‫به ن ـ ـ ـ ــام خدا‬
‫شبکه های کامپیوتری‬
‫جلسه پنجم‬
Wireless transmission
 Main Drivers
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Communication without a physical attachment such as wires or fibers.
Mobility of users
Ease of deployment
Reduce cost of communication (For example, providing connectivity
services to remote villages)
 Electromagnetic spectrum
 Higher frequencies (shorter wave lengths) have much larger available
bandwidth
 Radio Frequency Allocation
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Wireless transmission
 Propagation of waves
 In ground propagation,
 radio waves travel through the lowest portion of the atmosphere, hugging the earth.
 These low-frequency signals emanate in all directions from the transmitting antenna and follow
the curvature of the planet.
 Distance depends on the amount of power in the signal: The greater the power, the greater the
distance.
 In sky propagation,
 higher-frequency radio waves radiate upward into the ionosphere where they are reflected back to
earth.
 This type of transmission allows for greater distances with lower output power.
 In line-of-sight propagation,
 very high-frequency signals are transmitted in straight lines directly from antenna to antenna.
 Antennas must be directional, facing each other, and either tall enough or close enough together
not to be affected by the curvature of the earth.
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Wireless transmission
1.RadioWaves
 Radio range of frequencies (10KHz to 10MHz) usually used for broadcast
communication
 Long range, low attenuation, Multi directional
 Applications : AM and FM radio, television, maritime radio, cordless phones
2. Microwaves
 Microwave range of frequencies (10MHz to 10GHz) usually used for
point to point long distance transmission and local two-way
communications
 Medium range, affected by rain and weather conditions. Subject to multipath
fading.
 Applications: cellular phones , satellite networks and wireless LANs.
3.Infrared
 Infrared waves, with frequencies from 300 GHz to 400 THz, can be used
for short-range communication. Infrared waves, having high frequencies,
cannot penetrate walls.
 Applications: communication between devices such as keyboards, mice, PCs,
and printers.
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Wireless transmission
 Most radio frequency bands are regulated. Using frequency
spectrum for commercial purposes is usually subject to a
regulatory fee.
 Since 1994, US government has earned more than 60 billion
US$ from spectrum licensing fees
 Irancell has paid 300 million Euro for its spectrum license to
Communications Regulatory Agency (CRA)
 There are license free bands available in most countries (ISM
bands)
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Wireless Optical Transmission
 Idea: Light as the information carrier for free space communication
 Indoor applications:Wireless LAN
 Outdoor application: Building to building communication
 Can transmit high data rates to distances of a few kilometers
 Should cope with air turbulence effects and adaptively focus on target
receivers
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Satellite Communication
 Satellites can be used as a wireless node in the sky that can
receive, amplify, process and transmit communication signals
 They are mainly used in three orbit ranges and therefore have
different rotation period around earth.
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Satellite Communication
 GEO Satellites (GEOstationary)
 Line-of-sight propagation requires that the sending and
receiving antennas be locked onto each other’s location at all
times. To ensure constant communication, the satellite must
move at the same speed as the earth so that it seems to remain
fixed above a certain spot
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Satellite Communication
 Very Small Aperture Terminals (VSAT)
 Small terminals that transmit 1W of power and communicate to
each other through a Geostationary satellite and a HUB.
 Can usually transmit at 19.2 kbits/sec and receive at around 5.2
kbps
 Low cost provisioning of voice and/or data services
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Satellite Communication
 MEO Satellite
 Medium-Earth Orbit (MEO) satellites are positioned between
the two Van Allen belts. A satellite at this orbit takes
approximately 6 hours to circle the earth
GPS
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Satellite Communication
 LEO Satellites
 Low-Earth Orbit (LEO) satellites have polar orbits. The altitude
is between 500 to 2000 km, with a rotation period to 120 min
 ISL Inter Satellite Link
 GWL – Gateway Link
 UML – User Mobile Link
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Satellite Communication
• Application of LEO satellite
Iridium
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Globalstar
Satellites Versus Fiber
 Optical communication is the dominant technology for transmission of
data.
 From office LANs to intercontinental communication networks, optical
communication systems are used as a cost effective solution.
 Satellite communication serves some key demands
 Networks that require rapid deployment (disaster recovery, military
applications)
 Communication in places where terrestrial networks are not well developed
(jungles, countries with many islands, oil stations in the sea)
 Broadcasting (TV, Radio)
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Telephone system
 Public Switched Telephone Network (PSTN)
 Originally designed to transmit human voice
 Loop plant with Limited bandwidth and complex interference
environment
 Telephone system organization: hierarchical network with three
important components:
 Local loops
 Switching centers
 Trunks: connecting switching offices using various transmission
technologies
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