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WIRELESS TECHNICAL FUNDAMENTALS: TOPICS
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Conversions
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1 MHz = 1,000,000 Hz
1 GHz = 1,000 MHz
1 GHz = 1,000,000,000 Hz
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I.E.
- 2400 MHz is the same as 2.4 GHz
- 5800 MHz is the same as 5800 GHz
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Hz = Radio Spectrum
Bits = data
M = 1,000,000
G = 1,000,000,000
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| There is a total of 83.3 MHz of Spectrum available in the 2.4 GHz Band |
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- 902 to 928 MHz
- 2,400 to 2,483.5 MHz
- 5,725 to 5,850 MHz
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- 5,150 to 5,250 MHz
- 5,250 to 5,350 MHz
- 5,725 to 5,825 MHz
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- (902 to 928 MHz)
- Data Rates are up to 3 Mbps ( Typical 2 Mbps throughput)
- Non Line of Sight
- Unaffected by weather
- Only few Manufacturers for the products – proprietary technology
- Expensive – Compared to IEEE 802.X technologies
- Large bandwidth available (83.5 MHz)
- High data rates possible (up to 11 Mbps)
- With the 11g – 54 Mpps is possible
- Equipment reasonably priced
- Unaffected by weather
- Line of Sight required for long range (greater the several hundred yards)
- Used world-wide
- 5.725 – 5.850 GHz Spread Spectrum Band
- Virtually unlimited ERP on Point-to-Point links makes long range practical
- Very high data rates possible
- Equipment more expensive then 2.4 GHz equipment
- Still greatly underutilized
- High gain directional antennas focus signals
- Ideal for point-to-point wireless backbone
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Unlicensed National Information Infrastructure
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Band 1
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Band 2
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Band 3
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FREQUENCY
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5.15 - 5.25 GHz
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5.25 - 5.35 GHz
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5.725 - 5.825 GHz
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TX POWER
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23 dBm EIRP
(200 mW)
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30 dBm EIRP
(1 Watt)
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36 dBm EIRP
(4 Watts)
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APPLICATION
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INDOOR USE ONLY
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CAMPUS LINKS
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LONG RANGE
(Several Miles)
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Full Duplex
- Radio TX and RX data simultaneously
- Requires a pair of Frequencies (one for TX and one for RX)
- Fastest means of data transfer
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Both radios are transmitting and receiving at the same time on different radio frequencies.
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Half Duplex
- Used by IEEE 802.11a,b,g and others
- Time Division Duplex (TDD)
- Bi-directional system
- Only one radio transmits at a time
- Radio buffers the data until it is its turns to transmit
- Operates only on one radio channel
- Typically less then half thru-put of the over-the-air data rate
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Radios alternate transmit and receive on the same radio channel.
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dB
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Difference (or ratio) between two signal levels. Generally used used to describe the effect of system devices on signal strength.
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dBm
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A signal strength level. 0 dBm is defined as 1 mW of power. Small signals are negative numbers. (e.g. - 83 dBm)
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dBi
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The gain of an antenna relative to an isotropic radiator. Used in calculating ERP and range.
NOTE: dB, dBm and dBi are used because systems’ powers, gains and losses can be calculated by simply adding and subtracting.
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ERP
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Effective Radiated Power, equal to the antenna gain added to the power into that antenna. (Technically, called EIRP.)
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| Effective Radiated Power (ERP) |
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dBm
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0
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1
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2
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3
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6
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10
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15
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20
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30
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33
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36
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40
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50
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60
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Power
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1.0 mW
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1.3 mW
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1.6 mW
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2.0 mW
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4.0 mW
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10.0 mW
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32.0 mW
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100 mW
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1 Watt
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2 Watts
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4 Watts
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10 Watts
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100 Watts
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1.000 Watts
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dBm = 10 log
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dBm = 10 log |
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| dBm = 10 log |
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Example: Convert 250 mW to dBm
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10 log
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250
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= 10 x 2.39 = 24dBm |
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1 mW
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Double/Half Power:
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Add/Subtract 3dB |
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Ten Times/One-tenth Power:
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Add/Subtract 10dB |
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| Spread Spectrum Technologies |
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Frequency Hopping (FHSS)
- The radio carrier hops around the band.
Direct Sequence (DSSS)
- The radio carrier signal is “spread out” on a specific channel
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Each data packet sent on one of 82 channels.
- Next packet randomly hops to another channel.
- Signal itself not “spread” - occupies narrow bandwidth.
- Signals “hop” around interfering signals.
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| DSSS Spectrum - IEEE 802.11b |
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Direct Sequence Spread Spectrum (DSSS)
Frequency Channel in 2.4 GHz (IEEE 802.11)
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Standard
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Modulation
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Air Data Rate
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Frequency Band
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802.11
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FHSS
DSSS
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2 Mbps
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2.4 GHz
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802.11a
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OFDM
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54 Mbps
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UNII
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802.11b
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DSSS
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11 Mbps
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2.4 GHz
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802.11g
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OFDM
DSSS
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54 Mbps
11 Mbps
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2.4 GHz
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802.1x
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N/A
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Authentication and
Access Control
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All
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