Chapter 6-Wireless Networks and
Spread Spectrum Technology
• Frequency bands, channels and
technologies
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.
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
Pg 191
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
Pg 191
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
Pg 192
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
Pg 192
•
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
Pg 193
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
Pg 193
UNII-2 Middle
• 5.250 Ghz to 5.350 Ghz
– 250 mW IR from FCC
• Often used outdoors as well
Pg 194
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
Pg 194
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
Pg 194
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
Pg 195
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
Pg 197
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
Pg 197
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
Pg 197
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
Pg 197
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
Pg 200
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
Pg 200
Modulation
• Differential Binary Phase Shift Keying (DBPSK)
– Two phase shifts
• Differential Quadrature Phase Shift Keying
(DQPSK)
– 4 phase shifts
Pg 201
•
•
•
•
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
Pg 202
Orthogonal Frequency Division
Multiplexing (OFDM)
Pg 202
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
Pg 202
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
Pg 203
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
Pg 204
2.4 Ghz channels
Pg 204
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
Pg 206
Overlapping vs. Non-Overlapping
Pg 206
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
Pg 207
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
Pg 208
5 Ghz channels
Pg 208
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
Pg 210
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
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.