Once an RF wave leaves the antenna, the environment changes it. The CWNA exam expects you to recognize each propagation behavior, know which materials cause it and predict its effect on a wireless network. These behaviors explain why a floor plan that looks simple can produce dead spots, poor throughput or odd roaming.
Reflection happens when a wave hits a smooth surface larger than its wavelength, such as metal, a glass window with metallic coating or an elevator door, and bounces off. Reflection is the main cause of multipath indoors. Refraction is the bending of a wave as it passes through a medium of different density, for example air layers of different temperature or humidity; it mostly matters on long outdoor links. Diffraction is the bending of a wave around an obstacle, creating an RF shadow behind it, like light around the edge of a building. Scattering happens when a wave hits an uneven surface or many small objects, such as chain-link fencing, gravel, dust or foliage, and splits into many weaker reflections in different directions.
Absorption is the conversion of RF energy to heat as it passes through a material. Water and dense materials absorb strongly, so people, concrete, brick and water-filled objects all reduce signal. That is why a lecture hall full of people behaves differently from the same empty room during a survey. Materials are often described by their attenuation in decibels (dB), and higher frequencies usually suffer more absorption.
Free space path loss (FSPL) is the weakening of a signal simply because the energy spreads out over a larger area as it travels, even with no obstacles at all. A useful rule of thumb is the 6 dB rule: each time the distance doubles, FSPL increases by about 6 dB, which means about one quarter of the power. FSPL also rises with frequency, which matches what you learned about shorter wavelengths.
Multipath is the arrival of two or more copies of the same signal at a receiver at slightly different times, because some traveled directly and others were reflected. The time difference is called delay spread. Depending on phase, multipath can cause upfade (increased amplitude), downfade (decreased amplitude), nulling (cancellation) and data corruption from intersymbol interference. For older single-antenna radios, multipath was mostly harmful. MIMO (multiple-input, multiple-output) radios used by 802.11n and later actually take advantage of multipath to send several spatial streams at once.
When you troubleshoot, map the symptom to the behavior. Signal that is strong but data that is corrupted suggests multipath. Signal that drops sharply behind a concrete core or a crowd suggests absorption. Coverage that gets weaker simply with distance is FSPL.
Key terms
- Reflection
- A wave bouncing off a smooth surface larger than its wavelength, such as metal or coated glass.
- Diffraction
- A wave bending around an obstacle, leaving an RF shadow behind it.
- Scattering
- A wave splitting into many weaker reflections after hitting an uneven surface or many small objects.
- Absorption
- Loss of RF energy as a material converts it to heat; water, people and concrete absorb strongly.
- Free space path loss (FSPL)
- Signal weakening caused by the wavefront spreading out with distance, about 6 dB for each doubling of distance.
- Multipath
- Multiple copies of a signal reaching a receiver at different times over different paths.
A warehouse with metal racking shows good signal strength everywhere in a survey, yet older handheld scanners report many retries. The metal racks create heavy reflection and multipath. Replacing the scanners with MIMO-capable devices and adjusting AP placement reduces the retries.
Check yourself
Why does a crowded auditorium reduce signal compared with the same empty room?
Human bodies are mostly water, which absorbs RF energy and converts it to heat.
By roughly how much does free space path loss increase when you double the distance?
About 6 dB, which is about one quarter of the received power.
What four effects can multipath have on a signal?
Upfade, downfade, nulling and data corruption from intersymbol interference.