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Transmission media (wires and no wires)
Transmission media:
Guided media, the waves are guided along a physical
path; examples of guided media are twisted pair,
coaxial cable, and optical fiber.
Unguided media, also called wireless, provide a
means for transmitting electromagnetic waves but do
not guide them; examples are propagation through air,
vacuum, and seawater.
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 Guided Transmission Media
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Magnetic Media
Twisted Pair
Coaxial Cable
Fiber Optics
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 Magnetic Media
One of the most common ways to transport data from one
computer to another is to write them onto magnetic tape or
removable media (e.g., recordable DVDs), physically transport
the tape or disks to the destination machine, and read them
back in again.
A. PC => recordable DVD => another PC;
B.Ultrium tape: 200 gigabytes; A box (60X60X60 cm) can hold
about 1000 of these tapes =>200 terabytes or 1600 terabits
(24 hours by Federal Express, 1600 terabits/86400 sec =
19Gbps => if one hour away => 400Gbps.
C.Cost less than 5000 USD for 200 TB => less than 3 cents for
1GB
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 Twisted Pair (Copper Wires)
(a) Category 3 UTP.
(b) Category 5 UTP.
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 A coaxial cable (Copper Wires)
A coaxial cable.
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 Fiber Optics
(a) Three examples of a light ray from inside a silica fiber
impinging on the air/silica boundary at different angles.
(b) Light trapped by total internal reflection.
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 Transmission of Light through Fiber
Attenuation of light through fiber in the infrared region.
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 Fiber Cables
(a) Side view of a single fiber.
(b) End view of a sheath with three fibers.
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 Fiber Cables (2)
A comparison of semiconductor diodes and LEDs as light
sources.
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 Optical Fiber vs. Copper Wire
Comparison of the properties of wires and fiber:
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 Unguided Transmission Media
 The Electromagnetic Spectrum
 Radio Transmission
 Microwave Transmission
 Infrared and Millimeter Waves
 Lightwave Transmission
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 The Electromagnetic Spectrum
The electromagnetic spectrum and its uses for communication.
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 Radio Transmission
(a) In the VLF, LF, and MF bands, radio waves follow the curvature
of the earth.
(b) In the HF band, they bounce off the ionosphere.
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 Microwave Transmission
Microwaves have much bandwidth and are widely
used indoors (WiFi) and outdoors (3G, satellites)
– Signal is attenuated/reflected by everyday objects
– Strength varies with mobility due multipath fading,
etc.
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 Infrared and Millimeter Waves
infrared and millimeter waves: are widely
used for short-range communication. The
remote controls used on televisions, VCRs,
and stereos all use infrared communication.
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 Lightwave Transmission
Convection currents can interfere with laser communication
systems. A bidirectional system with two lasers is pictured here.
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 Wireless vs. Wires/Fiber
Wireless:
+ Easy and inexpensive to deploy
+ Naturally supports mobility
+ Naturally supports broadcast
• Transmissions interfere and must be managed
• Signal strengths hence data rates vary greatly
Wires/Fiber:
+ Easy to engineer a fixed data rate over point-to-point links
• Can be expensive to deploy, esp. over distances
• Doesn’t readily support mobility or broadcast
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