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

3
Types of Signals

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

4
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
  (quantization)
• Samples 44,100 times per second for each of two
  channels (sampling rate)
• 16441002 176,000 bps
• One hour of music requires 633Mb

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Calculate for a DVD

• Sampling rate 192,000/sec
• Quantization 24 bits
• Number of channels 2
• How much space (in Gb) is required for a 2 hour
  movie?

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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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MEO Satellites

• Exist between the first and second Van Allen
  Radiation belts
• Peak height is 9000 miles\
• Typical is about 4000 miles
• Need less power than GEO satellites to reach.
• Example GPS satellites

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Global Positioning Systems

• A constellation of 24 DoD satellites orbiting
  about 10,000 miles above earths surface
• First launched in 1978 complete set by 1994
  replaced every ten years or so..
• Solar-powered Each circles earth about twice a
  day
• Also have 5 ground stations (control segments)
• monitor the GPS satellites, checking both their
  operational health and their exact position in
  space.
• Five monitor stations Hawaii, Ascension Island,
  Diego Garcia, Kwajalein, and Colorado Springs.

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GPS Constellation
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How they work

• To determine position
• GPS satellites emit 3 bits of information in its
  signal (L1 for civilians L2 for military)
• Pseudorandom code (ID which identifies specific
  satellite)
• Ephemeris data (status of satellite and current
  data and time)
• Almanac data (tells exactly where that satellite
  and all others are supposed to be at any given
  time during the day)
• Finding your location
• Compare time a signal is transmitted to when it
  is received tells how far away satellite is
  receiver knows it is on the surface of an
  imaginary sphere centered around the GPS
  satellite
• With similar distance measurements from other
  satellites, receiver can determine location
  (intersection of at least three spheres)
• GPS receiver must lock on to 3 satellites to give
  2D location 4 satellites to give altitude as
  well.
• Accurate up to 10-15 meters DGPS and Augmented
  GPS can go down to a few centimeters.

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Sources of Error for GPS

• Ionosphere and troposphere delays The satellite
  signal slows as it passes through the atmosphere.
  
• Signal multipath This occurs when the GPS
  signal is reflected off objects such as tall
  buildings or large rock surfaces before it
  reaches the receiver. This increases the travel
  time of the signal, thereby causing errors.
• Receiver clock errors A receiver's built-in
  clock is not as accurate as the atomic clocks
  onboard the GPS satellites. Therefore, it may
  have very slight timing errors.
• Orbital errors Also known as ephemeris errors,
  these are inaccuracies of the satellite's
  reported location.
• Number of satellites visible The more
  satellites a GPS receiver can "see," the better
  the accuracy. Buildings, terrain, electronic
  interference, or sometimes even dense foliage can
  block signal reception, causing position errors
  or possibly no position reading at all. GPS units
  typically will not work indoors, underwater or
  underground.
• Satellite geometry/shading This refers to the
  relative position of the satellites at any given
  time. Ideal satellite geometry exits when the
  satellites are located at wide angles relative to
  each other. Poor geometry results when the
  satellites are located in a line or in a tight
  grouping.
• Intentional degradation of the satellite signal
  Selective Availability (SA) is an intentional
  degradation of the signal once imposed by the
  U.S. Department of Defence. The government turned
  off SA in May 2000, which significantly improved
  the accuracy of civilian GPS receivers.

Source http//www.pocketgps.co.uk/howgpsworks.php
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LEO Satellites

• Lowest of the satellites below the first
  radiation belt
• Typically orbit at 600 miles
• Much less power needed than for GEO and MEO
• Can be accessed using smaller devices such as
  phones.
• Available anywhere in the world.
• Geostationary?

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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