Tài liệu 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 Nguyen Tuan Nam/WNC 2010 2 Different Types of Wireless Networks •  Cellular network •  Satellite network •  Bluetooth •  802.11 •  Sensor •  Passive/active RFID •  Acoustic When to use which? Nguyen Tuan Nam/WNC 2010 3 Different Types of Wireless Networks Nguyen Tuan Nam/WNC 2010 4 Different Types of Wireless Network •  Mobile devices networks (WAN) •  WMAN – Wireless Metropolitan Area Network •  WLAN – Wireless Local Area Network •  WPAN – Wireless Personal Area Network •  Sensor –  –  –  –  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 –  –  –  –  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 –  –  –  Networks set up to provide wireless connectivity within finite coverage area: hospital, university, airport, … High data rate + Reach is restricted Wi-Fi, hotspot –  –  –  Wireless connectivity over distances of up to 10m or so Ad-hoc network (establish and dissolve) Bluetooth, IR –  Low power + ad hoc Nguyen Tuan Nam/WNC 2010 5 Different Properties •  Infrastructure vs. ad hoc •  Node movement: slow vs. fast •  Membership: –  Constant –  Dynamic –  Very dynamic •  Density: sparse vs. dense Nguyen Tuan Nam/WNC 2010 6 How do we Evaluate and Assess Network Performance? •  Real implementation •  Simulator Nguyen Tuan Nam/WNC 2010 7 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 Nguyen Tuan Nam/WNC 2010 9 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 Nguyen Tuan Nam/WNC 2010 10 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 Nguyen Tuan Nam/WNC 2010 11 Illustration What is the frequency in this case? Nguyen Tuan Nam/WNC 2010 12 Powers of Ten Nguyen Tuan Nam/WNC 2010 13 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 Nguyen Tuan Nam/WNC 2010 14 The Electromagnetic Spectrum Nguyen Tuan Nam/WNC 2010 15 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 Nguyen Tuan Nam/WNC 2010 16 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? Nguyen Tuan Nam/WNC 2010 17 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? Nguyen Tuan Nam/WNC 2010 18 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 Nguyen Tuan Nam/WNC 2010 19 Reflection Nguyen Tuan Nam/WNC 2010 20
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