Data can be transmitted through communication channels in one of two forms: analog or digital. Transmission of data in continuous wave form is referred to as analog transmission. An analog transmission can be likened to the waves created in a pan of still water when a stick is inserted. By sending "waves" down a wire electronically, messages are sent and received. In the past, analog transmission was the major means of relaying data over long distances. This was due largely to the type of telephone and telegraph lines provided by American Telephone and Telegraph (AT&T). Digital transmission, on the other hand, involves the transmission of data as distinct on and off pulses. Digital transmission tends to be faster and more accurate than analog transmission. The figure below illustrates the concepts of digital and analog transmission.
Analog transmission requires that the sender convert the data from the digital form in which it is stored to wave form before transmitting it. This conversion process is called modulation. The opposite conversion (from wave to digital form) is required at the receiving end before the data is understood by the computer. This conversion is called demodulation. Both modulation and demodulation are accomplished by devices called modems or data sets. The term modem is derived from the terms modulation and demodulation.
There are three types of modems: (a) acoustic coupler, (b) direct connect, and (c) internal. An acoustic-coupler modem is linked to a terminal and has a special cradle that holds a standard telephone handset (see picture below).
The modem processes audible analog tones that pass through the receiver, thus the term acoustic coupler. A direct-connect modem connects a computer directly to a telephone line (see picture below).
An internal modem consists of a circuit board that is plugged into the internal circuitry of the computer. This type of modem also connects a computer directly to a telephone line (see picture below).
Since the computer stores data in an on/off (digital) form, when digital transmission is used there is no need to convert data from wave to digital form. This reduces the time required to send messages and eliminates the data errors that frequently occur in the conversion process. Users can transmit large amounts of data faster and more reliably with digital transmission.
Last Updated Jan.7/99