Chapter 6-Wireless Networks and Spread Spectrum Technology

1. 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.

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. UNII-2 Middle • 5.250 Ghz to 5.350 Ghz • 250 mW IR from FCC • Often used outdoors as well Pg 194

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. Modulation • Differential Binary Phase Shift Keying (DBPSK) • Two phase shifts • Differential Quadrature Phase Shift Keying (DQPSK) • 4 phase shifts Pg 201

20. 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

21. Orthogonal Frequency Division Multiplexing (OFDM) Pg 202

22. 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

23. 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

24. 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

25. 2.4 Ghz channels Pg 204

26. 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

27. Overlapping vs. Non-Overlapping Pg 206

28. 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

29. 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

30. 5 Ghz channels Pg 208

31. 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

32. 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

33. 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.