallel transmission, the transfer rate of each individual path may be faster.
Timing skew can be a significant issue in these systems because the wires in parallel data transmission unavoidably have slightly different properties so some bits may arrive before others, which may corrupt the message.