802.11 Wireless Networks: The Definitive Guide, 2nd Edition

Chapter 11. The Frequency-Hopping (FH) PHY

Of all the physical layers standardized in the first draft of 802.11 in 1997, the frequency-hopping spread spectrum (FH or FHSS) layer was the first layer to see widespread deployment. The electronics used to support FH modulation are relatively cheap and do not have high power requirements. Initially, the main advantage to using frequency-hopping networks was that a greater number of networks could coexist, and the aggregate throughput of all the networks in a given area was high. At this point, however, FH networks are largely a footnote in the history of 802.11. Although it is a standard, only one vendor still manufactures and sells frequency-hopping systems, and they are being phased out. Higher-throughput specifications have demolished the advantage of aggregate throughput, and newer chipsets are less power-hungry.

This chapter describes the basic concepts used by the frequency-hopping PHY and the modulation techniques used. It also shows how the physical layer convergence procedure prepares frames for transmission on the radio link and touches briefly on a few details of the physical medium itself. At this point, the FH PHY is largely a footnote in the history of 802.11, so you may want to skip this chapter and move ahead to the next section on the direct-sequence PHY. However, understanding how 802.11 technology developed will give you a better feeling for how all the pieces fit together.

Frequency-Hopping Transmission

Frequency hopping ...