Semester One 2001/2002 Sheffield Hallam University1 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)

Semester One 2001/2002 Sheffield Hallam University2 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

Semester One 2001/2002 Sheffield Hallam University3 Information Sharing When travelling or at home, a modem can be used to access information held on a network at work s can be sent and received and office files can be worked on

Semester One 2001/2002 Sheffield Hallam University4 s 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

Semester One 2001/2002 Sheffield Hallam University5 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

Semester One 2001/2002 Sheffield Hallam University6 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

Semester One 2001/2002 Sheffield Hallam University7 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

Semester One 2001/2002 Sheffield Hallam University8 External Modems 2 If using an external modem with an older computer, check for 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

Semester One 2001/2002 Sheffield Hallam University9 Software Modems Uses sound functions to generate tones Requires a motherboard with integrated sound card Easiest to install Cheap

Semester One 2001/2002 Sheffield Hallam University10 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

Semester One 2001/2002 Sheffield Hallam University11 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

Semester One 2001/2002 Sheffield Hallam University12 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

Semester One 2001/2002 Sheffield Hallam University13 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

Semester One 2001/2002 Sheffield Hallam University14 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

Semester One 2001/2002 Sheffield Hallam University15 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

Semester One 2001/2002 Sheffield Hallam University16 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