Data Transfer

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November 13th, 2009 admin

Data Transfer, Data transmission, digital transmission or digital communications is the physical transfer of data (a digital bit stream) over a point-to-point or point-to-multipoint transmission medium. Examples of such media are copper wires, optical fibers, wireless communication media, and storage media. The data is often represented as an electro-magnetic signal, such as an electrical voltage signal, a radiowave or microwave signal or an infra-red signal.

While analog communications represents a continuously varying signal, a digital transmission can be broken down into discrete messages. The messages are either represented by a sequence of pulses by means of a line code (baseband transmission), or by a limited set of analogue wave forms (passband transmission), using a digital modulation method. According to the most common definition of digital signal, both baseband and passband signals representing bit-streams are considered as digital transmission, while an alternative definition only considers the baseband signal as digital, and the passband transmission as a form of digital-to-analog conversion.

Data transfer may be digital messages originating from a data source, for example a computer or a keyboard. It may also be an analog signal such as a phone call or a video signal, digitized into a bit-stream for example using pulse-code modulation (PCM) or more advanced source coding (data compression) schemes. This source coding and decoding is carried out by codec equipment.

Distinction between data tranfer related subjects

Courses and textbooks in the field of data transmission as well as digital transmission and digital communications have similar content.

Data Transfer, digital transmission or data transmission belongs to telecommunications and electrical engineering. Data transmission may also be covered within the subject of tele transmissions, which also includes computer networking or computer communication applications and networking protocols, for example routing, switching and process-to-process communication. Although the Transmission control protocol (TCP) involves the term “transmission”, TCP and other transport layer protocols are typically not discussed in a textbook or course about data transmission.

The term data communication involves analog as well as digital transmission. In most textbooks, the term analog transmission only refers to the transmission of an analog message signal (without digitization) by means of an analog signal, either as a non-modulated baseband signal, as a passband signal using an analog modulation method such as AM or FM, or as an analog-over-analog pulse modulatated baseband signal. In a few books, analog transmission also refers to passband transmission of bit-streams using digital modulation methods such as PSK and ASK. Note that the latter is covered in textbooks named digital transmission or data transmission, for example.

Asynchronous and synchronous data transfer

Asynchronous data transfer 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 data transfer 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.

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