Fundamentals of Networking

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Fundamentals of Networking
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Data Communications

• exchange of digital information between two
  devices using an electronic transmission medium

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Types of Signals

• Analog signals
• used for voice communication
• has a continuous waveform
• Digital signals
• discrete
• not continuous
• 0s and 1s

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Digital vs. Analog
Amplitude
Analog Transmission
Time
1
1
1
1
() voltage
Amplitude
0
0
0
0
0
(-) voltage
Time
Digital Transmission
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Advantages of Digital Signals

• Can be converted to decimal number
• Used for error detection and encryption
• Language of computers
• Easier to recover after distortion
• Signals weaken due to resistance in a medium
• Waveform shape gets distorted

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Signal Regeneration
1
1
1
1
() voltage
Amplitude
0
0
0
0
0
(-) voltage
Time
1
1
1
1
0
0
0
0
0
1
1
1
1
() voltage
Amplitude
0
0
0
0
0
(-) voltage
Time
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Converting Analog to Digital

• Pulse Code Modulation (PCM)
• Like getting a ticker quote every 10 minutes
• Approximates the actual signal curve
• In PCM
• Measure the signal height every 1/8000th of a
  second
• 8 bits used to report the height at each
  measurement
• 8800064,000 bits per second to provide
  approximation of analog signal
• 64Kbps represents a single voice line in digital
  telecommunications

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Pulse Code Modulation
1111 1111
Value transmitted
128 values This side
Sampling Interval 1/8000 second
128 values This side
0000 0000
Value transmitted
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How about a CD?

• Lasers etches lands and pits on the surface of a
  CD
• Uses 16 bits to measure height of signal
• Samples 44,100 times per second for each of two
  channels
• 16441002 176,000 bps
• One hour of music requires 633Mb

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Digital to Analog Conversion

• Needed to transmit computer signals over
  telephone lines
• Analog signal characteristics
• Amplitude
• Intensity of the wave (height)
• Wavelength
• Distance between comparable points on the wave
• Frequency
• Number of up and down cycles per second (Hz)
• Phase
• Relative state of the amplitude

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Wave Characteristics
Amplitude
Wavelength
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Amplitude Modulation
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Frequency Modulation
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Telecommunications System
Minicomputer
terminals
Main frame
Front-End Processor
modems
multiplexer
Remote location
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Multiplexing

• Allows multiple signals to be sent over same
  medium at same time
• Modes of multiplexing
• Frequency Division (FDM)
• Time Division (TDM)

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Frequency Division Multiplexing
X X X X
X X X X X X
Y Y Y Y Y Y
Y Y Y Y
Z Z Z Z Z Z
Z Z Z Z

• originally designed so multiple voice
• streams could be placed on same telephone line

• Multiple analog signals superimposed but on
• different frequency spectra

• Involves pair of multiplexers

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Time Division Multiplexing
X X X X
Y Y Y Y
X
Y
Z
X
Y
Z
X
Y
Z
Z Z Z Z

• Each signal allotted a time slot
• Creates a composite stream with slots dedicated
  to data sources
• If data source is not sending, slot goes unused
  wasteful
• Instead, use statistical TDM in which slots are
  dynamically allocated
• If there is big demand, buffers are used.

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Transmission Media

• the physical path along which the data is carried
• Types
• twisted pair
• coaxial
• fiber optics and free space
• satellite
• terrestrial

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Transmission Media

• Twisted Pair
• pair of wires twisted along entire length
• usually copper with an insulating coat
• Unshielded Twisted Pair (UTP) popular with LANs
• CAT3 (voice) and CAT5 are common
• CAT5 used for both voice and data
• 100Mbs transmission speed
• Limited segment length signals needs
  regeneration every 100 meters

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Transmission Media

• Coaxial cable
• thick insulated copper wire
• Longer segment lengths
• can carry up to 200 Mb/second
• less interference due to shielding
• Uses FDM to transmit 1000s of voice channels and
  100s of TV channels
• Not popular in LANS
• More difficult to work with than UTP

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Transmission Media

• Fiber Optics cable
• thousands of little fiber optic strands
• May be glass or plastic
• Thickness of a human hair
• Inner core surrounded by glass (cladding)
• Can be single mode or multimode
• Single mode
• Expensive, bigger capacity, long segment length
• 8/125
• Multimode
• Cheaper, less capacity
• 62.5/125
• Data transmitted as pulses of light
• 500 Kb/sec to several GB/sec

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A typical optic fiber

• Core made of silica and germania
• Optic cladding is pure silica
• Mix of different refractive indices allows for
• total internal reflection

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Point-to-point fiber optic system
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Advantages of fiber optics

• Nearly infinite capacity
• Single fiber can carry 40000 telephone calls or
  250 channels of television
• High transmission rates at greater distances
• Immune to interference and electricity
• Does not corrode (being glass)
• Smaller and lighter than coaxial or twisted pair
• Extremely secure

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Wireless Transmission

• Directional
• Focuses electromagnetic beam in direction of
  receiver
• Terrestrial microwave
• Satellite microwave
• Omni directional
• Spreads the electromagnetic signal in all
  directions
• AM and FM radio
• 3G networks
• Smart watches

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Terrestrial Microwave

• Parabolic dish antenna sends signal to receiving
  dish
• Line-of-sight
• Typically on towers to avoid obstacles
• Frequencies in the gigahertz range

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What is a telecommunications satellite?
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Telecommunications satellites

• Space-based cluster of radio repeaters (called
  transponders)
• Link
• terrestrial radio transmitters to satellite
  receiver (uplink)
• Satellite transmitters to terrestrial receivers
  (downlink)

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Orbits

• Mostly geostationary (GEO)
• Circular orbit
• 22,235 miles above earth
• Fixed point above surface
• Almost always a point on Equator
• Must be separated by at least 4 degrees

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Satellite services

• Wide Area Broadcasting
• Single transmitter to multiple receivers
• Wide Area Report-Back
• Multiple transmitters to a single receiver
• Example VSATs (very small aperture terminals)
• Also have microwave transmitters and receivers
• Allows for spot-beam transmission (point-
  to-point data communications)
• Can switch between beams upon request (Demand
  Assigned Multiple Access DAMA)
• Multi-beam satellites link widely dispersed
  mobile and fixed point users

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Earth-based equipment

• Original microwave transmitters and receivers
  were large installations
• Dishes measuring 100 feet in diameter
• Modern antennas about 3 feet in diameter

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A Modern GEO satellite (IntelSat 900 series)

• May have more than 72 separate microwave
  transponders
• Each transponder handles multiple simultaneous
  users (protocol called Time Division Multiple
  Access)
• Transponder consists of
• Receiver tuned to frequency of uplink
• Frequency shifter (to lower frequency to that of
  transmitter)
• Power amplifier

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IntelSat 902 (launched August 30, 2001)
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Frequency ranges

• Most transponders operate in 36MHz bandwidth
• Use this bandwidth for
• voice telephony (400 2-way channels/transponder)
• Data communication (120Mbs)
• TV and FM Radio

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C-band, Ku-band, Ka-band

• Most GEO satellites operate in the C-Band
  frequencies
• Uplink at 6 GHz
• Downlink at 4 GHz
• Ku-band also used
• Uplink at 14 GHz
• Downlink at 11 GHz
• Above bands best suited for minimal atmospheric
  attenuation
• Few slots left forcing companies to look at Ka
  band (uplink30 GHZ , downlink 20 GHz)

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Companies on the forefront Teledesic

• Offer Internet-in-the-Sky?
• Main shareholders Craig McCaw and Bill Gates
• McCaw also has taken over ICO Global
  Communications
• Wanted Iridium but has backed out

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Teledesic

• Again, series of LEO satellites
• 24 pole orbiting satellite rings, 15 degrees
  apart
• 12 satellites in each ring (total 288 LEO
  satellites)
• Worldwide switching.. Satellites pass on data
  through laser
• Will map IP packets on latitudes and longitudes
  .. Average will be 5 satellite hops in 75 ms
• Supposed to start in 2002 offer 2Mbps Internet
  access from terminals starting at 1000 each
• Postponed to 2005

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Optical Transmission

• Cutting edge
• Uses modulated monochromatic light to carry data
  from transmitter to receiver
• Optical wavelengths are suited for high rate
  broadband communications
• Laser-based (up to 1000 times faster than coaxial)

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Research Question for Next Class

• What is Abilene?

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Next Class

• More about networks