Using a process called modulation; a modem transforms the computer’s digital signals into analog tones that can be conveyed through the telephone system. On the receiving end, the process used is demodulation, in which the other modem transforms this signal from analog back to digital. Modems can play both roles, modulation and demodulation, which is where the name modems, comes from: It’s short fro modulator/demodulator.
Two types of modems are available: internal and external. An internal modem is designed to fit in one of your computer’s expansion slots. It gets its power from the computer’s expansion bus. An external modem has its own case and power supply. For this reason external modems are slightly more expensive.
Modems use a method of networking called asynchronous communication. In this method, data is sent one bit at a time, in a series (one after the other). So that the receiving computer can tell where one byte ends and the next one begins, start bits and stop bits are added to the data. This networking method is called asynchronous because the start and stop bits eliminate the need for some kind of synchronization signal. In synchronous communication, data exchange requires a synchronization signal that identifies the units of data being exchanged.
To establish communications, modems must conform to standards called modulation protocols. These protocols, set by international standards organizations, ensure that your modem can communicate with another modem, even if the second modem was made by a different manufacturer.
Several modulation protocols are in common use. Each protocol specifies all the necessary details of communication, including the data transfer rate. This is the rate by which the two modems can exchange data. The rate is measured in bits per second (bps). You may encounter the term baud rate when a modem’s data transfer rate is discussed, but the technical definitions of baud rate and bps rate differ. The correct measurement of a modem’s data transfer rate is the bps rate. The baud rate is the maximum number of changes that can occur per second in the electrical state of a communications circuit.
Modem protocols are governed by the International Telecommunications Union (ITU). The most recent modulation protocol, called V.90, enables modems to communicate at a maximum rate of 56 Kbps. (In practice, V.90 modems rarely achieve speeds higher than 42 Kbps.) The protocol also includes standards for data compression and error checking. The previous protocol, V.34, enables a rate of 28800 bits per second. An earlier standard, V.32 bis, established a rate of 14400 bps. Standards before V.32 regulated communication at 9600, 2400, 1200, and 300 bps.
Two modems can communicate only if both follow the same modulation protocol. If your modem follows the V.34 protocol, your modem cannot communicate at 28800 bps unless the modem on the other end observes the same protocol. Most modems, however, can fall back to a lower rate. When a modem attempts to establish a connection, it automatically negotiates with the modem on the other end. The two modems try to establish which protocols they share, and then they use the fastest rate that both modems have. If a computer with a 9600 bps modem is connected to a computer with a 14400 bps modem, data is transferred between the two computers at 9600 bps.
Facsimile transmission – or Fax, as it is popularly known – enables you to send an image of a document over the telephone lines to anyone who has a fax machine. The sending fax machine makes a digital image of the document. Using a built-in modem, the sending fax machine converts the image to an analog representation so that it can be transmitted through the analog telephone system. The receiving fax machine converts the analog signals to digital signals, converts the digital signals to an image of the document and prints that image. Fax usage didn’t take off until the ITU established modulation protocol standards for facsimile transmission. These standards differ from modem protocols. Fax protocols govern transmission at 9.6 Kbps (V.29) and 14.4 Kbps (V.17).
Some modems, called fax modems support fax as well as data modulation protocols. If your computer has a fax modem, you can send and receive faxes from your computer instead of a fax machine. However, you’ll need a scanner too if you want to fax something that’s printed or sketched on paper.