Chapter 4 - Transmission Media 9e
Transcript of Chapter 4 - Transmission Media 9e
Transmission MediaCEN 220/CIS 192 Advanced Data Communications and NetworkingData and Computer Communications, W. Stallings 9/E, Chapter 4
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Data Link Control Protocols “Communication channels in the animal world include
touch, sound, sight, and scent. Electric eels even use electric pulses. Ravens also are very expressive. By a combination voice, patterns of feather erection and body posture ravens communicate so clearly that an experienced observer can identify anger, affection, hunger, curiosity, playfulness, fright, boldness, and depression.”
—Mind of the Raven, Bernd Heinrich
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Overviewtransmission medium is the physical path between transmitter and receiverguided media – guided along a solid mediumunguided media – atmosphere, space, water characteristics and quality determined by medium and signal
guided media - medium is more important unguided media - bandwidth produced by the antenna
is more important
key concerns are data rate and distance
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Factors Determining Data Rate and Distance
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Electromagnetic Spectrum
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Transmission Characteristics of Guided Media
Frequency Range
Typical Attenuation
Typical Delay
Repeater Spacing
Twisted pair (with loading)
0 to 3.5 kHz
0.2 dB/km @ 1 kHz
50 µs/km 2 km
Twisted pairs (multi-pair cables)
0 to 1 MHz 0.7 dB/km @ 1 kHz
5 µs/km 2 km
Coaxial cable
0 to 500 MHz
7 dB/km @ 10 MHz
4 µs/km 1 to 9 km
Optical fiber
186 to 370 THz
0.2 to 0.5 dB/km
5 µs/km 40 km
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Guided Transmission Media
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Twisted Pair
Twisted pair is the least expensive and most widely used guided transmission medium.
– consists of 2 insulated copper wires in a regular spiral pattern
– a wire pair acts as a single communication link
– pairs are bundled together into a cable
– most commonly used in the telephone network and for communications within buildings
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Twisted Pair - Transmission Characteristics
susceptible to interference and noise
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Unshielded vs. Shielded Twisted Pair
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Twisted Pair Categories and Classes
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Near End Crosstalkcoupling of signal from one pair of conductors to another
occurs when transmit signal entering the link couples back to the receiving pair -
(near transmitted signal is picked up by near receiving pair)
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Signal Power Relationships
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Coaxial Cable
Coaxial cable can be used over longer distances and support more stations on a shared line than twisted pair.
– consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor
– is a versatile transmission medium used in a wide variety of applications
– used for TV distribution, long distance telephone transmission and LANs
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Coaxial Cable – Transmission Characteristics
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Optical Fiber
Optical fiber is a thin flexible medium capable of guiding an optical ray.
– various glasses and plastics can be used to make optical fibers
– has a cylindrical shape with three sections – core, cladding, jacket
– widely used in long distance telecommunications
– performance, price and advantages have made it popular to use
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Optical Fiber - Benefitsgreater capacity
data rates of hundreds of Gbps
smaller size and lighter weight considerably thinner than coaxial or twisted pair cable reduces structural support requirements
lower attenuation
electromagnetic isolation not vulnerable to interference, impulse noise, or crosstalk high degree of security from eavesdropping
greater repeater spacing lower cost and fewer sources of error
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Optical Fiber - Transmission Characteristicsuses total internal reflection to transmit light
– effectively acts as wave guide for 1014 to 1015 Hz (this covers portions of infrared & visible spectra)
light sources used:– Light Emitting Diode (LED)
● cheaper, operates over a greater temperature range, lasts longer
– Injection Laser Diode (ILD)● more efficient, has greater data rates
has a relationship among wavelength, type of transmission and achievable data rate
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Optical Fiber Transmission Modes
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Frequency Utilization for Fiber Applications
Wavelength (invacuum) range
(nm)
FrequencyRange (THz)
BandLabel
Fiber Type Application
820 to 900 366 to 333 Multimode LAN
1280 to 1350 234 to 222 S Single mode Various
1528 to 1561 196 to 192 C Single mode WDM
1561 to 1620 192 to 185 L Single mode WDM
WDM (wavelength division multiplexing)
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Attenuation in Guided Media
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Wireless Transmission Frequencies
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Antennaselectrical conductors used to radiate or collect electromagnetic energy
same antenna is often used for both purposes
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Radiation Patternpower radiated in all directions
does not perform equally well in all directions as seen in a radiation pattern diagram
an isotropic antenna is a point in space that radiates power
in all directions equally with a spherical radiation pattern
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Parabolic Reflective Antenna
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Antenna Gainmeasure of the directionality of an antenna
power output in particular direction verses that produced by an isotropic antenna
measured in decibels (dB)
results in loss in power in another direction
effective area relates to physical size and shape
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Terrestrial Microwave
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Terrestrial Microwave Applicationsused for long haul telecommunications, short point-to-point links between buildings and cellular systemsused for both voice and TV transmissionfewer repeaters but requires line of sight transmission1-40GHz frequencies, with higher frequencies having higher data ratesmain source of loss is attenuation caused mostly by distance, rainfall and interference
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Microwave Bandwidth and Data Rates
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Satellite Microwavea communication satellite is in effect a microwave relay stationused to link two or more ground stationsreceives on one frequency, amplifies or repeats signal and transmits on another frequency
frequency bands are called transponder channels
requires geo-stationary orbit rotation match occurs at a height of 35,863 km at the equator need to be spaced at least 3° - 4° apart to avoid interfering
with each other spacing limits the number of possible satellites
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Satellite Point-to-Point Link
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Satellite Broadcast Link
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Satellite Microwave Applicationsprivate business networks
– satellite providers can divide capacity into channels to lease to individual business users
television distribution– programs are transmitted to the satellite then broadcast down
to a number of stations which then distributes the programs to individual viewers
– Direct Broadcast Satellite (DBS) transmits video signals directly to the home user
global positioning– Navstar Global Positioning System (GPS)
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Transmission Characteristicsthe optimum frequency range for satellite transmission is 1 to 10 GHz
– lower has significant noise from natural sources
– higher is attenuated by atmospheric absorption and precipitation
satellites use a frequency bandwidth range of 5.925 to 6.425 GHz from earth to satellite (uplink) and a range of 3.7 to 4.2 GHz from satellite to earth (downlink)
– this is referred to as the 4/6-GHz band
– because of saturation the 12/14-GHz band has been developed (uplink: 14 - 14.5 GHz; downlink: 11.7 - 12.2 GHz
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Broadcast Radioradio is the term used to encompass frequencies in the range of 3 kHz to 300 GHz
broadcast radio (30 MHz – 1 GHz) covers– FM radio
– UHF and VHF television
– data networking applications
omnidirectional
limited to line of sight
suffers from multi-path interference– reflections from land, water, man-made objects
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Infraredachieved using transceivers that modulate non-coherent infrared lighttransceivers must be within line of sight of each other directly or via reflectiondoes not penetrate wallsno licenses requiredno frequency allocation issuestypical uses: TV remote control
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Frequency Bands
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Wireless Propagation Ground Wave
ground wave propagation follows the contour of the earth
and can propagate distances well over the visible horizon
this effect is found in frequencies up to 2MHz
best example of ground wave communication is AM radio
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Wireless Propagation Sky Wave
sky wave propagation is used for amateur radio, CB radio, and international broadcasts, e.g., BBC, Voice of America
a signal from an earth based antenna is reflected from the ionized layer of the upper atmosphere back down to earth
sky wave signals can travel through a number of hops, bouncing back and for the between the ionosphere and the earth’s surface
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Wireless Propagation Line of Sight
ground and sky wave propagation modes do not operate above 30-MHz
communication must be by line of sight
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Refractionvelocity of electromagnetic wave is a function of the density of the medium through which it travels
– ~3 x 108 m/s in vacuum, less in anything else
speed changes with movement between media
index of refraction (refractive index) is– sine (incidence) / sine (refraction)
– varies with wavelength
gradual bending– density of atmosphere decreases with height, resulting in
bending of radio waves towards earth
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Line of Sight Transmission
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Free Space Loss
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Multipath Interference
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Summarytransmission Media
– physical path between transmitter and receiver
– bandwidth, transmission impairments, interference, number of receivers
guided Media– twisted pair, coaxial cable, optical fiber
wireless Transmission– microwave frequencies
– antennas, terrestrial microwave, satellite microwave, broadcast radio
wireless Propagation– ground wave, sky wave, line of sight