Modems

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Modems

• Unit 5
• Data Communications

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What is a modem

• Device that provides an interface between a
  computer and a wide area network
• Traditional type of modem is used to connect to
  the Internet via dial-up telephone line
• Dial-up links use the analogue voice telephone
  network
• A modem converts the computers digital data into
  analogue signals using a process called
  modulation and demodulation
• Hence the name MoDem

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Traditional modem
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Steps in modem communication

• Dial the phone number of the receiving computers
  modem.
• The computers modem will answer and acknowledge
  the request to communicate.
• The modem converts (modulates) the computers
  digital binary data to an analogue carrier
  wave and the data is transmitted across the
  telephone line.
• Note a telephone line carries an analogue
  voice signal, but the data sent by a computer is
  digital in a sequence of 0s and 1s.
•  

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Steps in modem communication

• The receiving computer accepts the modulated
  transmission and then converts (demodulates) the
  transmission back into a binary sequence.
• The receiving computer modulates a reply to
  confirm acceptance of the transmission, and this
  loop continues until one of the computers is
  disconnected.

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Digital modems

• Although dial-up modems are still widely used,
  they are limited to a maximum transfer rate of
  56Kbps.
• This is quite slow for web surfing, especially
  when visiting sites with lots of graphics.
• Also rather slow for music downloads
• Broadband digital Internet connection allow much
  higher speeds, 512Kbps and higher.

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Digital modems

• With a broadband link an interface device is
  still required and it is still called a modem.
• Two main types of broadband Internet connection
• Using the telephone line, a system called
  Asymmetric Digital Subscriber Line (ADSL) allows
  a digital link
• Using a cable TV (CATV) connection.

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ADSL

• The ordinary analogue voice telephone connection
  only requires a very limited range of
  frequencies, roughly 0 to 3,400 Hz.
• The telephone wires have the potential to carry a
  much wide range of frequencies.
• ADSL takes advantage of this by allocating
  certain frequencies for voice traffic and others
  for internet traffic

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ADSL
Frequencies allocated to data upload
range of available frequencies
Frequencies allocated to data download
Frequencies allocated to voice traffic
As the diagram shows there is a greater range of
frequencies allocated to data download (i.e.
downloading data from the Internet to the users
computer) that there is to upload (i.e. data sent
from the users computer to the internet), hence
the term Asymmetrical.
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ADSL

• When using ADSL a device known as an ADSL modem
  provides the interface between the phone socket
  and the computer.
• Connection to the computer from the ADSL modem is
  usually via an RJ45 LAN connector or a USB
  interface
• Telephone sockets used for voice connection need
  to have a filter attached to prevent the digital
  data interfering with the voice traffic.

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ADSL

• One limitation of ADSL is that that the system
  will only work up to a certain distance from the
  telephone companies exchange
• As the distance increases the signal quality
  degrades and the connection speed goes down. The
  upper distance limit on ADSL is about 6Km,
  although technology improvements means this
  distance is increasing over time
• Therefore not all telephone subscribers are able
  to have an ADSL connection, and some may be
  limited to lower speed connections

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Cable modems

• A cable modem is a type of modem which provides a
  network connection over the cable TV network
• The cable TV (CATV) network was originally
  designed to deliver multiple TV channels to
  subscribers homes, using co-axial cables
• The many TV channels are transmitted on the cable
  using different frequencies, and the Internet
  service just occupies one of these frequency
  slots as if it were another TV channel

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Block diagram of a cable modem
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Cable modem functions

• The functions of the components shown in the
  block diagram are
• Tuner this connects directly to the coaxial
  cable which carries the CATV signal and tunes
  into the internet channel to separate it from the
  other TV channels.
• Demodulator Converts the analogue data received
  by the TV cable to digital data.
• Media access control Provides various protocol
  and timing services.
• Interface Provides the Ethernet and/or USB
  interface
• Modulator Converts the digital data to analogue
  for data being sent.