CS3502, Data and Computer Networks: the physical layer2

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CS3502,Data and Computer Networksthe physical
layer-2

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channel capacity

• channel - a path, contained in the transmission
  medium, through which signals/bits may pass
• a part of the medium, not all
• channel capacity - maximum number bits/sec the
  channel can support
• factors which determine channel capacity
• bandwidth
• number signal levels
• noise

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channel capacity

• basic channel capacity formulas
• 2 caseschannel requirement, channel capacity.
• Case 1 The channel capacity required to digitize
  an analog signal which contains the highest
  frequency Fmax
• is given by the Nyquist formula
• R 2 Fmax log2 (V),
• where
• R channel requirement in bps,
• Fmax maximum frequency in hertz
• V signal levels

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channel capacity

• examples
• 1. Fmax 3100 Hz, 8 signal levels. What is R?
• A R 2(3100) log(8) 18,600 bps
• 2. R 60 Kbps, Fmax is 6000 Hz. How many signal
  levels?
• A ?
• 3. Fmax 10KHz, V is 16. What is R?
• A ?

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channel capacity

• Case 2 Channel Capacity with noise present.
  Shannon formula.
• C W log2 (1 S/N)
• where
• C channel capacity in bps
  W band width in hz
• S signal strength in Watts
  N noise strength in Watts
• Note 1 upper bound, independent of signal
  levels.
• Note 2 S/N often given in decibels if so, must
  convert to
• absolute ratio using the formula
• S/N dB 10 log10 (S/N)

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channel capacity

• example
• 1. 30 dB 10 log10 S/N --gt S/N 103
  1000.
• 2. S/N 500, C 1Mb/s. What bandwidth needed?
• A 1 Mb/s W log2 (1500), appr.
• 1000000 W (9)
• W 111111 Hz (approx)
• 3. S/N 40dB, W 6200 Hz.
• A 81,840 (approx.)

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channel capacity

• note 1 Shannon formulas is an upper bound
  theoretical maximum. Actual data rates often much
  less.
• note 2 noise considered in Shannon is only
  thermal noise no other type of noise.
• note 3 data compression not considered. This can
  raise the data limits considerably.

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transmission media

• Guided Media
• twisted pair (copper)
• coaxial cable (copper)
• optical fibers (silicon... plastic or glass)
• Unguided Media
• broadcast radio frequencies
• terrestrial microwave
• satellite microwave
• Note take the tables in Text on data rates, etc.
  as a general guide, NOT as absolute truth

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transmission media twisted pair

• copper a good conductor of electricity
• (side note recent developments by IBM leading to
  use of copper on ICs - better chips)
• 2 copper wires used to form a circuit between
  Xmitter, Rcvr
• twisting gives better electrical properties
• backbone of the local telephone system
• also used for limited long distance telephones
• also heavily used in data comm., LANs
• used for both digital, analog signals

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transmission media twisted pair

• various quality levels voice grade, Cat 5
• data rates 1-100 Mbps, depending on quality
  voice grade at low end, Cat 5 top end.
• higher quality are more tightly twisted
• advantages
• mature - well known technology
• connections, splices easy
• production, installation techniques well known
• relatively cheap, easy to install

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transmission media twisted pair

• disadvantages
• cost of copper
• signal attenuation increases with frequency,
  starting at low frequencies
• often needs shield to reduce noise pickup
• susceptible to cross talk if lines close together
• susceptible to lightning strikes
• less bandwidth than most other media
• See text for further explanation

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transmission media coaxial cable

• a thick cable, consisting of an inner copper core
  surrounded by an insulator, surrounded by another
  conductor (braided shield), wrapped in a
  protective shield and an outer cover. (see
  diagram in text)
• Properties (approx.)
• bandwidth 500Mhz, analog
• data rates 500 Mbps or more
• repeater spacing 1-10 Km
• Two basic types
• broadband
• baseband

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transmission media coaxial cable

• broadband TV cable, analog signals
• baseband LANs, digital signals
• Uses
• long distance telephone
• cable TV
• LANs
• Note higher capacity than t.p., but also much
  bulkier and difficult to work with in limited
  spaces

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transmission media coaxial cable

• advantages
• lower attenuation than t.p. at high frequencies
• wider usable bandwidth
• better isolation (less susceptible to
  interference)
• easy to tap
• disadvantages
• physically larger, bulky
• limited bending radius
• heavier
• fire code restrictions on materials

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transmission media optical fiber

• development of OF a major milestone in
  communications made feasible by invention of
  laser 1960 first fibers developed 1970
• twisted pair 19th century coax 1930 radio
  1900 integrated circuits 1950...
• since about 1988, majority of all U.S. long
  distance traffic over OF, though only about 5 of
  cable is OF.
• due to OF, the networks have potential to be
  faster than the computer ---- a big flip flop

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transmission media optical fiber

• A thin, flexible medium of extremely pure
  plastic/glass. Thickness about 2-125 microns.
  Core often 62.5 microns.
• much higher data rates from 100M to several G.
• note prop. speed approximately 2/3 c, as with tp
  and coax bits much smaller
• repeater spacing much higher...
• FDDI, DQDB, and SONET all optical fiber standards
• principle each bit is transported by a tiny ray
  of light(darkness), guided by the medium.
• requires extremely accurate transmitters,
  receivers much finer degree of synchronization

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transmission media optical fiber

• principle total internal reflection
• Two major types of fiber
• 1. multi-mode
• step index
• graded index
• 2. single mode/monomode
• limitations
• modal dispersion (multimode)
• material dispersion (single mode)
• attenuation (single mode, at very high data rates)

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transmission media optical fiber

• advantages
• much higher bandwidth, real and potential
• very low radiation, noise pickup shielding not
  needed, crosstalk not a problem
• very low attenuation, and little variation in
  .85,1.3,and 1.55 micro- meter range
• not susceptible to lightning, etc.
• small physical size and weight
• cost will decrease
• very difficult to tap

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transmission media optical fiber

• disadvantages
• cost
• technology less mature
• splicing difficult and critical
• installation more difficult
• Key note fiber has literally made the network
  faster than the computer. We have far to go
  before we reach the potential data rates of
  fiber....

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unguided media broadcast radio TV

• lower frequency ranges roughly 30KHz-1GHz
• omni-directional
• data rates not as high as microwave, so less
  useful for data, but good for broadcast radio
• better propagation characteristics less
  attenuation, less interference from rain, etc.

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unguided media

• lower frequency ranges broadcast radio
• 300-3000 kHz MF AM radio
• 3-30 MHz HF shortwave radio, CB
• 30-300 MHz VHF FM radio, VHF TV
• ...
• microwave frequency ranges 1 to 40 GHz
• infrared just below visible light frequency
  1011 - 1014

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unguided media terrestrial microwave

• focused beam, 1-2 degrees
• high frequencies 3-40 GHz --gt high data rates
• paraboloid shaped antennas
• better repeater spacing than cable
• high data rates
• more susceptible to rain, clouds, dust, etc.
  than others

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unguided media satellite microwave

• high frequency ( same as terr. uwave)
• geosynchronous satellite --gt repeater in sky
• broadcast media
• 22,300 miles --gt 35,000 Km
• receives, xmits on diff. frequencies to avoid
  interference
• need spacing of 4 deg. between satellites
• significant prop delay 250 ms
• less difficulty with atmosphere
• 3 major differences with terr. microwave