CWNA

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Author:
Tjr31
ID:
33192
Filename:
CWNA
Updated:
2010-09-14 09:01:15
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cwna
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Description:
ch 3
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  1. Components of RF
    communications
    • Transmitter
    • Antenna
    • Receiver
    • isotropic radiator
    • Intentional Radiator (IR)

    •Equivalent Isotropically Radiated Power (EIRP)

    »
  2. Transmitter
    • •Initial component in the creation of a
    • wireless signal

    •Receives data from the system and begins RF communication

    •Encodes the data

    •Modulates the AC signal

    •Sends modulated signal to the antenna
  3. Antenna
    Collects the Modulated signal

    •Radiates the waves into the air

    •Captures RF waves

    •Passes the AC signal to the receiver

    •Acts as a bi-directional passive amplifier

    • •Shapes the coverage cell based on antenna
    • type and design
  4. Receiver
    Final component in wireless signaling

    •Converts carrier signal into 1’s and 0’s

    • •Receive amplitude (receive strength) is
    • weaker than transmit amplitude
  5. Isotropic Radiator
    •Radiates in all directions, like the Sun

    • •Omni-directional antennae are not Isotropic
    • Radiators

    • •There is not an isotropic antenna type,
    • although Omni-directional antennae seem to provide such coverage
  6. Intentional Radiator
    • A device that intentionally emits RF energy
    • by radiation or induction

    • •Regulatory bodies (governments) regulate the
    • maximum IR power

    • •IR power is measured from the transmitter to
    • the base of the antenna including all components except the antenna it’s self (basically the amount of power being sent
    • into the antenna for transmission onto the medium)
  7. Equivalent Isotropically
    Radiated Power (EIRP)
    • The highest RF signal strength that is
    • radiated by a specific antenna

    • •Regulatory bodies (governments) regulate the
    • maximum EIRP just like the IR

    •Measured after the antenna

    • (the amount of power the total system places
    • on the medium)
  8. Units of Power and
    Comparison
    Watt

    •Milliwatt

    •Decibel (dB)

    •dBi


    •dBd


    •dBm


    •Inverse Square Law
  9. Watt
    •An absolute measure of power

    •Named after James Watt 18th century Scottish inventor

    • •A Watt (W) is the basic unit of power
    • 1W=1000mW


    •1W=1 ampere (amp) of power flowing at 1 volt.
  10. Milliwatt (mW)
    •An absolute measure of power

    •1mW=1/1000W

    • •Most 802.11 devices transmit between 1wW and
    • 100mW

    • •Bridge devices may often exceed 1000mW or 1W
    • in transmissions


    • (In comparison, microwaves (which use RF energy to cook) use
    • thousands of Watts. It is no wonder that
    • they often cause layer one interference for 802.11 devices)
  11. Decibel (dB)
    •Not an absolute unit of measure

    •Used as a comparison

    • •Derived from the term bel from
    • Bell Labs.

    • •Normally used to measure change in signal
    • strength with either gain or loss

    • •Bel’s can
    • be examined using logarithms

    dB= 10xlog10(P1/P2)
  12. Decibels Isotropic (dBi)
    • •Not
    • an absolute unit of measure

    • •Decibels
    • Isotropic is a value compared to what an isotropic radiator would generate

    • •dBi is usually found in discussions about
    • antenna gain

    • •Also
    • phrased as change in power relative to an antenna

    • •Always
    • positive or 0 (no gain)
  13. Decibels Dipole (dBd)
    •Not an absolute unit of measure

    • •An additional reference used by antenna
    • makers to describe change related to antenna gain

    • •Denotes change relative to a dipole antenna
    • (the proper name for an Omni-directional antenna)

    •Most antennae are labeled in dBi


    •1 dBd=2.14 dBi
  14. dBm
    • Not
    • an absolute unit of measure

    • •Compares
    • a given signal against 1mW

    – 0 dBm = 1mW

    –10dBm = 10mW

    –20dBm = 100mW

    –30dBm = 1000mW = 1 W

    –36dBm = 4000mW = 4 W

    –See formula on page 74
  15. Inverse Square Law
    •Developed by Isaac Newton

    • •States that the change in power is equal to 1
    • divided by the square of the change in distance. Page 76

    • •This can be used to accurately calculate
    • expected Free Space Path Loss

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