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Chapter 6-Wireless Networks and Spread Spectrum Technology
• Frequency bands, channels and technologies
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Exam Essentials• Know the technical specifications of all the ISM and UNII
bands. – Make sure that you know all of the frequencies, bandwidth uses,
channels, and center channel separation rules.• Know spread spectrum.
– Spread spectrum can be complicated and has different favors. Understand FHSS, DSSS, and OFDM (although OFDM is not a spread spectrum technology, it has similar properties and you have to know it). Understand how coding and modulation work with spread spectrum and OFDM.
• Understand the similarities and differences between the transmission methods discussed in this chapter. – There are differences and similarities between many of the
topics in this chapter. Carefully compare and understand them. Minor subtleties can be difficult to recognize when taking the test.
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Industrial, Scientific and Medical Bands (ISM)
• 802.11, 802.11b, 802.11g all use same bands– 2.4 Ghz to 2.4835 Ghz
• ISM also has– 902-928 Mhz (26 Mhz)– 2.4000-2.4835 Ghz (83.5 Mhz)– 5.725-5.875 Ghz (150 Mhz)
• Specified by the ITU– Each country manages themselves
• All License Free
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900 Mhz ISM Band• 26 Mhz wide
• Was used for wireless– Not much used for wireless anymore
• Alos used by GSM in many countries
• 802.11 doesn’t use it
• Popular for wireless ISPs– Good through foliage
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2.4 Ghz ISM Band• Most common• Most 802.11 standards support it
– 802.11 (FHSS clause 14 or DSS clause 15)– 802.11 b (HR-DSSS clause 18)– 802.11 g (ERP Clause 19)– 802.11 n (HT Clause 20)
• Also used by microwave, cordless phones, baby monitors, wireless cameras– Lots of interference
• Each country manages range differentlyPg 192
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5.8 Ghz ISM Band• Similar consumer devices to 2.4 Ghz
• Not the same as UNII-3
• 802.11 a– Can work on ISM channel 165-5.825 Ghz
• Often used for outdoor long distance wireless bridging– Less restrictions on power
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Unlicensed National Information Infrastructure (UNII) Bands
• Original specs of 802.11 a had 3 bands of 4 channels– Lower-UNII-1– Middle-UNII-2– Upper-UNII-3– All three are 100 Mhz wide
• 802.11 h designated more– UNII-2 Extended-11 more channels– 255 Mhz wide
• 802.11a (OFDM Clause 17)• 802.11h (TPC and DFS)• 802.11n (HT Clause 20)• Each country will be different
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UNII-1 Lower• 5.150 Ghz to 5.250 Ghz
– 50 mW IR from FCC
• Original FCC specs had permanent antenna– Since changed to unique connector
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UNII-2 Middle• 5.250 Ghz to 5.350 Ghz
– 250 mW IR from FCC
• Often used outdoors as well
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UNII-2 Extended• 5.470 to 5.725 Ghz
– 255 Mhz wide– Max of 250 mW IR from FCC
• Introduced in 802.11h– Also set up TPC and DFS to avoid radar
transmission
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UNII-3 Upper• 5.725 Ghz to 5.825 Ghz
– 100 Mhz wide– Max of 1000 mW IR from FCC
• Mostly for outdoors
• Overlaps with 5.8 Ghz ISM band
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Narrowband and Spread Spectrum• Different ways of transmitting over RF• Narrowband uses little bandwidth, but high
power– 2 Mhz @ 80 Watts– Easier to block/jam
• Spread Spectrum uses more bandwidth than needed and spreads the signal– 22 Mhz at 100 mW– Harder to jam
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Transmission issues• Multipath
– When a reflected signal arrives at receiving antenna after the primary signal
• Delay between main and reflected signal is the delay spread– If delay spread is long enough to interfere with next
part of main signal it is intersymbol interference (ISI)
• Spread Spectrum technologies try to avoid ISI by spreading– More tolerant than narrowband
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Transmission issues• 802.11 (DSSS) and 802.11 b(HR-DSSS)
can tolerate 500 nanoseconds of delay– But it does affect performance– 802.11 b will drop to a lower rate to
compensate
• 802.11 g (OFDM) can maintain 54 Mbps with 150 nanoseconds of delay
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FHSS• Used in 802.11 prime• 1 and 2 Mbps in 2.4 Ghz ISM• Original spec for 79 Mhz between 2.402 and 2.480
– Mostly between 1997 and 1999
• Transmits small amount and then hops– Dwell time is amount of time on each frequency
• Hopping sequences need to sync between devices– Original spec of 1 Mhz hop– 802.11 standard included for hopping sequence information to
be sent in the beacon frame to client stations
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FHSS• Dwell Time
– Amount of time to transmit on a specific frequency• Max of 400 ms during 20 sec• Usually 100 to 200 ms
• Hop Time– Measure of how long it takes radio to change
frequency– Usually 200 to 300 microseconds – Wasted time-overhead
• Modulation– Gaussian Frequency Shift Keying to encode data– Two or 4 level
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Direct Sequence Spread Spectrum (DSSS)
• 802.11 prime– 1 and 2 Mbps in 2.4 Ghz ISM– Clause 15
• 802.11b– 5.5. and 11 Mbps in 2.4 Ghz ISM– HR-DSS clause 18
• Set to a single channel, but spread across
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Data Encoding• Data is encoded and sent as multiple bits• Adding additional bits is called processing
gain– Create chips
• Chips are then spread across a wide space
• Receiving devices de-spreads– With barker coding, 9 out of 11 chips can be
corrupt, but still be able to interpret
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Modulation• Differential Binary Phase Shift Keying (DBPSK)
– Two phase shifts• Differential Quadrature Phase Shift Keying
(DQPSK)– 4 phase shifts
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Orthogonal Frequency Division Multiplexing (OFDM)
• Used in both wired and wireless• OFDM at 5 Ghz• ERP OFDM at 2.4 Ghz• Not technically a spread spectrum
technology– Uses 52 subcarriers per channel– 312.5 Khz each– Lower data rates– ISI is less likley
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Orthogonal Frequency Division Multiplexing (OFDM)
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Orthogonal Frequency Division Multiplexing (OFDM)
• 48 out of 52 subcarriers are for data
• -21, -7, +7 and +21 are pilot carriers for phase and amplitude reference
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Convolutional Coding & Modulation• Convolutional Coding
– Form of error correction to avoid narrowband interference
• Forward error correction• Many types
• Modulation– Binary Phase Shift Keying– Quadrature Phase Shift Keying– 16-QAM and 64 QAM for higher speeds
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2.4 Ghz channels• 2.4 Ghz ISM range is set up in 14
channels– Regulations specify which channels are
available to be used. In US, 11 channels
• Each channel is 22 Mhz wide– Carrier +- 11 Mhz– Distance between carriers is 5 Mhz
• Lots of overlap
– Channels 1, 6 and 11 have least overlapPg 204
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2.4 Ghz channels
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Overlapping vs. Non-Overlapping• Specification of overlap has changed since
original 802.11– 25 or 30 Mhz between carrier frequencies– However, sidebands still cause interference
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Overlapping vs. Non-Overlapping
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Overlapping vs. Non-Overlapping• Sidebands are many dB less, but must still
be accounted for
• Place AP far enough apart so overlap is quiet enough
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5 Ghz channels• UNII-1, UNII-2, UNII-2 Extended, UNII-3
– Center of channels is 30 Mhz from the nads edge in UNII 1 and 2
– 20 Mhz in UNII-3
• All channels are non-overlapping– Spectral mask is about 20 Mhz
• Sideband are more likely to interfere than with ISM
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5 Ghz channels
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Adjacent, Nonadjeacent, and Overlapping
• Terminology is somewhat unclear
• DSSS needs 30 Mhz for non-verlap
• HR-DSSS and ERP need 25 Mhz
• 5 Ghz OFDM uses 20 Mhz
• Important when setting up overlapping cell areas to allow for roaming in an ESS– Channel reuse
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Throughput vs. Bandwidth• Don’t confuse frequency Bandwidht (size
of channels) and data bandwidht (speeds for transmission)– Also different from throughput, which is actual
data performance
• Since wireless is half duplex, most of the time you get 50% or the “bandwidth”
• Since it is shared, if 5 stations are sharing 20 Mbps, each will get about 4Mbps of performance
Pg 210
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Exam Essentials• Know the technical specifications of all the ISM and UNII
bands. – Make sure that you know all of the frequencies, bandwidth uses,
channels, and center channel separation rules.• Know spread spectrum.
– Spread spectrum can be complicated and has different favors. Understand FHSS, DSSS, and OFDM (although OFDM is not a spread spectrum technology, it has similar properties and you have to know it). Understand how coding and modulation work with spread spectrum and OFDM.
• Understand the similarities and differences between the transmission methods discussed in this chapter. – There are differences and similarities between many of the
topics in this chapter. Carefully compare and understand them. Minor subtleties can be difficult to recognize when taking the test.