Transcript CS412 Computer Networks - Winona State University

CS412 Introduction to
Computer Networking &
Telecommunication
Physical Layer –
Transmission Media
Chi-Cheng Lin, Winona State University
Topics

Guided Transmission Media

Wireless Transmission

Communication Satellites
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Transmission Media
Physical layer: Transport a raw bit
stream
 Physical media

Guided media
Information transmitted on wires by varying
some physical property such as voltage or
current
Copper wire, fiber optics
Unguided media
Information transmitted wirelessly by
electromagnetic waves
Radio, lasers
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Guided Media
Twisted pair
 Coaxial cable
 Fiber optics

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Twisted Pair Cable
Oldest, but still most common
 Two twisted insulated copper wires

Why twisted?
To reduce electrical interference
Telephone system
 Repeater needed for longer distances

Repeater: device that extends the distance
a signal can travel by regenerating the signal

Adequate performance at low cost
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Twisted Pair
(a) Category 3 UTP.
(b) Category 5 UTP.
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Coaxial Cable

Better shielding than twisted pairs
Span longer distances at higher speeds
Lower error rate

Widely used for
Cable TV
WAN (Internet over cable)
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Coaxial Cable
A coaxial cable.
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Fiber Optics

Light
Electromagnetic energy traveling at 3108 m/s
Refraction
Critical angle
Reflection
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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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Figure 7.10
Bending of light ray
Figure 7.11 Optical fiber
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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 Optics

Optical transmission system:
Light source: LED or lasers
Transmission medium: fiber optic cable
Detector: converting detected light to
electrical pulse

Propagation modes
Multimode
Step-index
Grade-index
Single mode
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Figure 7.13
Modes
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Single Mode
All beams received “together” and
signal can be combined with little
distortion
 Widely used for longer distance
 More expensive
 Currently 50 Gbps for 100 km w/o
amplification

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Fiber Optics Vs. Copper Wire

Pros
Higher bandwidth
Less attenuation  less repeater needed (about
every 50 km, copper 5 km)
Noise resistance: no interference, surge, ...
Thin and lightweight
Excellent security

Cons
Fiber interface costs more
Less familiar technology
Fragility
Unidirectional
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Wireless Transmission

Electromagnetic Spectrum
Electron movement creates
electromagnetic wave
Frequency: number of oscillations per
second of a electromagnetic wave
measured in Hertz (Hz)
Wavelength: distance between two
consecutive maxima (or minima)
Speed of light: C = 3  108 m/sec
C = wavelength  frequency, i.e., C = λf
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Electromagnetic Spectrum
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Figure 7.18
Propagation methods
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Table 7.4 Bands
Band
Range
Propagation
Application
VLF
3–30 KHz
Ground
Long-range radio navigation
LF
30–300 KHz
Ground
Radio beacons and
navigational locators
MF
300 KHz–3 MHz
Sky
AM radio
HF
3–30 MHz
Sky
Citizens band (CB),
ship/aircraft communication
VHF
30–300 MHz
Sky and
line-of-sight
VHF TV,
FM radio
UHF
300 MHz–3 GHz
Line-of-sight
UHF TV, cellular phones,
paging, satellite
SHF
3–30 GHz
Line-of-sight
Satellite communication
EHF
30–300 GHz
Line-of-sight
Long-range radio navigation
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Radio Transmission
Easy to generate
 Travel long distance
 Penetration
 Interference

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Microwave Transmission
MCI?
 Straight line travel
 Higher towers for longer distances
 Multipath fading problem, absorption by rain
 Advantages:

Right of way not needed
Inexpensive

Industrial/Scientific/Medical (ISM) bands
No license needed
Garage door opener, cordless phone, etc
Bluetooth, 802.11 wireless LANs
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Infrared and Millimeter Waves
Remote control
 Directional, cheap, easy to build
 Cannot pass through solid walls

Good or bad?

Limited use on desktop
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Applications of Wireless Media

Radio waves
Multicast communications
Radio, television, and paging systems

Microwaves
Unicast communication
Cellular telephones, satellite networks, and
wireless LANs.

Infrared signals
Short-range communication in a closed
area using line-of-sight propagation
Wireless keyboards, mice, printers
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Lightwave Transmission
Lasers
 High bandwidth, low cost, easy to
install
 Aiming is hard
 No penetration through rain or thick fog

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Communication Satellite
Big microwave repeater in the sky
 Transponders, each

Listens to some portion of spectrum
Earth to satellite: Uplink
Amplifies incoming signal
Rebroadcast it at another frequency
Earth to satellite: Downlink
 Bent pipe mode
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Figure 7-34
Satellite Communication
Uplink
WCB/McGraw-Hill
Downlink
 The McGraw-Hill Companies, Inc., 1998
Communication Satellites
Communication satellites and some of their properties,
including altitude above the earth, round-trip delay time
and number of satellites needed for global coverage.
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Communication Satellites
VSATs using a hub.
VSATs: Very small Aperture Terminals
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Communication Satellite

Low-Earth Orbit Satellites
Iridium: 66 satellites
Goal:
 Provide worldwide telecommunication service using
hand-held devices that communicates directly with
the Iridium satellites
Current status?
 Broke, auctioned, restarted
Globalstar: 48 LEOs using bent-pipe design
Teledisc:
Goal: provide Internet users with high
bandwidth using VSAT-like antenna
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Iridium vs. Globalstar


(a) Iridium: Relaying in space.
(b) Globalstar: Relaying on the ground.
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Satellites Vs. Fiber
Availability
 Mobility
 Broadcasting
 Geographically issue
 Right of way
 Rapid deployment
 Future?

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