Transcript EE 3760 chapter 7 - Seattle Pacific University

Transmission Media:
Wires, Cables, Fiber Optics, and
Microwaves
Based on Chapter 4 of William Stallings, Data and
Computer Communication
Kevin Bolding
Electrical Engineering
Seattle Pacific University
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Transmission Media
No. 1
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 signal’s direction
• Microwaves, broadcast radio waves
• Lasers, Infrared
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Transmission Media
No. 2
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
• We’re in the real world…
• Number of receivers
• Broadcast vs. point-to-point
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Transmission Media
No. 3
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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Transmission Media
No. 4
Attenuation Curves
Attenuation per 100ft for UTP/Coax
Attenuation per
100ft (dB)
25
20
Cat-5 UTP
RG58 Coax
RG6 Coax
15
10
5
0
1
10
100
1000
MHz
Attenuation is very dependent on conductor size
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Cat-5: 0.21 mm2
RG58: 0.64 mm2
RG6: 1.0 mm2
Transmission Media
No. 5
Frequency of various signals
Frequency (Hz)
102 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015
Power/
Telephone
Radio
Microwave Infrared
Visible
Light
Twisted Pair
Coax
AM Radio FM Radio/TV
Microwave
Trans.
Optical
Fiber
106 105 104 103 102 101 100 10-1 10-2 10-3 10-4 10-5 10-6
Wavelength (Meters)
Source: Stallings, Fig. 4.1
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Transmission Media
No. 6
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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Transmission Media
No. 7
Differential Signaling
Signals gather noise when travelling in a cable
Evil Noise
(0.6v)
Receiver
5.6V
GND
Source
5.0V
If the signal and GND are both sent together, they
both experience the same noise. Computing the
difference removes the noise.
Evil Noise
(0.6v)
Receiver
5.6V
5.0V
0.6V
Source
5.0V
GND
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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Transmission Media
No. 8
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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signal
Interference prop. to area
return
signal
Better…
return
Trace on PCB
Ground return
Transmission Media
No. 9
Twisted Pair Cables
Adjacent Loops
Out 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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Transmission Media
No. 10
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: <16Mbps, Cat-5 UTP: 100Mbps, Cat-6 UTP:
1000Mbps
• Shielded – Includes a grounded shield
(source: Microsoft Networking Essentials)
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Transmission Media
No. 11
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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Transmission Media
No. 12
Optical Fiber
• Relies on total internal
reflection
• Light waves bounce of
edge of fiber
• Channels waves to
(Source: Stallings, Fig. 4.4)
destination
• 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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Transmission Media
No. 13
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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Transmission Media
No. 14
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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Transmission Media
No. 15
Unguided Media Attenuation
Transmitted wave spreads out over a spherical surface
Power density at receiver: S  PT
4d 2
Received power depends on the
2 Thus
PT
2
PR  S
PR 

2
receiver antenna’s aperture:
4
4d 4
Free Space
Path Loss =
Ptrans (4d ) 2 (4fd ) 2


2
Prcv

c2
Pt
(4fd ) 2
10 log10
 10 log10
Pr
c2
f = frequency (Hz)
d = distance (m)
= wavelength (m)
c = speed of light (m/s)
4fd
loss(dB)  20 log10
 20 log10 f  20 log10 d  147.56dB
c
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Transmission Media
No. 16
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
 4
4 
h2
d (km)  3.57
h1 
h2 
3 
 3
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Transmission Media
Ionosphere
No. 17
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
http://www.mike-willis.com/Tutorial/gases.htm
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Transmission Media
No. 18