The Evolution of The Modem

The Evolution of The Modem

The word modem is an acronym for Modulator Demodulator. Modems accept digital data supplied by the personal computer, and convert it to a modulated analog waveform that can be transmitted over an analog telephone line. Modems also work in reverse. They take a modulated analog wave from the phone line, then convert it to a digital form, then pass it onto the personal computer (PC). Modems, at connection, send tones to each other to negotiate the fastest mutually supported modulation method that will work over whatever quality of line has been established for that call. There are two main differences in the types of modems for the PC, internal and external.

Modem data transfer rates have gone from 300bps to 1,200bps to 9,600bps to 14.4kbps to 28.8kbps and to 33.6kbps. Now new modem standards are emerging, reaching speeds of 56kbps. Viewing graphics or downloading sound files improves dramatically with 56kbps. The experts say that the next major improvement will have to come from the telephone companies, with the fiber optic cables so we can have an integrated services digital network (ISDN). Digital connections are more efficient because of the frequent disconnects and modem freezing of the analog modem. These errors can be caused by line noise, which can come from many sources such as lightning storms, sunspots, and other electromagnetic phenomena. Modems will automatically reduce their speed to avoid introducing data errors.

The transmission of speed sometimes causes confusion. The root of this problem seems to stem from the term's baud and bits are used interchangeably. A baud is a change in a signal from positive to negative or vice-versa that is used as a measure of transmission speed and bits per second is a measure of the number of data bits (digital 0's and 1's) transmitted each second in a communications channel.

Synchronous and Asynchronous data transfer are two methods of sending data over the telephone line. In synchronous data transmission, data is sent via a bit stream, which sends a group characters in a single stream. In order to do this, modems gather groups of characters into a buffer, where they are prepared to be sent as such a stream. In order for the stream to be sent, synchronous modems must be in perfect synchronization with each other.