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© 2008 Cisco Systems, Inc. All rights reserved.WLAN-RF-Principles-1 Wireless LAN RF Principles
© 2008 Cisco Systems, Inc. All rights reserved.IUWNE v Wireless Spectrum Wireless networks use RF signals RF are electromagnetic waves Spectrum defines waves sizes, grouped by categories Wireless network radio range is in the microwave segment
© 2008 Cisco Systems, Inc. All rights reserved.IUWNE v Frequency The frequency determines how often a signal is seen 1 cycle per second is 1 Hertz Low frequency travel farther in the air than higher frequencies
© 2008 Cisco Systems, Inc. All rights reserved.IUWNE v The signal generated in the transmitter is sent to the antenna The electrons movement generate an electric field which is a wave The size of the cycle pattern is called the wavelength Wavelength
© 2008 Cisco Systems, Inc. All rights reserved.IUWNE v Amplitude is the vertical distance, or height, between crests For the same wavelength and frequency, different amplitude can exist It represents the quantity of energy injected in the signal This value is usually regulated as it can affect the receivers Amplitude
© 2008 Cisco Systems, Inc. All rights reserved.IUWNE v Free Path Loss As the wave spreads away from the emitter, it gets weaker: –The quantity of energy declines as the distance increases: the available quantity of energy available on each point of the circle is less as the circle is larger; the receiver catches only part of this energy Determining a range is determining the energy loss depending on the distance
© 2008 Cisco Systems, Inc. All rights reserved.IUWNE v Absorption Absorption takes energy from the wave This energy is dissipated in heat in the obstacle When 100% of the energy is taken, the wave stops The effect of absorption is to reduce amplitude The signal is therefore less powerful, but keeps the same wavelength and frequency
© 2008 Cisco Systems, Inc. All rights reserved.IUWNE v Reflection Part of the energy is reflected Part may be transmitted The angle of reflection is the same as the initial angle Reflection depends on the material roughness relative to the wavelength and the angle Amplitude has no impact
© 2008 Cisco Systems, Inc. All rights reserved.IUWNE v Multipath Occurs when the signal reflects on surfaces and arrives to the receiver at different times Delayed multiple copies of the same signal hit the receiver Depends on the wavelength and the position of the receiver
© 2008 Cisco Systems, Inc. All rights reserved.IUWNE v Multipath: Phase 2 signals are in phase when their cycles crests coincide Being out of phase weakens both signal or cancel them if amplitude and wavelength are the same
© 2008 Cisco Systems, Inc. All rights reserved.IUWNE v Occurs when micro particles deviate the wave in multiple directions Affects shorter wavelengths more than longer ones Can weaken the signal or block it Scattering
© 2008 Cisco Systems, Inc. All rights reserved.IUWNE v Refraction Occurs when the wave passes from one medium to another: direction change Minor effects on indoor networks Can have high impacts on outdoor long range links
© 2008 Cisco Systems, Inc. All rights reserved.IUWNE v Necessary for good signal transmission Earth curvature plays a role in outdoor links from a few miles (depends on transmitter and receiver elevation Visual obstacles may or may not prevent radio line of sight Line of Sight
© 2008 Cisco Systems, Inc. All rights reserved.IUWNE v Fresnel Zone Determines an area around radio line of sight where reflections have most negative impact on the signal Should be at least 60% free from obstacles
© 2008 Cisco Systems, Inc. All rights reserved.IUWNE v RSSI and SNR RSSI is the signal strength indicator dBm value transformed from a a vendor dependent grading coefficient Usually negative value, the closer to 0 the better SNR is signal strength relative to noise level The higher the better
© 2008 Cisco Systems, Inc. All rights reserved.IUWNE v Summary The wireless spectrum expresses the range of radio waves Frequency is how often the same cycle repeats per second Wavelength is the distance of the same points in a signal cycle Amplitude is how strong the signal is, and how high the wave Once radiated, some energy will be lost as the signal spreads, incurring Free Path Loss As the signal goes through obstacles, absorption weakens it It can also bounce on objects, which is reflection
© 2008 Cisco Systems, Inc. All rights reserved.IUWNE v Summary (cont.) When reflection occurs and several signals arrive at the receiver, a multipath issue occurs Scattering occurs when the signal is reflected in many directions by small obstacles Refraction occurs when the signal goes from one medium to another and changes direction For a signal to be transmitted in good conditions, radio line of sight should exist between endpoints Around the line of sight, a zone, the first Fresnel zone, should be mostly free from obstacles RSSI will determine received signal strength and SNR the signal- to-noise ratio at the receiver
© 2008 Cisco Systems, Inc. All rights reserved.IUWNE v
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