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Modems

Modems. allow computers to exchange information over the telephone network by: converting outgoing data from digital (binary) form to analogue (wave) form in a process known as modulation converting incoming data from analogue to digital form in a proces known as demodulation

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Modems

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  1. Modems • allow computers to exchange information over the telephone network by: • converting outgoing data from digital (binary) form to analogue (wave) form in a process known as modulation • converting incoming data from analogue to digital form in a proces known as demodulation • hence MODEM (MODulation, DEModulation) Sheffield Hallam University

  2. Modems and the Internet • A modem lets you connect to the Internet and online services like: • America On Line (AOL) • Demon • CompuServe • Freeserve • This lets you access information on computers all over the world and join groups of people with similar interests called newsgroups Sheffield Hallam University

  3. Information Sharing • When travelling or at home, a modem can be used to access information held on a network at work • Emails can be sent and received and office files can be worked on Sheffield Hallam University

  4. Emails and Faxes • Most modems can send and receive faxes • A document can be created on a computer and sent to another computer or fax machine • When a computer gets a fax it appears on the screen • It can be viewed or printed, but not edited • You need Optical Character Recognition software to convert it so that it can be edited Sheffield Hallam University

  5. Phone Line • You don’t need a separate phone line • telephone and modem can share the same line • But a second line is more conveneient if you don’t want to miss a call • Broadband allows both to work on one line Sheffield Hallam University

  6. Internal Modems • This type is a circuit board (card) that plugs into an expansion slot on the motherboard • Usually cheaper than an external modem • Saves space on desk • Can be more difficult to set up than an external type Sheffield Hallam University

  7. External Modems • A small box plugs into the back of your computer • Costs more, uses some deskspace • Can transfer to another PC easily • Status lights tell you what is happening • receiving data • sending data Sheffield Hallam University

  8. External Modems 2 • If using an external modem with an older computer, check for 16550 UART Chip • Older serial ports cannot transfer information fast enough • Universal Asynchronous Receiver Transmitter is the chip that controls the flow of information to and from the modem Sheffield Hallam University

  9. Software Modems • Uses sound functions to generate tones • Requires a motherboard with integrated sound card • Easiest to install • Cheap Sheffield Hallam University

  10. Modem Speed • Determines how fast data can be sent and received • Measured in bits per second • 14,400 bps is minimum • 28,800 OK • 33,600 better • Usually expressed in kbps (kilobits per second) • 33,600 bps is likely to be referred to as 28.8 kbps Sheffield Hallam University

  11. Time is Money! • Get the fastest modem you can afford • Faster information transfer will cut-down telephone bills and online service charges • Reduced frustration of waiting for completion of data transfer Sheffield Hallam University

  12. Phone Line Quality • Modem speed isn’t the only factor in determining transmission rates • A 28.8 kbps modem may not actually deliver at that speed if the phone line quality is not good • Watch out! • Modem speeds are usually quoted as download speeds • upload speeds are much slower • fine for surfing the net • poor for managing web sites Sheffield Hallam University

  13. Handshake • Before exchanging information modems perform what is known as a handshake • to establish how they will manage the exchange • e.g. a transmission speed has to be set • this will be the speed o the slower modem • NB some of the online service providers only use the lower modem speeds Sheffield Hallam University

  14. Online/Offline • When your modem has successfully connected to another modem it is said to be online: • this means that the modems are ready to exchange information • when the modem is not connected, you are offline Sheffield Hallam University

  15. Data Compression • To reduce transmission costs, a modem will compress data • The receiving modem ill decompress the data at the other end • Error control procedures are used to ensure that there has been no loss or corruption of data Sheffield Hallam University

  16. ISDN • There are other ways of sending data along phone lines • Integrated Services Digital Network • carry data in binary form at four times the speed of a typical modem • no modulation or demodulation required, hence improvement in speed Sheffield Hallam University

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