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Last modified: Friday, September 10, 2004 

(mō´dem) (n.) Short for modulator-demodulator. A modem is a device or program that enables a computer to transmit data over, for example, telephone or cable 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.

    Also see Dial-Up Modem Standards in the Quick Reference section of Webopedia.

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

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    56K.COM - 56K modem home page 
    Provides information and resources for the 56K owner and buyer. Offers newsflashes, a primer, upgrade and buyer's guides, modem initialization strings, drivers, and firmware updates.

    Article on cable modems 
    This Byte magazine article, from September 1996, provides information on the development of technologies that will solve bandwidth problems, with information on cable modems, ADSL, copper vs. coaxial, phone and cable problems, and infrastructure debates.

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

    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."

    U.S. Robotics home page 
    The is the home page for US Robotics, a leading modem manufacturer. This page offers links to company information, as well as products and services for the home, office and business user.

    Baseband modems
    From Patton Electronics, the page provides information on two baseband modem products.

    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.

    Extended AT command set reference
    This resource page, from, provides links to general modem operation and modem control links for AT commands and functions.

    FAQ on purchasing a modem
    Provides an answer to the question "What modem should I buy?", and provides information on internal vs. external modems, low-speed vs. high-speed, and an explanation of baud rates and bits per second.

    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.

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

    Modem Protocols
    Provides an explanation of modem protocols.

    Modem Tutorial
    Provides a detailed description of how modems work and the different types of modems that are available. Also describes different modulation techniques. home page
    Contains extensive descriptions of modem technology and products.

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

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

    Young Design home page
    Contains information on Young Design's line of wireless radio and spread spectrum radio modems, wireless systems, data communication and systems and interface equipment products.

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    AeroComm: Wireless Equipment & Supplies - Provides wireless data communications to original equipment manufacturers, including RF modules, wireless modems, and wireless print sharing.

    Automation Direct: Wireless Modems - Offers a variety of industrial modem solutions, including wireless ethernet modems, wireless serial radio modems, telephone modems, and accessories.

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    Peripheral Devices

    related terms

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    answer-only modem



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    communications protocol

    communications software


    data compression

    dial-up access


    flash memory



    host-based modem





    modulation fallback




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