CWNA Chapter 3 – Radio Frequency Components, Measurements, and Mathematics

My Notes from chapter 3 of the CWNA study guide

RF Components

  • Transmitters
    • Initial component in the creation of wireless medium.
    • Transmitters job is to begin the RF communications
    • Transmitter starts AC signal oscillating  at the RF frequency
    • Transmitter takes data provided and modifies the AC signal using a Modulation technique
  • Antenna
    • Two functions when connected to the transmitter it collect the AC signal received from the Transmitter and directs or radiates it out.
    • When connected to the receiver it takes the signal and directs the AC signal to the receiver.
    • Two ways to increase the power output of an antenna
      • Generate more power at the transmitter
      • Direct or focus the RF signal that is radiating from the antenna
  • Receiver
    • Takes the carrier signal received from the antenna and translate the modulated signals into 1s and 0s
    • Then passes the data onto the network/computer/device
  • Intentional Radiator (IR)
    • FCC defines and intentional Radiator as “a device that intentionally generates and emits radio frequency energy by radiation or induction”
    • Regulatory bodies limit the amount of power that is aloud to be generated by an IR
    • Components making up the IR:
      • Transmitter
      • All cables
      • Connectors
      • Any other equipment (grounding, lightning arrestors, amplifiers, attenuators, and so forth) between the transmitter and the and antenna
    • IR power measured at the connector that provides the input to the antenna
    • Power usually measured in Milliwatt or Decibels relative to 1 Milliwatt (dBm)
  • Equivalent Isotopically Radiated Power (EIRP)
    • Highest RF signal strength that is transmitted from a particular antenna
    • Regulatory bodies limit the amount of EIRP of an antenna

Units of Power and Comparison

  • Watt ( W )
    • Unit of power
    • One watt is equal to 1 ampere (A) of current flowing at 1 volt (V).
    • Watt ( W ) = Volt (V) x Ampere (A)
  • Milliwatt (mW)
    • Unit of power
    • 1 Milliwatt = 1/1,000 watts
  • Decibel (dB)
    • Unit of comparison
    • Difference between two values. dB is a relative expression used to represent a difference between two values
    • In wireless networking dBs are often used to compare the power of two transmitters or more often to compare the difference or loss between the EIRP output of a transmitters antenna and the amount of power received by the receivers antenna
  • dBi
    • Used to compare the output of one antenna to another
    • The gain or increase of power from an antenna when compared to what an isotropic radiator would generate is known as decibels isotropic (dBi)
  • dBd
    • Relative measurement and not a unit of power
    • Antenna industry users two dB scales to describe the gain of antenna
      • dBi
      • dBd
    • dBd decibel gain relative to a dipole antenna
    • dBd value is the increase in gain of an antenna when compared to the signal of a dipole antenna
  • How to compare two antennas one in dBi and other in dBd
    • Standard dipole antenna has dBi of 2.14
    • If antenna has value of 3 dBd, this means it is 3 dB greater than a dipole antenna
    • Because value of dipole antenna is 2.13dBi all you need to do is add 3 to 2.14
      • So a 3dBd antenna is equal to a 5.14dBi antenna
  • dBm
    • Compares a signal to 1 Milliwatt of power
    • Decibels relative to 1 Milliwatt
    • 0dBm = 1 Milliwatt
    • dBm = 10 x log10(PmW)
    • +6dB doubles the distance of the usable signal
    • -6dB halves the distance of the usable signal
    • dBm makes it easy to calculate the effects of antenna gain on a signal.
  • Inverse Square Law
    • Originally developed by Isaac Newton
    • Law states that the change in power is equal to 1 divided by the square of the change in distance
      • As the distance from the source of the signal doubles the energy is spread out over four times the area, resulting in one-fourth of the original intensity of the signal.
    • Free space path loss formula:
      • FSPL = 36.6 + (20log10(F)) + (20log10 (D))
        • FSPL = Free space path loss
        • F = Frequency in MHz
        • D = Distance in miles between antennas
      • FSPL = 32.4 + (20log10 (F)) + (20log10 (D))
        • FSPL = Free space path loss
        • F = Frequency in MHz
        • D = Distance in Kilometres between antennas
    • FSPL is based on Newtons inverse square law.

RF Mathematics

  • Rules of 10s and 3s
    • For ever 3dB of gain (relative), double the absolute power (mW)
    • For every dB of loss (relative), halve the absolute power (mW)
    • For every 10 dB of gain (relative), multiply the absolute power (mW) by a factor of 10
    • For every 10 dB of loss (relative), divide the absolute power (mW) by a factor of 10
  • Noise Floor
    • Noise floor is the or background level of radio energy on a specific channel.
    • This can include modulated or encoded bits from nearby 802.11 transmitting radios or unmodulated energy coming from non-802.11 devices such as microwave ovens, Bluetooth device
    • The Amplitude of the noise floor varies in different environments.
    •  2.4GHz will have higher Noise floor than 5GHz as the bands are more crowded.
  • Signal-to-Noise Ratio (SNR)
    • Is the difference in decibels between the received signal and the background noise level (noise floor), not actually a ratio.
    • Example:
      • Radio receives a signal of -85dBm and the noise floor is measured at -100dBm the difference is 15bD therefore the SNR is 15dB

SNR.png

  • Received Signal Strength Indicator
    • The power level of an RF signal required to be successfully received by the receiver radio.
    • The lower the power level that a receiver can successfully process the better the receive sensitivity.
    • In WLAN equipment the receive sensitivity is usually defined as a function of the network speed.
  • Link Budget
    • Link budget is the sum of all the planned and expected gains and losses from the transmitting radio, through the RF medium, to the receiver radio

Link Budget.png

  • Fade Margin / System Operating Margin
    • Fade Margin is a level of desired signal above what is required.
    • Effectively is a margin added to the required signal level to account for outside factors causing the signal level to fluctuate
    • Normally used for outdoor WLAN bridge links.
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