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 differentlyPg 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 overlapPg 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.