Operating (Orthogonal) Channels in 802.11b Wireless Networks

1. Operating (Orthogonal) Channels in 802.11b Wireless Networks Saleh Al-Harthi

2. OUTLINE 802.11 FAMILY DS PHY SYSTEMS OPERATING CHANNELS FH PHY SYSTEMS OPERATING CHANNELS

3. 802.11 Family • Comparison of 802.11 standards PHY = Physical Layer, DS = Direct Sequence, FH = Frequencey Hopping, OFDM = Orthogonal Frequency Division Multiplexing

4. DS PHY Systems Channels • FCC: ISM band 2.4--2.4835GHz, referred to as the 2.4GHz band • There are 14 channels (USA) in the 2.4GHz band,each 5 MHz wide • Channel 1 is placed at center frequency 2.412GHz, channel 2 at 2.417GHz, …and so on. • In USA, only channels 1 through 11 are allowed (by FCC) • Channel 10 is allowed by all (world) regulatory authorities and is the default channel for Access Points by most manufacturers • Channel Energy Spread in 802.11 • 11-MHz chip clock is chosen (Minimal sequence allowed by FCC, and minimizing frequency saves power…) • The effect of the chip sequence is to “spread” the transmitted bandwidth by a ratio of 11:1, and to “spread” energy out from the channel center in multiple of 11-MHz (Fourier Analysis) • The peak power of the signal is reduced by an identical ratio • RF power is confined mostly to a 22-MHz by a transmitting Mask (Figure, Next)

5. DS PHY Systems Channels • Spectral Mask • -30 dBr = reducing power by 1000 • -50 dBr = reducing power by 100,000

6. FH PHY Systems Channels • 802.11 FH PHY divides the ISM band into a series of channels each of 1-MHz wide • Ch. 0 at center frequency 2.400 GHz • Ch. 1 at center frequency 2.401 GHz … • Ch. 95 at center frequency 2.495 GHz • In the US, only channels 2 through 79 are allowed • ISM emission rules (FCC, US): (examples) • 1. There must be at least 75 hopping channels in the band • 2. Hopping channels can be no wider than 1-MHz • 3. Devices must use all available channels equally. In a 30-second period, no more then 0.4 seconds may be spent using any one channel

7. FH PHY Systems Channels (cont.) • Hopping patterns: North America as an example • There is a “base”-hopping sequence b(i): For north America, for example, b(i) is defined as: b(i) = {0, 23, 62, 8, 43, 16, 71, 47, 19, 61…} 79 elements • An FH pattern number x, denoted Fx, consists of a permutation of all frequency channels • Fx = {fx(1), fx(2), …, fx(79)}, where for i=1, 2, 3, …, 79 and x is the “hopping-pattern number” given in three sets: Set 1: x = {0, 3, 6, 9, 12, …, 75}  26 elements Set 2: x = {1, 4, 7, 10, 13, …, 76}  26 elements Set 3: x = {2, 5, 8, 11, 14, …, 77}  26 elements

8. FH PHY Systems Channels (cont.) • Joining an 802.11 Frequency-Hopping Network: The Operating Channel? • Beacon frames on FH networks include a “timestamp” and the “FH Parameter Set Element” • The “FH Parameter Set Element” includes the “hop pattern number” and a “hop index” • Once the station knows the “hop pattern number” and a “hop index”, it knows the next channel-hopping order…because “hop patterns” (or sequences) are standardized… • Hop “times” are also well-defined: The hop occurs when the “timestamp” modulo “dwell time” (included in the beacon) is 0.