Chapter 3

Three Requirements For Successful Communications

1. Two or more devices want to communicate 2. Medium, means, or method to communicate 3. Set of rules to use when communicating. • These rules apply to all forms of communications

Intentional Radiator

FCC defines intentional radiator as - A device that intentionally generates and emits radio frequency energy by radiation or induction • Basically, something that is specifically designed to generate RF • IR = All components from the transmitter to the antenna (excluding the antenna) • Regulatory bodies limit amount of power allowed to be generated by an IR (measured to the input to the antenna)

dBm

• Absolute measurement • Decibels relative to 1 milliwatt • Set dBm to 0 and equate it to 1 milliwatt of RF power • dBm = 10 x log10 (Pmw) • Pmw=10(dBm/10)

Noise Floor

• Ambient or background level of radio energy • Noise floor includes - Includes modulated signals from nearby 802.11 devices - Unmodulated energy from non-802.11 devices (microwave ovens, portable telephones, etc.) - Potentially anything electromagnet. • 2.4 GHz and 5 GHz - Typical noise floor around -100 dBm - Noisier environment around -90 dBm - 5 GHz typically less noisy than 2.4 GHz

Watt (W)

• Basic unit of power • Name after James Watt, 18th- century inventor • Watt = 1 ampere of current flowing at 1 volt • Watt = volts times amps

Inverse Square Law

• Change in power is equal to 1 divided by the square of the change in distance • 2 x distance = 1/(2)^2 • FSPL is based on Inverse Square Law • Power at a specific distance = P/(4π^2), where P = initial EIRP power and r = initial distance

Why Use Decibels

• Decibels provide a linear scale to RF power, whereas milliwatts uses a logorithmic scale •An indoor 802.11 device may transmit at 100 mW or 20 dBm • That signal may be received at as low as 0.00000001 mW or -80 dBm

Signal to Noise Ratio (SNR)

• Difference in dB between received signal and background noise (noise floor) • Low SNR will likely cause data corruption • SNR > 25 db is considered good • SNR < 10 dB is considered very poor

Receiver

• Final component in the wireless medium • Takes carrier signal from the antenna • Signal is often altered due to interference • Translates the signal into 1s and 0s • Passes the data to the computer to be processed

Transmitter

• Initial component in the creation of wireless medium • Receives data from the computer • Generates AC signal • AC signal determines the frequency of the transmission • Modulates the AC signal to contain the date (carrier signal) • Determines the power/amplitude of the signal

Fade Margin

• Level of desired signal above what is required • Comfort zone • Received signal fluctuates due to outside influences and interference • Protects reception of signal due to fluctuation of the received signal • 10 dB to 25 dB buffer is common practice • System operating margin (SOM) is the difference between the actual received signal and the signal required for communications.

RF Mathematics

• Logarithms are needed only for exact calculations • Math skills necessary for RF math - Addition and subtraction using 3 and 10 - Multiplication and division using 2 and 10 • Math is based on the rule of 10s and 3s

Units of Comparison

• Measures difference between signals • Relative power measurements • Units of comparison - decibel (dB) - decibels relative to an isotropic radiator (dBi) - decibels relative to a dipole antenna (dBd)

Units of Power

• Measures transmission amplitude and received amplitude • Absolute power measurements • Units of power - watt (W) - milliwatt (mW) - decibels relative to a milliwatt (dBm)

Antenna

• Performs two functions - Collects the carrier signal from the transmitter and directs or radiates it - Takes the RF waves that it receives through the air and directs the AC signal to the receiver • Antennas direct or focus the RF signal • Antenna transmissions are usually referenced to an isotropic radiator Isotropic radiator = a point source that radiates equally in all directions.

Received Signal Strength Indicator

• Power level of an RF signal required to be successfully received by the receiver radio • Weakest signal that transceiver can decode (no considering the noise floor) • Defined by IEEE as a relative metric used by 802.11 radios to measure signal strength • Metric values and scales can vary between vendors • Indicator often used to initiate roaming or dynamic rate switching (DRS)

Rule of 10s and 3s

• Provide and approximate value (fairly accurate) • Four basic rules - For every 3 db gain, double the power - For every 3 db loss, halve the power - For every 10 db gain, power times 10 - for every 10 db loss, power divided by 10

dBi

• Relative measurement • Decibel gain referenced to an isotropic radiator • Think change in power cause by an antenna • Typical rubber-encased half-wave dipole antenna has a dBi value of 2.14

dBd

• Relative measurement • Decibel gain relative to a dipole antenna • 3 dBd = 3 dB greater than a dipole antenna (so 3 dB greater than 2.14 dBim or 5.14 dBi)

Equivalent Isotropically Radiated Power (EIRP)

• The highest RF signal strength that is transmitted from a particular antenna. • The maximum effective focused signal strength radiated from an antenna. • Also known as - Equivalent isotropic radiated power - Effective isotropic radiated power • Regulatory bodies limit the amount of effective power radiate.

Link Budget

• The sum of all gains and losses from the transmitting radio to the receiver radio • Used to guarantee that the received signal is above the receiver sensitivity threshold.

Decibel (dB)

• Unit of comparison • Measurement of change in power • Derived from the term bel • 1 bel= ratio of 10 to 1 between the power of two transmitters • dB = 1/10 of a bel • 10^2=100 inverse is log10(100)=2 • dB = 10 x log10 (Power1/Power2)

Milliwatt (mW)

• Unit of power used to measure 802.11 RF • 1/1,000 of a watt • 802.11 indoor typically transmit at power levels between 1 mW and 100 mW • 802.11 outdoor equipment commonly transmit at power levels up to 300 mW