unit 1 topic 5 Transmission Media _2_

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unit 1 topic 5 Transmission Media _2_ Powered By Docstoc
					CS 1302-
Data Communications &

         Topic 5 – Transmission Media
Relation to Internet Model

 Actually located below the physical layer
 Directly controlled by the physical layer

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 Data must be converted into electromagnetic signals to be
  transmitted from device to device
 Signals can travel through a vacuum, air, or other media
 May be in the form of power, voice, radio waves, infrared
  light, and gamma rays
 Each of these forms constitutes a portion of the
  electromagnetic spectrum

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Categories of Media

 Broad categories:
     Guided Media – media with a physical boundary
        Twisted   pair, coaxial, and fiber-optic
     Unguided Media – no physical boundaries
        Radio   waves, infrared light, visible light and gamma
        Sent by microwave, satellite, and cellular transmission

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Classes of Transmission Media

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Guided Media

 Provides a conduit from one device to another
 Signal is directed and contained by physical limits
  of medium
 Twisted-pair and coaxial use copper conductors
  to accept and transport signals in form of
  electrical current
 Optical fiber is glass cable that accepts and
  transports signals in form of light

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Twisted-Pair Cable
 Two conductors surrounded by insulating material
 One wire used to carry signals; other used as a
  ground reference
 Twisting wires reduces the effect of noise
  interference or crosstalk since both wires will likely be
  equally affected
 More twists = better quality
      Limits interfences
 No. of twists / unit length determines the quality of the

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Unshielded Twisted Pair (UTP)
 Most common type; suitable for both voice and
  data transmission
 Categories are determined by cable quality
      Cat 3 commonly used for telephone systems (up to 10
       Mbps - 10 Base T)
      Cat 5 usually used for data networks (up to 100 Mbps
       – 100 Base T)
 Performance is measured by attenuation versus
  frequency and distance
 Adv: cheaper, flexible, easy to install
 UTP connectors - RJ45

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Categories of UTP cables

Category     Bandwidth        Data Rate             Digital/Analog      Use

    1         very low        < 100 kbps                   Analog     Telephone

    2         < 2 MHz          2 Mbps                Analog/digital   T-1 lines

    3         16 MHz           10 Mbps                     Digital     LANs

    4         20 MHz           20 Mbps                     Digital     LANs

    5        100 MHz           100 Mbps                    Digital     LANs

 6 (draft)   200 MHz           200 Mbps                    Digital     LANs

 7 (draft)   600 MHz           600 Mbps                    Digital     LANs

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UTP example

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Shielded Twisted Pair (STP)
 A metal foil or braided-mesh
    covering encases each pair of
    insulated conductors to prevent
    electromagnetic noise called
   Crosstalk occurs when one line
    picks up some of the signals
    traveling over another line
   Uses RJ-45 connectors
   More expensive but less
    susceptible to noise
   Supports high Bandwidth over
    long distances

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Coaxial Cable
 Has a central core conductor
  enclosed in an insulating
  sheath, encased in an outer
  conductor of metal foil
 RG numbers denote
  physical specs such as wire
  gauge, thickness and type of
  insulator, construction of
  shield and size/type of outer
        RG-8, RG-9, and RG-11
         used in thick Ethernet
        RG-58 used in thin
        RG-59 used for TV
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Coaxial Cable Connectors

 Most common is barrel connector (BNC)
 T-connectors are used to branch off to secondary cables
 Terminators are required for bus topologies to prevent
  echoing of signals

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Coaxial Applications & Performance
 Analog and digital phone networks
 Cable TV networks
 Traditional Ethernet LANs
 Home Networks-phone line , power line.
 Higher bandwidth than twisted-pair
 Attenuation is higher and requires frequent use of
 Single coax carries 10000 voice signals & digital
  data upto 600 Mbps.

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Fiber-Optic Cable

 Made of glass; signals
  are transmitted as light
  pulses from an LED or
 Light is also a form of
  electromagnetic energy
 Speed depends on
  density of medium it is
  traveling through; fastest
  when in a vacuum,
  186,000 miles/second

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Refraction and Reflection

 Refraction often occurs
  when light bends as it
  passes from one medium
  to another less dense
 When this angle results in
  a refraction great enough,
  reflection occurs and the
  light no longer passes into
  the less dense medium

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 Optical fibers use reflection to guide light through a
 Information is encoded onto a beam of light as a series of
  on-off pulses representing 1s and 0s

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Propagation Modes

 Method for transmitting optical signals:
     Multimode
        Multimode step-index fiber
        Multimode graded-index fiber

     Single Mode

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 Multiple beams from light source move through
  core at different paths
 Multimode step-index fiber
     Density remains constant from center to edges
     Light moves in a straight line until it reaches the
     Some beams penetrate the cladding and are lost,
      while others are reflected down the channel to the
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Multimode (cont)
 As a result, beams reach the destination at
  different times and the signal may not be the
  same as that which was transmitted
 To address this problem and to allow for more
  precise transmissions, multimode graded-index
  fiber may be used
 Index refers to the index of refraction
 Graded-index refers to varying densities of the
  fiber; highest at center and decreases at edge
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Multimode Graded-Index Fiber (cont)

 Since the core density decreases with distance
  from the center, the light beams refract into a
 Eliminates problem with some of the signals
  penetrating the cladding and being lost
 Also signals intersect at regular intervals

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Single Mode

 Only one beam from a light source is transmitted
  using a smaller range of angles
 Smaller diameter and lower density
 Makes propagation of beams almost horizontal;
  delays are negligible
 All beams arrive together and can be recombined
  without signal distortion
 Uses stepped index fiber

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Propagation Modes

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Light Sources & Connectors
 Light source is light-emitting diode (LED) or a
 LEDs are cheaper but not as precise
  (unfocused); limited to short-distance use
 Lasers can have a narrow range, better control
  over angle
 Receiving device needs a photosensitive cell
  (photodiode) capable of receiving the signal
 Uses SC- Subscriber channel & ST-straight tip

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Applications of Fiber Optics

 Backbone networks due to wide bandwidth and
  cost effectiveness
 Cable TV
     100Base-FX (Fast Ethernet)
     1000Base-X

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Advantages of Fiber Optics

 Higher bandwidth than twisted-pair and coaxial
  cable; not limited by medium, but by equipment
  used to generate and receive signals
 Noise resistance
 Less signal attenuation
 More resistant to corrosive materials
 Lightweight
 Greater security

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Disadvantages of Fiber Optics

 Installation/maintenance
 Unidirectional
 Cost

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7.2 Unguided Media: Wireless

 Wireless communication; transporting electromagnetic
  waves without a physical conductor

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Wireless Propagation Methods

 Ground – radio waves travel through lowest
  portion of atmosphere, hugging the Earth
     Distance depends on power of signal
 Sky – higher-frequency radio waves radiate
  upward into ionosphere and then reflect back to
 Line-of-sight – high-frequency signals transmitted
  in straight lines directly from antenna to antenna

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Propagation Methods

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Wireless Transmission Waves

 Radio Waves
 Microwave
 Infrared

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Radio Waves

 Frequency ranges: 3 KHz to 1 GHz
 Omni directional
 Susceptible to interference by other antennas
  using same frequency or band
 Ideal for long-distance broadcasting
 May penetrate walls
 Propagate in SKY mode
 Used for multicast communication such as radio,
  TV etc
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Band       Range          Propagation                      Application
VLF      3–30 KHz            Ground             Long-range radio navigation
                                                        Radio beacons and
LF      30–300 KHz           Ground
                                                       navigational locators
MF     300 KHz–3 MHz           Sky                          AM radio
                                                    Citizens band (CB),
HF       3–30 MHz              Sky
                                                ship/aircraft communication
                            Sky and                         VHF TV,
VHF     30–300 MHz
                          line-of-sight                     FM radio
                                                  UHF TV, cellular phones,
UHF    300 MHz–3 GHz      Line-of-sight
                                                     paging, satellite

SHF      3–30 GHz         Line-of-sight           Satellite communication

EHF     30–300 GHz        Line-of-sight       Radar, satellite communication
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 Frequencies between 1 and 300 GHz
 Unidirectional
 Narrow focus requires sending and receiving
  antennas to be aligned
 Issues:
     Line-of-sight (curvature of the Earth; obstacles)
     Cannot penetrate walls

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Parabolic Dish Antenna

 Incoming signals - Signal
  bounces off of dish and is
  directed to focus
 Outgoing signals –
  transmission is broadcast
  through horn aimed at
  dish and are deflected

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Horn Antenna

 Outgoing transmissions
  broadcast through a stem
  and deflected outward
 Received transmissions
  collected by a scooped
  part of the horn and
  deflected downward into
  the stem

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Microwave Applications

 Unicasting – one-to-one communication between
 sender and receiver
     Cellular phones
     Satellite networks
     Wireless LANs

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 Frequencies between 300 GHz and 400 THz
 Short-range communication
 High frequencies cannot penetrate walls
 Requires line-of-sight propagation
 Adv: prevents interference between systems in
  adjacent rooms
 Disadv: cannot use for long-range communication
  or outside a building due to sun’s rays
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Infrared Applications

 Wide bandwidth available for data transmission
 Communication between keyboards, mice, PCs,
  and printers

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Media selection

 Each media has advantages and disadvantages. Some of
  the advantage or disadvantage comparisons concern the
      Cable length
      Cost
      Ease of installation
      Susceptibility to interference

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 Ethernet Media standard

 The cables and connector specifications used to support
  Ethernet implementations are derived from the Electronic
  Industries Association and the Telecommunications
  Industry Association (EIA/TIA) standards body.
 The categories of cabling defined for Ethernet are derived
  from the EIA/TIA-568 (SP-2840) Commercial Building
  Telecommunications Wiring Standards.

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Lab on Cable connectorization

  EIA/TIA specifies an
   RJ-45 connector for
   unshielded twisted-pair
   (UTP) cable. The letters
   RJ stand for registered
   jack, and the number 45
   refers to a specific
   wiring sequence.

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Straight through Cable
 Maintain the pin connection all the way through the cable.
 Wire connected to pin 1 is the same on both ends.
 Used to connect such devices as PCs or routers to other
  devices such as hubs or switches.

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Cross over cable
 Cross the critical pair to properly align, transmit, and
  receive signals on devices with like connections.
 Pin 1 connected to pin 3, pin 2 connected to pin 6.
 Used to connect similar devices: switch to switch, switch
  to hub, hub to hub, router to router, PC to PC.

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