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Acronym for modulator-demodulator. A modem is a device or program that enables a computer to transmit data over telephone lines. Computer information is stored digitally, whereas information transmitted over telephone lines is transmitted in the form of analog waves. A modem converts between these two forms.

Fortunately, there is one standard interface for connecting external modems to computers called RS-232. Consequently, any external modem can be attached to any computer that has an RS-232 port, which almost all personal computers have. There are also modems that come as an expansion board that you can insert into a vacant expansion slot. These are sometimes called onboard or internal modems.

While the modem interfaces are standardized, a number of different protocols for formatting data to be transmitted over telephone lines exist. Some, like CCITT V.34, are official standards, while others have been developed by private companies. Most modems have built-in support for the more common protocols -- at slow data transmission speeds at least, most modems can communicate with each other. At high transmission speeds, however, the protocols are less standardized.

Aside from the transmission protocols that they support, the following characteristics distinguish one modem from another:

  • bps : How fast the modem can transmit and receive data. At slow rates, modems are measured in terms of baud rates. The slowest rate is 300 baud (about 25 cps). At higher speeds, modems are measured in terms of bits per second (bps). The fastest modems run at 57,600 bps, although they can achieve even higher data transfer rates by compressing the data. Obviously, the faster the transmission rate, the faster you can send and receive data. Note, however, that you cannot receive data any faster than it is being sent. If, for example, the device sending data to your computer is sending it at 2,400 bps, you must receive it at 2,400 bps. It does not always pay, therefore, to have a very fast modem. In addition, some telephone lines are unable to transmit data reliably at very high rates.
  • voice/data: Many modems support a switch to change between voice and data modes. In data mode, the modem acts like a regular modem. In voice mode, the modem acts like a regular telephone. Modems that support a voice/data switch have a built-in loudspeaker and microphone for voice communication.
  • auto-answer : An auto-answer modem enables your computer to receive calls in your absence. This is only necessary if you are offering some type of computer service that people can call in to use.
  • data compression : Some modems perform data compression, which enables them to send data at faster rates. However, the modem at the receiving end must be able to decompress the data using the same compression technique.
  • flash memory : Some modems come with flash memory rather than conventional ROM, which means that the communications protocols can be easily updated if necessary.
  • Fax capability: Most modern modems are fax modems, which means that they can send and receive faxes.
  • To get the most out of a modem, you should have a communications software package, a program that simplifies the task of transferring data.

    Figure

    Communications Protocols



    Protocol
    Maximum Transmission Rate

    Duplex Mode
    Bell 103 300 bps
    Full
    CCITT V.21 300 bps
    Full
    Bell 212A 1,200 bps
    Full
    ITU V.22 1,200 bps
    Half
    ITU V.22bis 2,400 bps
    Full
    ITU V.29 9,600 bps
    Half
    ITU V.32 9,600 bps
    Full
    ITU V.32bis 14,400 bps
    Full
    ITU V.34 36,600 bps
    Full
    ITU V.90 56,000 bps
    Full



    For internet.com pages about modem, . Also, check out the following links!


    More Information

    Outstanding Page HardwareCentral: Modems
    Provides tutorials on installing different types of modems, increasing modem speed, and troubleshooting modem problems.

    Outstanding Page Serial Communication Overview
    Outlines the technology behind the PC modem, the associated CCITT standards and higher performance solutions such as ISDN, and xDSL. Includes graphics and diagram. This page is from "The PC Technology Guide." Updated on Aug 5, 1998

      Curt's High Speed Modem FAQ
    This large FAQ provides information on modems, bits, bytes and baud rates, settings, initialization strings, links to modem manufacturer pages, and news. Updated on Aug 1, 1998

      Database of Modem Control Strings
    This site, from Shiva, contains a searchable database of modem control strings. Updated on Jan 10, 1998

      Modem and PPP
    Modem and PPP information, help, tuning, and troubleshooting.

      Modem Express
    Home page of Modem Express, a data communication repair company who has purchased the rights to Hayes warranty and repair inventory. For more information regarding Hayes and Hayes products, see the news section of their site. Updated on Aug 4, 1998

      Modem Fault Finder
    Contains a series of questions that walk you through modem set-up and provides troubleshooting and optimization suggestions. Updated on May 17, 1998

      Modem Help: Support for Modems
    Comprehensive directory of modem related information. Includes support forums, daily news, manufacturer listings, product reviews and tutorials.

      Modem Optimization
    Contains tips on optimizing your modem for the Internet. Updated on May 17, 1998

      Modem Protocols
    Provides an explanation of modem protocols. Updated on Aug 5, 1998

      Modem Resources
    Contains information on modem manufacturers, their products and related links along with general information on modem drivers, initialization strings, newsgroups listings, FAQs, and a section on 56Kbps modems. Updated on Jul 31, 1998

      Modem Tutorial
    Provides a detailed description of how modems work and the different types of modems that are available. Also describes different modulation techniques. Updated on Aug 1, 1998

      PC Computing article about 56K modems
    PC Computing article describing X2 and K56Flex and comparing specific modems supporting these technologies. Updated on Aug 4, 1998

      PC Mechanic's Modem Page
    Step-by-step instructions on how to install internal and external modems. Updated on Apr 18, 1998

      Software modems
    Contains information about the AltoCom company and their line of software modems (SoftModems) and software-based signal processing intensive communications products. Updated on Apr 7, 1998

      Troubleshooting Modems
    Discusses some basic troubleshooting of problems related to modems. This page is from "The PC Guide."

      Troubleshooting Modems in Windows 95
    Describes how to configure a modem in Windows 95. Maintained by Electronic Arts, this site discusses device conflicts, how to determine which COM port your modem uses, modem initialization strings, and other topics. Updated on Apr 4, 1998

      Yahoo!'s modem buyer's guides
    Yahoo!'s directory of modem buyer's guides. Updated on Aug 2, 1998

      Yahoo!'s modem companies page
    Yahoo!'s directory of modem companies. Updated on Aug 4, 1998

      Yahoo!'s modem page
    Yahoo!'s directory of modems.

      ZDNET's Modem Help
    Excellent resource for finding information about and troubleshooting modems.

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