Computer Networks

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Computer Networks

• Dr. Taek Mu Kwon
• Department of Electrical and Computer
• Engineering

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Outline

• ECE 4321
• Network architecture
• Optical fiber
• DSL

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ECE 4321 Computer Networks

• ECE Technical Elective Course
• 3 Credits
• Network Lab (MWAH 60)
• Network Programming Project (3 4)
• Written Exams
• Course Objective On completion of this course
  students are expected to learn and understand
  characteristics of network transmission media,
  OSI model, TCP/IP, data link protocols, routing
  algorithms, various LAN technologies, WAN
  technologies, and network programming.

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Integrated Service Revolution (1990 - )

• The fundamental difference between data
  processing (computer) and data communication
  (transmission and switching equipment) have
  blurred. Ex) Half of OS is concerned with
  computer networks.
• The fundamental difference among data, voice, and
  video communication have blurred.
• The line between single-processor computer,
  multi-processor computer, local network,
  metropolitan network, and long-haul network have
  blurred.
• WWW became an integration point of all types of
  communications data, voice, video.

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OSI Model
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Electromagnetic Spectrum
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Transmission Characteristics of Guided Media
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Attenuation of Typical Guided Media
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Optical Fiber

• Invented in Aug. 1970 by Dow Corning Scientists
  Dr. Donald Keck, Dr. Bob Maurer and Dr. Peter
  Schultz
• Invention was motivated by space limitation of
  telephone wires
• First large commercial use nationwide
  long-distance telephone network by MCI using
  single-mode optical fiber, 1983

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What is Optical Fiber?

• A thin flexible medium capable of conducting an
  optical ray. The basic material consists of
  silicon dioxide, silica, and plastic.

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Internal Structure of Optical Fiber
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Snells Law
qt
Cladding Refractive index n2
qi
Fused Silica Refractive index n1
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Optical Fiber Types

• Multimode (Orange Color)
• Stepped index
• Grade index
• Singlemode (Yellow Color)
• The fiber diameter is one wavelength of light
  (1um)

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Wavelengths Used in Optical Fiber(Infrared
region)

• 850 nm --- used in most LAN, travel shorter
  distance, generated by LEDs
• 1300 nm --- higher data rates, longer distances,
  LED or Laser
• 1550 nm --- highest data rates, longest distance,
  Laser

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Wavelength Division Multiplexing (WDM)

• Multiple beams of different wavelengths are
  transmitted over the same fiber
• Transmission of one Tbps was achieved by 100
  beams each operating at 10 Gbps (1997 Bell Lab)

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Characteristics of Optical Fiber

• Greater bandwidth
• Smaller size and light weight
• Lower attenuation
• EM isolation
• Greater repeater spacing
• Greater security

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DSL, ADSL (Asymetric Digital Subscriber Line)

• Connecting new wires for high-speed transmission
  to subscribers new home is expensive
• Voice graded telephone lines are band limited to
  4KHz by Local Branch Office, but capable of
  transmitting 1MHz or more.
• ADSL provides high-speed transmission over
  ordinary telephone line (typ. 2Mbps, but up to
  9Mbps)

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ADSL Design

• Uses FDM in a novel way
• Allocates the lowest 25KHz for voice
• Smaller upstream and larger downstream band
• A single bit stream is split into multiple
  parallel bit streams, where each portion is
  carried in a separate frequency

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Discrete Multi-tone (DMT)

• The available transmission band is divided into
  4KHz subchannels (0 to 60Kbps).
• DMT modem tests each channels signal to noise
  (S/N) ratio.
• More bits are assigned to high S/N ratio
  channels
• Present ADSL/DMT employs 256 downstream
  subchannels.
• Theoretical Limit 25660K15.36 Mbps
• (In practice 1.5 to 9Mbps)

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Discrete Multi-tone (DMT)
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xDSL Comparison