Lecture 14

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					INTRODUCTION TO COMPUTER                                    LECTURE NO 14

In Network communications media refers to the wires, cables and other means
by which data travels from its source to destination. The most common media for
data communication are twisted pair wire, coaxial cable, fiber optic cable and
wireless links.
Twisted pair wire
Twisted pair wire normally consists of four or eight copper stands of wire,
individually insulated in plastic, then twisted around each other in braided pairs
and bound together in an other layer of plastic insulation, Except for the plastic
coating , nothing shields this type of wire from outside interference, so it is also
called unshielded twisted pair (UTP) wire. Sometimes network media are
compared by the amount of data they can transmit each second, the difference
between the highest and lowest frequencies of a transmission channel is known
as bandwidth. As more users transmit data over a network, the bandwidth
reduces, thereby slowing down all transmissions. Bandwidth is expressed in
cycle per second (Hertz) or in bits per second. Twisted-pair wire was once
considered a low bandwidth media, but networks based on twisted-pair wires
now support transmission speeds up to 150 megabits per second (Mbps), and
even faster speeds are on the horizon.
Coaxial Cable
Coaxial cable, some times called coax is widely used for cable TV and is used in
some networks. There are two conductors in coaxial cable, one is a single wire in
the center of the cable, and the other is a wire mesh shield that surrounds the
first wire with an insulator in between. Two types of coaxial cable are used with
networks: thick and thin.
Thick coax is the older standard and is seldom installed in new networks.
Fiber Optic Cable
A fiber optic is thin strand of glass that transmits beams of light rather than
electric frequencies. When one end of the strand is exposed to light, the strand
carries the light all the way to the other end bending around corners with only a
minute loss of energy along the way. Because light travels at a much higher
frequency than electrical signals, fiber optic cable can easily carry data at more
than billion bits per second usually 13oo mbps.
The disadvantage of fiber optic cable is that it is more expensive than twisted
pair and coax, and it is more difficult to install because it does not bend around
corners as easily.
Wireless Links
Today wireless communication is competing with twisted pair, coaxial and fiber
optic cable. The advantage of wireless communication is the flexibility that is
offers in term of the network layout.
There are four common uses of wireless communication in network
    1.      Office LANs can use radio signals to transmit data between nodes

INTRODUCTION TO COMPUTER                                 LECTURE NO 14

  2.      Laptops can be equipped with cellular telephone equipment and
          modem so that business man can stay in touch with the office network
  3.      Corporate WANs often use microwave transmission to connect two
          LANs with same metropolitan area.
  4.      WANs that cover long distances often use satellites and microwave


      The alternative to using dedicated media is to use the telephone system
  for data communication. This is possible because the telephone system is
  really just a giant electronic network owned by the telephone companies.
  Although it is designed to carry two way electronic information, the network
  of telephone lines is significantly different from a typical computer network.
  The phone system was originally designed to carry voice messages, which are
  analog signals. Increasingly, however, phone lines are being used to send
  digital data.
     Although digital telephone lines are gaining popularity, most people still
  have analog phone lines attached to their homes and businesses. Attaching a
  computer to an analog phone line requires a modem, so it is important to
  know a few things about how modems work and what to look for when you
  buy one
  How a Modem Works
      A modem short for modulator and de-modulator is needed        to translate
  these digital signals into analog signals that can travel over    a standard
  telephone lines. In its modulation phase, the modem turns the     computer’s
  digital signals into analog signals. In its demodulation phase,   the reverse
  takes place.
  Choosing a Modem
       When you buy a modem, there are four areas to consider:
  1.   Transmission Speed
  2.   Data Compression
  3.   Error Correction
  4.   Internal versus external
  Connecting With Modems
      A computer equipped with a modem and connected to a standard
  telephone line can dial up and communicate with any other computer that
  has a modem and a telephone connection. These are some common uses for
  data communication over the phone lines.
       Direct connections with other users
       Connections with office LANs

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