- 1. Wireless Fundamentals Introducing WLAN RF Principles
2. Wireless Spectrum
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- Wireless networks use RF signals.
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- Radio frequencies are electromagnetic waves.
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- Spectrum defines wave sizes, grouped by categories.
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- Wireless network radio range is in the microwave segment.
3. Frequency
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- The frequency determines how often a signal is seen.
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- One cycle per second equals 1 Hz.
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- Low frequencies travel farther in the air than high
frequencies.
4.
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- The signal generated in the transmitter is sent to the
antenna.
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- Themovement of theelectronsg enerates an electric field, which
isthe electromagneticwave.
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- The size of the cycle pattern is called the wavelength.
Wavelength 5.
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- Amplitude is the vertical distance, or height, between
crests.
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- For the same wavelength and frequency, different amplitude scan
exist.
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- Amplitude represents the quantity of energy injected in the
signal.
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- The value of the amplitude is usually regulated because it can
affect the receivers.
Amplitude 6. Free Path Loss
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- As the wave spreads away from the emitter, it gets weaker.
-
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- 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.
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- Determining a range is determining the energy loss depending on
the distance.
7. Absorption
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- Absorption takes energy from the wave.
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- This energy is dissipated as heat in the obstacle.
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- When 100% of the energy is taken, the wave stops.
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- The effectof absorption is to reduce amplitude.
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- The signal is therefore less powerful, but the same wavelength
and frequencyare maintained.
8. Reflection
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- Part of the energy is reflected .
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- Partof the energymay be transmitted .
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- The angle of reflection is the same as the initial angle .
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- Reflection depends on theroughness of thematerial relative to
the wavelength and the angle .
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- Amplitude has no impact .
9. Multipath
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- O ccurs whenasignal reflectsfromsurfaces andsignalsarriveatthe
receiver at different times
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- Delayed multiple copies of the same signal hit the
receiver
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- D epends on the wavelength and the position of the
receiver
10. Multipath: Phase
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- Two signals are in phase when the crests of their cycles
coincide.
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- Being out of phase weakens both signals or cancels them if
amplitude and wavelength are the same.
11.
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- Scattering o ccurs when microparticles deviate the wave in
multiple directions .
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- It a ffects shorter wavelengths more than longer ones .
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- It c an weaken the signal or block it .
Scattering 12. Refraction
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- Refraction o ccurs whenawave passes from one medium to another
, causing the wave to change direction.
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- Refraction has a m inor effect on indoor networks .
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- It c an havea bigimpact on outdoor long - range links .
13.
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- Line of sight is necessary for good signal transmission.
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- 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).
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- Visual obstacles may or may not prevent radio line of
sight.
Line of Sight 14. Fresnel Zone
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- Determines an area around radio line of sight where reflections
have most negative impact on the signal
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- Should be at least 60% free from obstacles
15. RSSI and SNR
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- RSSI is the signal strength indicator.
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- The dBm value is obtained from a signal grading coefficient,
which is determined by the vendor.
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- RSSI usually a negative value, the closer to 0 the better.
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- SNR is signal strength relative to noise level.
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- The higher the SNR, the better.
16. Summary
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- The wireless spectrum expresses the range of radio waves .
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- Frequency is how often the same cycle repeats per second .
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- Wavelength is the distance of the same points in a signal cycle
.
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- Amplitude is how strong the signal is and how high the wave
.
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- Once radiated, some energy will be lostasthe signal spreads,
incurringf reep athl oss .
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- As the signal goes through obstacles, absorption weakens it
.
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- The signalcan also bounce on objects, which is reflection
.
17. Summary (Cont.)
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- When reflection occurs and several signals arrive at the
receiver, a multipath issue occurs.
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- Scattering occurs when the signal is reflected in many
directions by small obstacles.
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- Refraction occurs when the signal goes from one medium to
another and changes direction.
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- For a signal to be transmitted in good condition, radio line of
sight should exist between endpoints.
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- Around the line of sight, a zone, the first Fresnel zone,
should be mostly free from obstacles.
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- RSSI determines received signal strength and SNR determines the
signal-to-noise ratio at the receiver.
18.