Transmission Media: Wires, Cables, Fiber Optics, and Microwaves

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Transmission MediaWires, Cables, Fiber Optics,
and Microwaves

• Based on Chapter 4 of William Stallings, Data and
  Computer Communication, 8th Ed.

Kevin BoldingElectrical EngineeringSeattle
Pacific University
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Transmission Media

• A signal must be transmitted through some medium

• Guided Media determine the path of the signal
• Wires (cables, twisted pair, coax)
• Fiber Optics
• Other things

• Signals Propagate in all directions in Unguided
  Media
• The medium is usually free space (air), but the
  signal type gets the name
• Refers to transmitting signals through passive
  media that does not change the signals direction
• Microwaves, broadcast radio waves
• Lasers, Infrared

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Media Issues

• Frequency range
• Some media support higher frequencies than others

• Impairments
• Different media deform signals differently
• Some are more susceptible to noise and distortion

• Cost
• Were in the real world

• Number of receivers
• Broadcast vs. point-to-point

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How Fast/How Far can a Signal be Sent?

• The question
• Given a source signal with a given power, how far
  can it go before it is attenuated so much that
  the SNR is too low to be usable?

• As far as media is concerned, the main issue is
  attenuation

• Attenuation increases with distance. Usually
  expressed in dB/m, dB/100ft, etc.

• Attenuation usually increases with frequency.

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Attenuation Curves
Cat-5 0.21 mm2RG58 0.64 mm2 RG6 1.0 mm2
Attenuation is very dependent on conductor size
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Frequency of various signals
Source Stallings, Fig. 4.1
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Guided Media

• Guided media control the path of the signal wave
• Electrical Signal needs conductor and ground
• Differences are in how ground/conductor interact
• Twisted pair
• Coax
• Striplines on PCBs

• Optical Signal is sent using internal
  reflection
• Differences are in light sources and fiber
  diameter

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Differential Signaling
Differential signaling works best when the two
signal conductors are routed as close as possible
to each other so they experience the same
external noise.
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Electrical Cables

• Keep the two parts of the signal close together

• Electromagnetic interference (EMI)
• Loops make great antennas
• Antenna strength proportional to the area inside
  of the loop
• Worse for shorter wavelengths

• Common ground systems (such as PCBs with ground
  planes)
• Return path directly below signal
• Minimizes loop area

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Twisted Pair Cables
Adjacent LoopsOut of phase

• Twist the signal and ground together
• Both sides experience similar noise effects
• Loop size proportional to twist size
• Adjacent twists are 180 degrees out of phase
• Tend to cancel out
• Varying the twist size helps to minimize crosstalk

• Data rates
• Over long distances, about 1-3 Mbps
• Short distances 1Gbps and higher

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Shielding

• Twisted pair usually comes bundled with several
  pairs in a cable
• Unshielded Just a plastic (teflon) jacket
• For distances of around 100m -
• Cat-3 UTP lt16Mbps, Cat-5 UTP 100Mbps, Cat-6
  UTP 1000Mbps
• Shielded Includes a grounded shield

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Coaxial Cables

• Concentric mesh wire for ground
• Acts as an excellent shield
• Very little interference or radiation

• Center conductor can be large (low resistance),
  reducing attenuation
• Better data rates over long distances than
  twisted pair

• The downside
• Expensive to manufacture
• More difficult to install

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Optical Fiber

• Relies on total internal reflection
• Light waves bounce of edge of fiber
• Channels waves to destination

(Source Stallings, Fig. 4.4)

• Varieties
• Multi-mode (wide fiber)
• Light waves bounce off at different angles
• Some have shallow angles (straight path), while
  others have steeper angles (crooked path)
• Results in pulse spreading
• Single-mode (narrow fiber)
• Only a straight shot down the middle is allowed
• Requires a laser source

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Fiber has its advantages

• Advantages
• No electromagnetic interference
• Very little attenuation
• Extremely high bandwidth (THz)
• Small, lightweight

• Disadvantages
• More expensive transceivers
• More difficult to install

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Wireless (Unguided) Media

• Omnidirectional
• Signal radiates in all directions
• Good for broadcast
• Inexpensive antenna

• Directional
• Signal radiates in a single direction
• Usually requires parabolic (dish) antenna
• 2-40 GHz (microwave)
• Also works with lasers

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Unguided Media Attenuation
Transmitted wave spreads out over a spherical
surface
Power density at receiver
Received power depends on the receiver antennas
aperture
Thus

• f frequency (Hz)
• d distance (m)
• wavelength (m)
• c speed of light (m/s)

Free Space Path Loss
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Terrestrial Radio (All forms)

• Ground-wave propagation follows the curvature of
  the earth
• Frequencies below 2MHz
• AM radio (550-1600KHz)

• Sky-wave propagation relies on the ionosphere and
  the surface of the earth to refract waves
  back-and-forth
• Frequencies 2MHz-30MHz
• Short-wave Radio, HAM radio

• Line of site is point-to-point in a nearly
  straight line
• Frequencies 30MHz and up
• FM radio, TV, Mobile phones, etc.
• Max distance between antennas with height h1 and
  h2

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Satellite Radio

• Requires satellite in geosynchronous orbit
• 35,784 km
• Delay of ¼ second (round-trip)
• Satellites spaced 4 degrees apart

• Above 10GHz, signal is attenuated by atmosphere
• Higher frequencies use smaller dishes, though