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© 2008 Cisco Systems, Inc. All rights reserved. IUWNE v1.0—1-1 Wireless Fundamentals Introducing WLAN RF Principles

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  • 1. Wireless Fundamentals Introducing WLAN RF Principles

2. Wireless Spectrum

    • Wireless networks use RF signals.
    • Radio frequencies are electromagnetic waves.
    • Spectrum defines wave sizes, grouped by categories.
    • Wireless network radio range is in the microwave segment.

3. Frequency

    • The frequency determines how often a signal is seen.
    • One cycle per second equals 1 Hz.
    • Low frequencies travel farther in the air than high frequencies.


    • The signal generated in the transmitter is sent to the antenna.
    • Themovement of theelectronsg enerates an electric field, which isthe electromagneticwave.
    • The size of the cycle pattern is called the wavelength.

Wavelength 5.

    • Amplitude is the vertical distance, or height, between crests.
    • For the same wavelength and frequency, different amplitude scan exist.
    • Amplitude represents the quantity of energy injected in the signal.
    • The value of the amplitude is usually regulated because it can affect the receivers.

Amplitude 6. Free Path Loss

    • As the wave spreads away from the emitter, it gets weaker.
      • The quantity of energy declines as the distance increases; the quantity of energy available on each point of the circle is less as the circle is larger, and the receiver catches only part of this energy.
    • Determining a range is determining the energy loss depending on the distance.

7. Absorption

    • Absorption takes energy from the wave.
    • This energy is dissipated as heat in the obstacle.
    • When 100% of the energy is taken, the wave stops.
    • The effectof absorption is to reduce amplitude.
    • The signal is therefore less powerful, but the same wavelength and frequencyare maintained.

8. Reflection

    • Part of the energy is reflected .
    • Partof the energymay be transmitted .
    • The angle of reflection is the same as the initial angle .
    • Reflection depends on theroughness of thematerial relative to the wavelength and the angle .
    • Amplitude has no impact .

9. Multipath

    • O ccurs whenasignal reflectsfromsurfaces andsignalsarriveatthe receiver at different times
    • Delayed multiple copies of the same signal hit the receiver
    • D epends on the wavelength and the position of the receiver

10. Multipath: Phase

    • Two signals are in phase when the crests of their cycles coincide.
    • Being out of phase weakens both signals or cancels them if amplitude and wavelength are the same.


    • Scattering o ccurs when microparticles deviate the wave in multiple directions .
    • It a ffects shorter wavelengths more than longer ones .
    • It c an weaken the signal or block it .

Scattering 12. Refraction

    • Refraction o ccurs whenawave passes from one medium to another , causing the wave to change direction.
    • Refraction has a m inor effect on indoor networks .
    • It c an havea bigimpact on outdoor long - range links .


    • Line of sight is necessary for good signal transmission.
    • Earth curvature plays a role in the quality of outdoor links, even with a distance of a few miles (depending on the elevation of the transmitter and receiver).
    • Visual obstacles may or may not prevent radio line of sight.

Line of Sight 14. 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

15. RSSI and SNR

    • RSSI is the signal strength indicator.
    • The dBm value is obtained from a signal grading coefficient, which is determined by the vendor.
    • RSSI usually a negative value, the closer to 0 the better.
    • SNR is signal strength relative to noise level.
    • The higher the SNR, the better.

16. 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 lostasthe signal spreads, incurringf reep athl oss .
    • As the signal goes through obstacles, absorption weakens it .
    • The signalcan also bounce on objects, which is reflection .

17. 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 condition, 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 determines received signal strength and SNR determines the signal-to-noise ratio at the receiver.