Transmission Media Reading Assignment : Stallings Chapter 3

1
Transmission Media Reading Assignment
Stallings Chapter 3

• Transmission Media
• physical path between transmitter and receiver
• electromagnetic wave
• Guided transmission
• solid medium, copper twisted pair, coaxial cable,
  optical fiber
• data rate mainly determined by medium
• Unguided transmission
• atmosphere, outer space
• wireless transmission
• low frequencies omnidirectional (all directions)

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• high frequencies possible to focus signal
• transmission characteristics determined by
  bandwidth
• Factors determining data rate and distance
• bandwidth
• greater bandwidth higher data rate
• transmission impairments
• attenuation
• twisted pair gt coaxial cable gt optical fiber
• interference
• by unshielded cables
• from competing signals in overlapping frequency
  bands
• number of receivers
• Each attachment introduces some attenuation and
  distortion on the line.

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• Guided transmission media
• point-to-point vs. multipoint
• Twisted pair
• Physical description
• least expensive, most widely used
• Spiral pattern decreases crosstalk interference.
• Neighboring pairs have different twist length (2
  - 6 in).
• bundled together in hundreds
• limitation data rate and distance
• Applications
• analog
• telephone systems
• private branch exchange (PBX)
• end office (of phone company)
• digital
• digital switch or digital PBX (64 kbps)
• local area network 10 Mbps or 100 Mbps (limited
  length)
• long distance (4 Mbps)

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• Transmission characteristics
• amplifiers needed
• analog signals 5 - 6 km
• digital signals 2 - 3 km
• high attenuation for higher frequency
• susceptible to interference and noise
• shielding to reduce interference from power lines
• twisting to reduce low-frequency interference
• different twist lengths
• point-to-point analog signals
• bandwidth 250 k Hz a few voice channels
• long distance point-to-point digital signals
• 4 Mbps
• short distance digital signals
• 100 Mbps
• Unshielded twisted pair (UTP)
• Category 3 and 5 UTP
• Shielded twisted pair (STP)

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• Coaxial cable
• Physical description
• Applications
• TV distribution (cable TV)
• long distance telephone transmission
• inferior to optical fibre
• short-run computer system links
• high speed I/O
• local area networks (LAN)
• Transmission characteristics
• much better than twisted pair in attenuation,
  thermal noise, and intermodulation noise
• analog signals amplifiers every few km
• digital signals repeaters every km
• Optical fiber
• Physical description
• core
• thin strand of fibre of glass or plastic
• cladding
• glass or plastic

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• Advantages over twisted pair or coaxial cable
• greater capacity
• smaller size, lighter weight
• lower attenuation
• electromagnetic isolation
• no radiation of energy, little interference to
  others, difficult to tap - security
• greater repeater spacing
• Applications
• long-haul trunks
• 900 miles, 20 k - 60 k voice channels
• replacing coaxial cables in telephone networks
• metropolitan trunks
• joining telephone exchanges in a metropolitan
  area
• average 7.8 miles, 100 k voice channels
• rural-exchange trunks
• subscriber loops
• from central exchange to a subscriber
• still more expensive than twisted pair
• local area networks

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• Transmission characteristics
• multimode variety of reflecting angles
• single mode a single angle can pass -- the
  axial ray
• light source
• light-emitting diode (LED) cheaper, longer
  operational life, greater temperature range
• injection laser diode (ILD) more expensive,
  higher data rates
• wavelength windows
• 850, 1300, 1550 nm, all infrared
• Wireless transmission
• antenna
• directional transmission
• focused electromagnetic beam, higher frequency
• omnidirectional transmission

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• Frequencies
• broadcast radio
• VHF, part of UHF
• microwave
• part of UHF, all of SHF
• 2 G Hz 40 G Hz
• directional, point-to-point
• 30 M Hz 1 G Hz
• omnidirectional, broadcast
• infrared
• 3 X 1011 2 X 1014 Hz
• point-to-point, confined areas
• Terrestrial microwave
• Physical description
• parabolic dish, 10 ft in diameter, installed
  high above ground, rigidly fixed, focused on next
  receiving antenna / relay tower
• e.g., 100 m high gt 82 km apart
• Applications
• long-haul telecommunications service
• requiring line-of-sight transmission

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• short point-to-point links between buildings
• private short-haul microwave link to bypass local
  telephone company
• also used to propagate cable TV to local
  installations, and then to subscribers via
  coaxial cable
• Transmission characteristics
• Attenuation varies as the square of the distance.
• For twisted pair and coaxial cable, loss varies
  logarithmically with distance.
• I.e, repeaters or amplifiers are farther apart
  for microwave systems.
• Attenuation increases with frequency.
• Attenuation increases with rainfall.
• Assignment of frequency bands is strictly
  regulated to avoid interference.
• Higher frequency gt higher potential bandwidth gt
  higher data rate
• for short distance
• smaller and cheaper antenna

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• Satellite microwave
• Physical description
• satellite microwave relay station
• earth / ground station
• uplink frequency band ? downlink frequency band
• transponder channels / transponders
• point-to-point and broadcast
• geostatic satelites height 35,784 km, period of
  rotation earths period
• 4? angular spacing with respect to earth for 4/6
  G Hz band, 3? for 12/14 G Hz band.
• Applications
• TV distribution
• direct broadcast satellite (DBS) video signal
  directly to home user
• long-distance telephone transmission
• point-to-point trunks between exchange offices
• private business networks
• Divide total capacity into a number of leased
  channels to individual business users.
• Very small aperture terminal (VSAT) a low cost
  private network

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• Transmission characteristics
• 1 - 10 G Hz
• below 1 G Hz galactic, solar, atmospheric noise
• above 10 G Hz atmospheric attenuation
• typical frequency 4 / 6 G Hz band
• uplink 5.925 - 6.425 G Hz
• downlink 3.7 - 4.2 G Hz
• propagation delay about 0.25 sec from one
  ground station to another
• inherently broadcast nature