Data transmission

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Data transmission is essentially the same thing as digital communications, and implies physical transmission of a message as a digital bit stream, represented as an electro-magnetic signal, over a physical point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibers, wireless communication channels, and storage media.

Data transmission is a subset of the field of data communications, which also includes computer networking or computer communication applications and networking protocols, for example routing and switching.

Contents

[edit] Applications and history

The first data transmission applications in modern time were telegraphy (1809) and teletypewriters (1906). 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.

Data transmission is utilized in computers in computer buses and for communication with peripheral equipment via parallel ports and serial ports such us RS-232 (1969), Firewire (1995) and USB (1996). The principles of data transmission is also utilized in storage media for Error detection and correction since 1951.

Data transmission is utilized in computer networking equipment such as modems (1940), local area networks (LAN) adapters (1964), repeaters, hubs, microwave links, wireless network access points (1997), etc.

In telephone networks, digital communication is utilized for transferring many phone calls over the same copper cable or fiber cable by means of Pulse code modulation (PCM), i.e. sampling and digitalization, in combination with Time division multiplexing (TDM) (1962). Telephone exchanges have become digital and software controlled, facilitating many value added services. For example the first AXE telephone exchange was presented in 1976. Since late 1980th, digital communication to the end user has been possible using Integrated Services Digital Network (ISDN) services. Since the end of 1990th, broadband access techniques such as ADSL, Cable modems, fiber-to-the-building (FTTB) and fiber-to-the-home (FTTH) have become wide spread to small offices and homes. The current tendency is to replace traditional telecommunication services by packet mode communication such as IP telephony and IPTV.

The digital revolution has also resulted in many digital telecommunication applications where the principles of data transmission are applied. Examples are second-generation (1991) and later cellular telephony, video conferencing, digital TV (1998), digital radio (1999), telemetry, etc.

[edit] Protocol layers and theoretical topics

OSI Model
7 Application layer
6 Presentation layer
5 Session layer
4 Transport layer
3 Network layer
2 Data link layer
1 Physical layer

Courses and books in the field of data transmission or digital communications typically deal with the following protocol layers and topics:

Courses and literature in computer networking and data communications typically also deal with the other protocol layers in the seven layer OSI model.

[edit] Types of data transmission

Serial transmission bits are sent over a single wire individually. Whilst only one bit is sent at a time, high transfer rates are possible. This can be used over longer distances as a check digit or Parity bit can be sent along it easily.

Parallel transmission Multiple wires are used which can transmit multiple bits simultaneously, which allows for higher data transfer rates than can achieved than with Serial transmission. This method is used internally within the computer, for example the internal buses, and sometimes externally for such things as printers, The major issue with this is "skewing" because the wires in parallel data transmission have slightly different properties (not intentionally) so some bits may arrive before others, which means that the original "meaning" of the message is lost and thus leads to corruption, a parity bit can help to reduce this. How ever parallel data transmission is therefore unsuitable for long distances (as already mentioned) because skewing is far more likely.

[edit] Types of communication channels

[edit] Asynchronous and synchronous data transmission

Asynchronous transmission uses start and stop bits to signify the beginning bit ASCII character would actually be transmitted using 10 bits e.g.: A "0100 0001" would become "1 0100 0001 0". The extra one (or zero depending on parity bit) at the start and end of the transmission tells the receiver first that a character is coming and secondly that the character has ended. This method of transmission is used when data are sent intermittently as opposed to in a solid stream. In the previous example the start and stop bits are in bold. The start and stop bits must be of opposite polarity. This allows the receiver to recognize when the second packet of information is being sent.

Synchronous transmission uses no start and stop bits but instead synchronizes transmission speeds at both the receiving and sending end of the transmission using clock signals built into each component. A continual stream of data is then sent between the two nodes. Due to there being no start and stop bits the data transfer rate is quicker although more errors will occur, as the clocks will eventually get out of sync, and the receiving device would have the wrong time that had been agreed in protocol (computing) for sending/receiving data, so some bytes could become corrupted (by losing bits). Ways to get around this problem include re-synchronization of the clocks and use of check digits to ensure the byte is correctly interpreted and received.

[edit] References

  • Simon Haykin, "Digital Communications", John Wiley & Sons, 1988. ISBN-13: 9780471629474

[edit] See also