Networks Part 2

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Networks Part 2

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Network Hardware

• For a network to function and share information
  there needs to be a physical, or logical link
  between the sender and the receiver.
• A physical link includes a network card
  or adapter in the back of the workstation,
  a cable is linked between this and
  other devices (or nodes). The other devices
  could be a workstation, a printer, a server, a
  repeater, a hub, or a router.
• A logical link is where a wireless
  connection such as radio or satellite
  communication is used.

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Data

• When data is sent from one computer to another,
  it is normally broken up into smaller manageable
  segments with a to address and a from address
  attached.
• These parts of the complete data are known as
  data packets.
• Devices are used on networks to control what
  happens to these packets while they are in
  transit.

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Repeater

• A Repeater is used to regenerate or replicate a
  signal that is traveling along a long network
  cable to prevent it being distorted by
  transmission loss.
• A digital repeater reconstructs a signal to near
  its original quality, it doesnt just amplify it
  as the distortion and noise will also be
  amplified.
• Hubs can operate as repeaters by relaying
  messages to all
  connected computers.

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Bridge

• A Bridge will connect two segments of a LAN, but
  it also acts as a repeater.
• A bridge will read the destination address of a
  packet of data being transmitted to determine
  whether to let it cross the bridge to the other
  segment or to keep it within the original
  segment.
• This helps keep the traffic on the network lower
  by not having packets lost in the wrong segments.

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Segment A
Segment B
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Router

• A Router can do the job of a repeater and a
  bridge but it also has special software that
  reads the destination address.
• The device then makes a decision as to the best
  path the data packet must take to get closer to
  its destination.
• A router table database is used to help choose
  the correct path, but heavy transmission or a
  fault on a line would be detected by the router
  and an alternative path would be given.

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Using a Router to determine which route to send
the Message.
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Gateway

• A Gateway is an enhanced router, usually used to
  connect a series of LANs to a WAN.
• The specialised software on the gateway is used
  to read different protocols from different
  operating systems and different hardware an be
  able to connect them all together.

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Gateway
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Gateways Connecting dissimilar LANs
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Multiplexing

• Multiplexing is where one signal channel is used
  for data transmission instead of several.
• The signals are broken into packets and the
  packets take turns to be sent. At the receiving
  end they are reconstructed.

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Bandwidth Error Detection

• Bandwidth is a measure of how much data can be
  carried at one time, measured in Megabits per
  second (Mbps).
• The higher the bandwidth the more data
  transmitted.
• The bandwidth is determined by the transmission
  media (cabling) and the protection around it to
  lessen interference.
• Error checking is done at the receiving end to
  ensure the data sent is the same as data
  received, the most common being a parity check.

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Bandwidth
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Even Parity
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Transmission Media
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Categories
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Twisted Pair

• Twisted pair cable is shielded or unshielded.
• UTP, unshielded twisted pair is the most common.
• Most common in the telephone network.
• Suitable for data and voice.
• Consists of two copper conductors with their own
  sleeves of insulating material i.e. plastic.
• High quality costly cable can give very high
  speeds.

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Twisted Pair Cable
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Shielded Twisted Pair

• STP Shielded twisted pair has a metal foil or
  braided mesh covering for each pair of insulated
  conductors.
• Has the same quality as unshielded cable, but the
  cost is greater.
• The extra casing helps prevent the penetration of
  noise.
• Eliminates the problems of cross-talk.
• The extra shield must be grounded.

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Shielded Twisted Pair
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Co-Axial cable

• Constructed differently to twisted pair.
• Carries signals of higher frequency.
• Consists of a central core conductor of solid
  copper, insulated in a sleeve, encased in an
  outer conductor of metal foil or braiding.
• The metal braid is an extra shield from noise,
  and a second conductor.
• The whole cable is then protected by a plastic
  covering.
• Good quality transmission at high speed,
  Ethernet, television.

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Co-Axial Cable
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Fibre Optic Cable

• Optical Fibre is made of a glass or plastic
  hollow tube.
• A pulse of light is shone through the tube.
• The speed of propagation in a vacuum is 186,000
  miles per second.
• The higher the density of the tube the slower the
  light travels.

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Fibre Optic Cable
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Unguided Media

• Wireless connections using radio waves and
  microwaves.
• Terrestrial Microwave almost line of sight (can
  be up to about 30 miles apart), used for cell
  phones.

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Satellite

• Communications satellite in geo-synchronous
  orbit (rotating at the same speed as earth,
  therefore stationary relative to Earth).

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Ethernet

• Ethernet uses a system of sending signals called
  Carrier Sense Multiple Access / Collision
  Detect CSMA/CD.
• When a piece of data is to be sent, it is broken
  into smaller packets, then data to be sent waits
  until the communication line is clear, and then
  sends.
• If two different devices send at exactly the same
  time, there will be a collision and the packet
  will become corrupted.
• The system detects the collision and puts a stop
  to all signals being sent until the communication
  line is clear, devices are then able to send at
  random intervals, and the destroyed packet can be
  resent.
• If the communication line is very busy some
  signals never make it and have to be resent by
  the operator.
• Ethernet speeds are about 10 Mbps.

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Token Ring

• A ring network may use a Token to carry a message
  from one device to another.
• The sender waits for the Token to arrive, sticks
  the message on to it and the Token goes around
  the ring visiting every device until the message
  finds its destination.
• Occasionally the Token may get lost and if it
  travels around the ring a second time it gets
  destroyed and a new one created.
• The system avoids collisions as only one Token is
  allowed on the network, it is also fair as busy
  devices are serviced the same as less busy ones.