| | | | Abbreviated from Modulator/Demodulator. A modem is a device (normally the term refers to hardware rather than software) that converts or "modulates" an outgoing digital signal from a computer to an analogue carrier signal prior to sending it over telephone or power cables (for example) and converts an incoming analogue signal to digital, so that it can be read by the computer. Modems are able to communicate with eachother via communcations lines that carry other information such as telephone voice calls by using i) timing signals so that they can recognise "relevant" data and ignore the rest - in the case of synchronous transmissions - and ii) using a number of protocols (e.g. TCP/IP) for error-correction in the case of asynchronous ones. Wireless modems use radio signals. The set of commands a computer uses to control a modem is called the Hayes Command Set and remain largely unchaged since their introduction by Hayes Communications in the early 1980s when, for the first time, their SmartModem was able to connect and disconnect from the phone line and dial out/hang up without manual human intervention. Modems may be external, in the form of a box that connects to the computer using the ubiquitous RS-232 port, internal, using an expansion slot such as PCI or increasingly, built in to the onboard circuitry of the motherboard as standard. Voiceband modems operate at a maximum speed of 56kbit/s and have gone through a series of "standards" as technology and the speeds possible have increased over the years, including: V.32, V.34, V.70, V.90 and the latest, V.92. With the huge uptake of broadband, however, many ISPs have never bothered upgrading their services to the V.92 standard due to the costs involved, the falling requirement for dialup connections and the fact that the maximum speed did not increase from the 56kbit/s of V.90. | | | |
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