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ITNW 1351 Fundamentals of Wireless LANs

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ITNW 1351 Fundamentals of Wireless LANs

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    1. ITNW 1351 Fundamentals of Wireless LANs Chapter 2 Radio Frequency (RF) Math

    2. 2 RF Mathematics Consideration of the following calculations is required to determine whether RF links are compliant with the power limitations set by the FCC Each of the following factors must be taken into account when planning a wireless LAN All of the following factors are related mathematically

    3. 3 RF Mathematics Important Power Calculations… Power at the transmitting device Loss and gain of connectivity devices cables, connectors, amplifiers, attenuators, splitters, etc. (Between Tx and antenna) Power at last connector before signal enters antenna (IR) Power at the antenna element (EIRP)

    4. 4 RF Mathematics Important Power Calculations… Power lost by the signal as it travels (i.e. difference between transmitted and received power) Loss and gain of connectivity devices between receiving antenna and receiving device.

    5. 5 Units of Measure… Watt (W) Milliwatt (mW) Decibel (dB) dBm dBi dBd

    6. 6 Units of Measure… Watt = unit of power One ampere at one volt Milliwatt (mW) = 1/1000th watt Access points generally radiate 30 to 100 mW Decibels = based on a logarithmic relationship to a previously explained linear measurement of power (watts) – used to determine gain and loss measurements

    7. 7 The Watt (W)… The basic unit of power One ampere at one volt P = I * E (Where P = power, I = amps, and E = volts) The FCC allows up to 4 watts of power to be radiated from an antenna in a point-to-multipoint wireless LAN* *Using 2.4 GHz band, spread spectrum equipment

    8. 8 The Milliwatt (mW)… 1/1000th watt 1 mW = 0.001 W Most 802.11 devices use between 1 & 100 mW of power Access points generally radiate 30 to 100 mW

    9. 9 The Decibel (dB)… A general-purpose unit for expressing the difference between two measurements e.g. gain and loss in signal strength (mW) between two RF signals A logarithmic measurement (non-linear) Logarithm = exponent to which the number 10 must be raised to reach some value Log 1000 = 3 (because 103 = 1000 Measures (or expresses) the relative strength between two signals

    10. 10 The Decibel (dB)… Power Difference (dB) = 10 * log (Power A/Power B) Ex: A = 100 mW & B = 50 mW log of (100 mW/50 mW) = log of 2 = 0.3 10 * 0.3 = 3 dB Since A > B, we have a 3 dB loss

    11. 11 The Decibel (dB)… Relationships: +3 dB will double the watt value -3 dB will halve the watt value +10 dB will increase watt value ten-fold -10 dB will decrease the watt value to 1/10th Ex: A 20 mW signal increases by 10 dB… …the output signal would be 200 mW

    12. 12 The Decibel (dB)… Gains and losses in dB will always add algebraically… A certain signal starts at 60 dB. There is a 5 dB loss in the cable and the antenna introduces a 10 dB loss. 60 dB + (-5 dB) + (-10 dB) = 45 dB

    13. 13 Decibels (The dBm)… dBm = the reference point that relates the dB scale to the linear watt… 1 mW = 0 dBm This gives us an absolute power level We have normalized the decibel Now we can use the decibel to determine amounts of power

    14. 14 Decibels (The dBm)… We have normalized the decibel Now we can use the decibel to determine amounts of power and vice versa Ex: 100 mW = how many dBm? 100mW = 10 * 10 = +10 dB + 10 dB = + 20 dB Therefore, 100 mW = 20 dBm

    15. 15 Decibels (The dBm)… We have normalized the decibel Now we can use the decibel to determine amounts of power and vice versa Ex: 30 mW = how many dBm? 30 mW = 10 * 3 = +10 dB + 3 dB = 13 dB Therefore, 30 mW = 13 dBm

    16. 16 Decibels (The dBm)…

    17. 17 Decibels (The dBi)… Defn = the unit that is used to measure the gain of an antenna* Calculate power level @ a certain distance in the preferred direction Compare to power level @ same distance of an isotropic radiator Difference between the two power levels = dBi *Most 802.11 antennae are measured in dBi

    18. 18 Decibels (The dBi)…

    19. 19 Decibels (The dBd)… Defn = the gain of a real antenna relative to a theoretical half-wave dipole antenna* Standard gain of 2.14 dBi 0 dBd = 2.14 dBi *Most commonly used for gain of UHF & VHF antennas

    20. 20 Signal Strength… Why measure signal strength? To determine whether or not to roam to a different (stronger signal) access point To determine what data rate to transmit (based on transmitter’s received signals) To determine whether any other station is transmitting on the medium… Both determine whether it should begin receiving Both determine whether it’s safe to transmit

    21. 21 Received Signal Strength Indicator (RSSI)… 802.11 only requires a report of signal strength using RSSI Not required to report in dBm or mW Not measured in or expressed in any sort of unit No standard Ex: one vender – 1 to 255, another 1 to 32 Often indicated in percentage of signal strength

    22. Questions?

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