Chapter 6-Wireless Networks and Spread Spectrum Technology

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Chapter 6-Wireless Networks and Spread Spectrum
Technology

• Frequency bands, channels and technologies

2
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 doesnt 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 differently

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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 _at_ 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 overlap

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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

• Dont 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

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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.