Transcript CS412 Computer Networks

CS 313 Introduction to
Computer Networking &
Telecommunication
Physical Layer –
Transmission Media
Chi-Cheng Lin, Winona State University
Topics

Guided Transmission Media

Wireless Transmission

Communication Satellites
2
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
3
Guided Media
Twisted pairs
 Coaxial cable
 Power lines
 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, Ethernet
 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
Category 5 UTP cable with 4 twisted pairs
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Transmission Direction Modes



100-Mbps Ethernet uses two pairs – one for
each direction
1-Gbps Ethernet uses all four pairs in both
directions simultaneously.
Terminologies: transmission direction modes
Simplex
Data only travel in one direction
Half-duplex
Data can travel in either direction, but not
simultaneously
Full-duplex
Data can travel in both directions simultaneously
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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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Fiber Optics

Light
Electromagnetic energy traveling at 3108 m/s
Refraction
Critical angle
Reflection
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Fiber Optics
(Less dense)
(More dense)
cladding
core
I
(critical
angle)
cladding
(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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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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Modes
Core diameter
200 micrometers
50 - 100
micrometers
~ 10 micrometers
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©The McGraw-Hill Companies, Inc., 2004
Single Mode
All beams received “together” and
signal can be combined with little
distortion
 Widely used for longer distance (over
550 m)
 More expensive
 Currently 100 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, as wiretapping is harder

Cons
Fiber interface costs more
Fragility
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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
Ground
Sky
Line-of-sight
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Radio Transmission (1)
In the VLF, LF, and MF bands, radio waves follow the curvature
of the earth
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011
Radio Transmission (2)
In the HF band, they bounce off the ionosphere.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011
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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The Politics of the Electromagnetic
Spectrum
ISM and U-NII bands used in the
United States by wireless devices
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011
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
Satellite to Earth: Downlink
 Bent pipe mode
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Communication Satellites
(Geostationary Earth Orbit)
(Medium Earth Orbit), app.: GPS
(Low Earth Orbit), voice/data communication
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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Geostationary Satellites (1)
The principal satellite bands
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011
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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Low-Earth Orbit Satellites (1)
The Iridium satellites form six necklaces
around the earth.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011
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?

“It's tough to make predictions, especially
about the future.” Yogi Berra (?)
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