 power words, corresponding to the 8 input bits that define control. These features are required for operation in most the chosen CCK block are allowed. They are found in European jurisdictions, and the recent FCC decisions the following fashion, shown schematically on spectrum in the United States will also require such Figure 5.1-12. (Figure 5.1-12 is headed "even-numbered capability for operation in most of the 5-GHz bands. symbol" because alternate code words have slightly In the next three subsections we examine the im- different phase definitions, important for controlling the portant 802.11b and 802.11a PHY layers and briefly spectrum of the output but confusing when one is trying touch on their admixture in 802.11g. We also provide to figure out how the scheme works.) The input bits are a cursory discussion of WLAN (in)security and some grouped in pairs (dibits). Each dibit defines one of four cures for the deficiencies revealed therein. We must re- possible values of an intermediate phase Q. The in- grettably refer the reader to the IEEE web sites described termediate phases are then added up (modulo-2) as in section 5.1.3.7 for more details on the other task shown in the chart to determine each QPSK symbol C of groups within 802.11. the transmitted code word. Thus, the last symbol c7 shares the phase of 41 (and is used as the phase reference 5.1.3.5 The Wi-Fi PHY (802.11b) for the remainder of the code word). The phase of c4 is (1 + 02 + 43). Symbols c3 and c6 The 802.11b physical layer uses the same 2.4-GHz band have TT radians added to the argument (i.e., they are and channelization as the classic PHY. Furthermore, the multiplied by 41). basic signaling structure of 11 megasamples/second This procedure is a bit confusing to describe but easy (Msps) of either BPSK or QPSK symbols is unchanged, to implement and ensures that the resulting allowed code and so it may be expected that the frequency spectrum words are uniformly distributed among all the possible of the transmitted signals will be similar to 