Communication is also an important activity, as most of the organizations now are widespread and we need to have the data communicated, else we need to maintain separate data files at different locations for the same data. Thus, to answer the problems, the communication systems have their birth.

We can take data communication as a process of sending data electronically from one Geographic Location to other, which is done usually by linking these locations using a network.

This networking, not only helps the data communications, but also permits one to share the available resources (information), update and analyze it. Computers to communicate are either linked through a direct cable, if they are in same premises, or using any other of networking, if they are wide spread, usually telecommunication lines are used for networking.

Here, the data transmitted, as a continuous wave from one terminal to other. For example, the telephone system use analog data transmission, where the signals are transmitted in a continuous wave form, the following figure illustrates Analog Data Transmission.

This method of data transmission, uses distinct “on” and “off” electricity states. The data is represented as “presence” i.e.,. “On” and “absence” i.e. “off” of electric pulses. These are represented using 1 and 0 respectively. The Digital Data Transmission is faster, when compared to analog. As most of the communication lines i.e. telephone lines, do work on analog data transmission, we use modem, an instrument, which converts analog data into digital form.

Data in computer system exists as digital pulses, where as telephone lines are made to transmit analog pulses. When the data has to be transmitted through telephone lines, between computers, we need to do the conversion of data formats.

Modulation: Before data transmission from one computer to other, the data has to be converted from digital to analog, if telephone lines are used. The Process of Converting Digital Data, before transmission into analog data is known as modulation. We use the modem to this process.

Demodulation: The receiving computer has to reconvert the analog data received into digital form, which again is done by the modem.  This process of reconverting an analog signal into digital pulses is called demodulation.  The device which does the Modulation and Demodulationgets the name Modem, which signifies, the work.

For data transmission, various methods are used, the following are fine methods used:

This method of data transmission involves blocks of characters that are transmitted at regular time sequences. Each of the blocks of characters is marked with synchronization characters. The receiving end, accepts the data block, till it detects the ending character or after a predetermined number of characters are received, indicating end of the message. This transmission uses the faster channels and is used when the data transfer requirements exceed several thousand bits per second. Synchronous transmission is adapted in direct to Computer Communication for large computer systems because of the requirements for high speed data transfer.

For error detection, in data transmission, Synchronous Transmission uses a special method known as Cyclic Redundancy Check (CRC). This method involves, application of an algorithm to the data blocks before they are transmitted, the CRC, identifiers, the result of the computation is added to the data block. On the receiving end, the same algorithm is used and the results area compared with the CRC received, if they are matching, it implies that the data is transmitted error free, else error has occurred.

This Method of Data Transmission involves transmission of data, character by character at a time. These characters are transmitted and controlled using start and stop bits. Every character is enclosed between start and stop bits. At the receiving end, these characters synchronize to complete the data transfer. This transmission is usually slow, but it less expensive method of data transmission.