My Study notes from Chapter 2 of CWNA Study Guide
RF Characteristics
- Wavelength
- A wavelength is the distance between the two successive crescents (peaks) or two successive throughs (valleys)
- Frequency
- Is how often an RF signal cycles in a certain time period
- Standard measurement of frequency is hertz (Hz). An event that occurs once in a second has a frequency of 1 Hz.
- 1 Hertz (Hz) = 1 Cycle per second
- 1 Kilohertz (KHz) = 1000 cycles per second
- 1 megahertz (MHz) = 1,000,000 (million) cycles per second
- 1 gigahertz (GHz) = 1,000,000,000 (billion) cycles per second
- Amplitude
- The height of the wave.
- Can be characterised simply as the signal’s strength or power
- Phase
- Is not a property of just one signal but instead involves the relationship between two or more signals that share the same frequency.
RF Behaviours
- Wave Propagation
- The way the wave moves is known as wave propagation.
- This can vary drastically depending on the type of material the signal is traversing.
- Absorption
- Most common RF behaviour
- If signal does not bounce off an object, move around an object, or pass through an object, then 100% absorption has occurred.
- Most materials absorb some RF signal, the amount depends on the material.
- Reflection
- Most important RF behaviour to be aware of.
- Causes serious performance problems in legacy 802.11a/b/g WLANs
- When wave hits smooth object that is larger than the wave itself, dependant on the material the signal may bounce in another direction.
- Two Types:
- Sky wave reflection: Signals below 1 GHz – signal bounces off charged partials in the ionosphere in the earth’s atmosphere.
- Microwave: 1GHz – 300 GHz, can bounce off smaller objects like a metal door
- Scattering
- Described as multiple reflections
- Is reflection off an object with multiple sides (think mirror ball)
- Two types:
- Lower level has lessor effect on signal quality and strength
- Second type occurs when RF signal encounters some type of uneven surface and is reflected into multiple directions (Chain link fences, wire mesh in stucco walls)
- Refraction
- Signal is bent into behaviour known as refraction.
- when signal passes through a medium with different density thus causing the wave to change direction.
- Three most common causes:
- Water Vapour
- Changes in air temperature
- Changes in air pressure
- Diffraction
- Signal is bent around an object
- Sitting directly behind the object is an area known as the RF shadow. Depending on the change in direction of the signal this area can become a dead zone of coverage.
- Loss (Attenuation)
- Decrease in amplitude
- Signal may lose strength on the wire or in the air
- Signal can be absorbed into materials it passes through.
- Free Space Path Loss (FSPL)
- Because of the laws of physics the signal will attenuate as it travels despite the lack of attenuation caused by obstructions, absorptions, reflection, diffraction and so on.
- Loss of signal strength caused by the natural broadening of the waves.
- Multipath
- Occurs when 2 or more signals arrive at the receiving station at the same time or within nanoseconds of each other.
- Four results of multipath
- Upfade
- Multiple RF signal paths arrive at the same time and are in phase (phase differences of 0 to 120 degrees) will cause upfade
- Results in increased signal strength
- Downfade
- Multiple RF signals arrive at the same time but are out of phase (phase difference of 121 to 179 degrees)
- Results in decrease signal strength
- Nulling
- This is signal cancellation
- Multiple RF signals arrive at the same time and are 180 degrees out of phase of each other
- Data Corruption
- Multiple signals arriving but not at the same time the receiver might have trouble demodulating the signal.
- Upfade
- Gain (Amplification)
- Increase in amplitude or signal strength
- Two types:
- Active – increase to signal on the transmitter or transceivers side through the use of an amplifier. IE more power is applied
- Passive – done by focusing the antenna, the inner workings of the antenna make the signal stronger.