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ITNW 1351
Fundamentals of Wireless
LANsChapter 3
Spread Spectrum Technology
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www.ntia.doc.gov/osmhome/allochrt.pdf
http://www.fact-index.com/t/ty/types_of_radio_emissions.html#Bandwidth
http://www.kmj.com/proxim/pxhist.html
http://www.ntia.doc.gov/osmhome/allochrt.pdfhttp://www.fact-index.com/t/ty/types_of_radio_emissions.htmlhttp://www.kmj.com/proxim/pxhist.htmlhttp://www.kmj.com/proxim/pxhist.htmlhttp://www.fact-index.com/t/ty/types_of_radio_emissions.htmlhttp://www.ntia.doc.gov/osmhome/allochrt.pdf -
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Narrowband Transmission
Frequency
P
o
w
e
r
1. Uses only enoughfrequency to carry
data (& NO MORE)
2. High peak power
(more power required
to use a smaller
frequency range)
3. Signal must stand
out above inherent
RF noise level (noise
floor) in order to be
received
4. Can be jammed
easily
5. Can experience
interference easily
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Narrowband Transmission
AMBroadcast
Example:
10 kHz (5 kHz above &
5 kHz below fC)
fC
1340 kHz
1345 kHz1335 kHz
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Spread Spectrum Technology
Frequency
P
o
w
e
r
1. Information can besent using
narrowband carrier
signal spread out
over a much larger
frequency range
2. Probability of signal
corruption or
jamming is greatly
reduced
3. Peak power is very
low
4. Signal looks like
noise to most
receivers
three radio bands for
transmission under 1 Watt of
power:902-928 MHz
2400-2483.5 MHz
& 5752.5-5850 MHz
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Narrowband vs. Spread Spectrum
Technology
Narrow Band:
1. High peak
power
2. Interference
and jamming
a problem
Spread
Spectrum
Technology
1. Wide band
2. Low peak
power
3. Looks likenoise
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Uses of Spread Spectrum
1950s & 1960s - military
Since 1980s
Cordless phones Global positioning systems (GPS)
Digital cellular telephony (code division
multiple access CDMA)
Personal communications system (PCS)
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Uses of Spread Spectrum
Wireless LANS (WLANs) Wireless personal area network (WPAN)
Bluetooth technology
Wireless metropolitan area network(WMAN) Highly directional, high-gain antennas for long
distance
High-speed RF links with relatively low power Wireless wide area network (WWAN)
See WMAN above
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Uses of Spread Spectrum
The most common uses of spread
spectrum technology today lie in a
combination of wireless 802.11-
compliant LANs and 802.15-compliant
Bluetooth devices
The two function differently, play within
the same FCC rules, but can interfere witheach other
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Bluetooth
The most popular WPAN technologies
Specified by the IEEE 802.15 standard
Hops approximately 1600 times persecond
Other technologies frequency-hopping
HomeRF 2.0 approximately 50 times persecond
802.11 WLAN 5-10 times per second
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WMANs
Wireless links that span an entire cityusing high-power point-to-point links tocreate a network
Use licensed frequencies instead of theunlicensed frequencies used withWLANs
For frequency control i.e. organization willnot have to worry about an interferingnetwork
Same applies to WWANs
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FCC Specifications
Has specified two types of spread
spectrum technology
1. Direct Sequencing
2. Frequency Hopping
Title 47 Telegraphs, Telephones,
and Radiotelegraphs Part 15
Wireless LAN devices are called part 15
devices
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Frequency-hopping Spread
Spectrum (FHSS)
Uses frequency agility to spread data over a
minimum of 75 and a maximum of 79 carrier
frequencies
Frequency agility = a radios ability tochange transmission frequency abruptly
within the usable RF band
2.4 GHz Industrial Scientific Medical (ISM)band
The band is 83.5 MHz wide (802.11)
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How FHSS Works
A carrier changes frequency (hops)according to a pseudo-randomsequence
A list of several frequencies to which thecarrier will hop at specified intervalsbefore repeating pattern
Carrier remains at a certain frequencyfor a specified time (dwell time), then Uses small amount of time to hop to the
next frequency (hop time)
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How FHSS Works
When the list of frequencies has been
exhausted, the transmitter repeats the
sequence
The process of repeating the sequence
continues until the information is
completely received
The receiver radio is synchronized to
the transmitting radios hop sequence
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How FHSS Works
F
r
e
q
u
e
n
c
y
Elapsed Time
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Frequency-hopping is not totally immune
to narrowband interference
If a particular frequency is interfered with, only
that portion of the spread spectrum signal
would be lost Lost data would be retransmitted
In order for a frequency-hopping system tobe 802.11, it must operate in in the 2.4
GHz ISM band
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Frequency-hopping systems operate using
a specified hopping pattern called achannel
26 hop patterns or a subset thereof
Synchronized radios
12 co-located systems as a max
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Direct Sequence Spread Spectrum
(DSSS)
Vast majority of wireless LAN
equipment uses this technology
A method of sending data in which the
tx and rx systems are both on a 22
MHz-wide set of frequencies
Wide channel = more data at higher rate
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DSSS
2.401
GHz
2.473
GHz
P
ow
e
r
Frequency
1. 11 Channels
2. Each channel is 22 MHz wide
3. Center frequencies are 5 MHz apart
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DSSS Channel 1 Example
fC
= 2.412 GHz
11 MHz 11 MHz
22 MHz
2.401 GHz 2.423 GHz
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fC = 2.412 GHz
2.401 GHz 2.423 GHz
fC = 2.417 GHz
2.406 GHz 2.428 GHz
5 MHz
DSSS Ch 1 & Ch 2 Spacing
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DSSSCh #
FCCfC (GHz)
ETSI
fC (GHz)
1 2.412 N/A
2 2.417 N/A
3 2.422 2.422
4 2.427 2.427
5 2.432 2.432
6 2.437 2.437
7 2.442 2.442
8 2.447 2.447
9 2.452 2.452
10 2.457 2.457
11 2.462 2.462
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DSSS Nonoverlapping
channels
P
o
w
e
r
Frequency
22 MHz 3 MHz
Ch 1 Ch 6 Ch 11
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Comparing FHSS & DSSS
Factors to consider:
Narrowband interference present in environment
Co-location for failover, load balancing, or added
capacity Cost of FHSS & DSSS systems
Equipment compatibility and availability
Data rate and throughput
Security
Standards support
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Comparing FHSS & DSSS
Narrowband interference
FHSS = greater resistance
DSSS systems affected more because of the use of
22 MHz contiguous bands instead of 79 MHz usedby FHSS
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Comparing FHSS & DSSS
Cost
DSSS equipment cost much less than FHSS
Because of rapid adoption in the
marketplace
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Comparing FHSS & DSSS
Co-location for failover, load balancing, and
added capacity
FHSS has advantage because of its
ability for many more frequency-hoppingsystems to be co-located than DSSS
79 discrete carrier frequencies
DSSS has maximum co-location of threeaccess points
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Comparing FHSS & DSSS
Equipment compatibility & availability
Wireless Ethernet Compatibility Alliance
(WECA) tests 802.11b compliant DSSS
LAN equipment to insure compatibility
and interoperation (Wi-Fi standard)
No such tests for FHSS
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Comparing FHSS & DSSS
Data rate and throughput
Frequency-hopping slower than DSSS
Both systems have a throughput (actual
data sent) of about of their data rate
Due to interframe spacing between data
frames for control signals etc.
No data sent when FHSS is changingfrequencies
Expect 5 to 6 Mbps on the 11 Mbps setting
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Comparing FHSS & DSSS
Security???
Standard sets of hop sequences and
compliance with IEEE or WLIF make
breaking code of hop sequences fairlyeasy
The radio broadcasts the channel
number in clear text with each beacon
MAC address of AP can be see with
each beacon
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Comparing FHSS & DSSS
Standards support
DSSS has gained wide acceptance
because of low cost, high speed, and
Wi-Fis interoperability standards
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THE
END