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

• 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

• 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.