Transcript
- 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.
4.
- 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.
11.
- 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 .
13.
- 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.
18.
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