Modem
From ArticleWorld
A modem is a device that converts signals of digital data into their analogue form to make them suitable for transmission over analogue telecommunication systems. These devices receive and demodulate signals that are initially in a modulated form. They are used to support various forms of data communication such as email and facsimile.
Types of modems
Modems can be either voiceband or cable:
- Voiceband modems use telephone lines to communicate and operate around the narrow bandwidth conditions of the human voice, hence the name.
- Cable modems use wider bandwidth cables and fibre optic channels.
Standards used by modems
Modems communicate between data machines with the help of protocols which should match at both the reception and transmission ends. The International Telecommunication Union's Telecommunication standardisation sector has set worldwide recommendations for voiceband modems, which operate using telephone lines. The standards can be full-duplex or half-duplex, depending on whether the communication is simultaneously from both ends or occurs at different instances of time.
Modulation Methods
The wavelike fluctuations of analogue signals have to be modulated to arrive at digital signals that can be sent across. A number of modulation methods are employed for this purpose. The most important methods are the frequency-shift keying (FSK) and the phase-shift keying (PSK). The former involves the shifting of the carrier wave between two established frequencies. The latter involves the shifting of the carrier wave by as many as eight different phase angles. Each phase element represents up to three bits.