Modems

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)

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

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

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

5
Phone Line

• You dont need a separate phone line
• telephone and modem can share the same line
• But a second line is more conveneient if you
  dont want to miss a call
• Broadband allows both to work on one line

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

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

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

9
Software Modems

• Uses sound functions to generate tones
• Requires a motherboard with integrated sound card
• Easiest to install
• Cheap

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

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

12
Phone Line Quality

• Modem speed isnt 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

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

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

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

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