ts were deployed with the FH radio, and the FH Let us examine how these problems are addressed approach does offer certain advantages in terms of the in some current and upcoming WLAN and WPAN number of independent collocated networks that can be technologies. supported with minimal interference. However, in terms of current commercial importance, the DSSS physical layer is completely dominant, due to its later extension to 5.1.3 802.11 WLANs higher data rates in the 802.11b standard in 1999. Therefore, here we shall concentrate only on this phys- In 1985 the U.S. Federal Communications Commission ical layer. The IEEE standards bodies do many wonderful (FCC) issued new regulations that allowed unlicensed things, but creation of convenient nomenclature is not communications use of several bands, including the one of them; therefore, we introduce the terminology 2.4-GHz band, that had previously been reserved for 802.11 classic to refer to the original 1997 802.11 unintended emissions from industrial equipment. In- specification and its later releases and in our case pre- terest in possible uses of this band grew, and in the late dominantly to the DSSS variant. We can then un- 1980s researchers at NCR in Holland, who had ambiguously use the unmodified moniker "802.11" to Wireless local area networks CHAPTER 5.1 refers to the ESSID but is obviously somewhat am- BYTES: <1500 to 64 Start Destination Source Preamble Length Error biguous and potentially confusing. The ESS architec- address or type Payload Frame address check ture provides a framework in which to deal with the problem of mobile stations roaming from one BSS to Figure 5.1-2 An Ethernet (802.3) frame (Packet). another in the same ESS. It would obviously be nice if a mobile station could be carried from the coverage area of one access point (i.e., one BSS) to that of an- refer to all the 802.11 working group standards, including other without having to go through a laborious process the alphabet soup of elaborations released in later years. of reauthenticat