Lecture-04-introwireless-radiophysics

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Introduction to Wireless
Network
Dr. Nguyen Tuan Nam
Introduction to Wireless
Networks
• Why do we need to use wireless?
– Cannot use wire
– Scenario: mobility
– Environment: not permit the use of running-wire
• Wireless network:
– Telecommunication network whose interconnection between nodes is
implemented without the use of wire.
• Ok, we should use wireless, but how?
– Can we use existing protocols & technologies?
– If not, will the new one compatible with current Internet technology
– Security
• What type of wireless technology?
– Distance
– Power
– Licensed/non-licensed band
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Different Types of Wireless
Networks
• Cellular network
• Satellite network
• Bluetooth When to use which?
• 802.11
• Sensor
• Passive/active RFID
• Acoustic
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Different Types of Wireless
Networks
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Different Types of Wireless
Network
• Mobile devices networks (WAN)
– Include networks provided by the cell phone carriers
– Cellular voice services + data services
– Carriers determine where to provide coverage based on business strategy + control QoS
– Speed is less important + Reach is for public at large
• WMAN – Wireless Metropolitan Area Network
– WiMax
– Connecting Wi-Fi hotspots to Internet
– Wireless alternative to cable and DSL
– Scheduling algorithm: each subscriber station is allocated an access slot by base station
• WLAN – Wireless Local Area Network
– Networks set up to provide wireless connectivity within finite coverage area: hospital, university, airport, …
– High data rate + Reach is restricted
– Wi-Fi, hotspot
• WPAN – Wireless Personal Area Network
– Wireless connectivity over distances of up to 10m or so
– Ad-hoc network (establish and dissolve)
– Bluetooth, IR
• Sensor
– Low power + ad hoc
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Different Properties
• Infrastructure vs. ad hoc
• Node movement: slow vs. fast
• Membership:
– Constant
– Dynamic
– Very dynamic
• Density: sparse vs. dense
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How do we Evaluate and Assess
Network Performance?
• Real implementation
• Simulator
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A Practical Introduction to
Radio Physics
Dr. Nguyen Tuan Nam
Why Is It Important?
• Wireless communications make use of electromagnetic waves to send
signal along distances
• From a user’s perspective, wireless connections seems to be not
particularly different from wired connection
• But in reality, radio waves have some unexpected properties compared to
Ethernet cable
– Ethernet
• How do you find the path that an Ethernet cable takes?
• Is it ok to run many Ethernet cables alongside each other?
– Wireless
• How do you know where the waves emanating from your wireless card are going?
• What happens when these waves bounce off of objects and buildings?
• How can several wireless cards be used in the same area without interfering with each
other?
à Important to understand how radio waves behave in the real world
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What Is a Wave?
• Vibration or oscillation
– Ex:
• Pendulum
• Tree swaying in the wind
• String of a guitar
– Some medium or object
• Swinging in a periodic manner
• With a certain number of cycles per unit of time
à Mechanical wave: defined by the motion of an object or
its propagating medium
à Electromagnetic wave: requires no medium in which to
propagate (even through the vacuum of space)
• When such oscillations travel (swinging not stay
bound to one place) à waves propagating in space
– A singer is singing
– A stone is plunging into a lake
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Speed, Frequency and Wavelength
• Relation
Speed = Frequency * Wavelength
• Wavelength
– Referred to as lambda, λ
– Distance measured from a point on one wave to the equivalent part of the next
– Measured in meters
• Frequency
– The number of whole waves that pass a fixed point in a period of time
– Measured in cycles/second (Hertz, Hz)
• Speed
– Measured in meters/second
• Amplitude
– Distance from the center of the wave to the extreme of one of its peak
– Can be thought of as the height of a water wave
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Illustration
What is the frequency in this case?
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Powers of Ten
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The Electromagnetic Spectrum
• Electromagnetic waves span a wide range of frequencies (and,
accordingly, wavelengths)
• This range of frequencies and wavelengths is called the
electromagnetic spectrum
– Light, visible portion, ~ 3.8x1014 Hz and 7.5x1014
– Alternating Current (AC), 50/60Hz
– Ultraviolet
– Infrared
– Radio is used for the portion of electromagnetic spectrum
• Waves can be generated by applying alternating current to an antenna (3Hz
to 300GHz)
• In a more narrow sense of the term (FM, AM): 3Hz to 1GHz
– Microwave
• Between radio (narrow sense) and infrared
• 1GHz to 300GHz
– ISM band: Industrial, Scientific, and Medical à unlicensed use for most country
– Other parts of the spectrum: tightly controlled by licensing legislation. Example?
• Example: microwave oven
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The Electromagnetic Spectrum
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Bandwidth (in Radio Physics)
• Bandwidth is simply a measure of
frequency range
– Range of 2.40GHz to 2.48GHz is used by a
device à bandwidth would be 0.08 GHz (80
MHz)
– Closely related to the amount of data you can
transmit within it
• Different from the bandwidth in computer
term
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Behavior of Radio Waves
• The longer the wavelength, the further it
goes
• The longer the wavelength, the better it
travels through and around things
• The shorter the wavelength, the more
data it can transport. Why?
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Absorption
• When electromagnetic waves go through something
(material), they generally get weakened or dampened
– Frequency
– Material
• For microwaves, 2 main absorbent materials are
– Metal
– Water
– Change in weather?
– Trees and wood?
– Plastics
• Generally do not absorb a lot of radio energy
• How to tests?
– Human?
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Reflection
• Radio waves are reflected when they come in
contact with some materials:
– Metal
– Water surfaces
• In the eyes of a radio wave, a dense grid of bars
acts just the same as a solid surface
– As long as the distance between bars is small
compared to the wavelength
– At 2.4GHz, a 1-cm metal grid will act much the same
as a metal plate
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Reflection
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