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ECIS469: Lecture 2
Fundamentals of Networking
Data Communications
• exchange of digital information between
two devices using an electronic
transmission medium
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
• 8*8000=64,000 bits per second to provide approximation of
analog signal
• 64Kbps represents a single voice line in digital
telecommunications
Pulse Code Modulation
1111 1111
Value transmitted
128 values
This side
Sampling Interval = 1/8000 second
128 values
This side
0000 0000
Value transmitted
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
• 16*44100*2 = 176,000 bps
• One hour of music requires 633Mb
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
Wave Characteristics
Amplitude
Wavelength
Amplitude Modulation
Frequency Modulation
Telecommunications System
Minicomputer
Main
frame
Front-End
Processor
terminals
modems
multiplexer
Remote location
Multiplexing
• Allows multiple signals to be sent over
same medium at same time
• Modes of multiplexing
– Frequency Division (FDM)
– Time Division (TDM)
Frequency Division
Multiplexing
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-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
Time Division Multiplexing
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-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.
Transmission Media
• the physical path along which the data is
carried
• Types
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twisted pair
coaxial
fiber optics and free space
satellite
terrestrial
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
Transmission Media
• Coaxial cable
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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
Transmission Media
• Fiber Optics cable
– thousands of little fiber optic strands
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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
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
Point-to-point fiber optic system
Advantages of fiber optics
• Nearly infinite capacity
– Single fiber can carry 40000 telephone calls or 250
channels of television
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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
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
Terrestrial Microwave
• Parabolic dish antenna sends signal to
receiving dish
• Line-of-sight
• Typically on towers to avoid obstacles
• Frequencies in the gigahertz range
What is a telecommunications
satellite?
Telecommunications satellites
• Space-based cluster of radio repeaters
(called transponders)
• Link
– terrestrial radio transmitters to satellite
receiver (uplink)
– Satellite transmitters to terrestrial receivers
(downlink)
Orbits
• Mostly geostationary (GEO)
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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
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
Earth-based equipment
• Original
microwave
transmitters and
receivers were
large installations
– Dishes measuring
100 feet in
diameter
• Modern antennas
about 3 feet in
diameter
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
IntelSat 902 (launched August 30,
2001)
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
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 (uplink:30 GHZ , downlink: 20 GHz)
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
Teledesic
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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
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)
Research Question for Next Class
• What is Abilene?