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Operating Channels in 802.11b Wireless Networks - A Comprehensive Overview

Explore the operating channels in 802.11b wireless networks, including the DS PHY systems and FH PHY systems. Learn about the frequency band, channel spread, spectral masks, hopping patterns, and how devices join frequency-hopping networks.

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Operating Channels in 802.11b Wireless Networks - A Comprehensive Overview

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  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.

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