c and 802. 11b installed base. To overcome this obstacle while preserving the CHAPTER 5.1 Wireless local area networks 6.MHz spectrum symbol time offset (us) frequency (MHz) 54 Mbps 6 Mbps time (us) frame dB above average calculations from Agilent Signal Studio 802.11 Figure 5.1-20 Several views of an 802.11 OFDM frame. attractive high peak data rates of the 802.11a standard, OFDM. In this case there is additional overhead because task group "G" provided two enhancements to the Wi-Fi of the low data rate of the older preamble but no addi- PHY. The first variant is essentially the transfer of the tional packet exchange. The final chip of the preamble is 802.11a OFDM physical layer in toto to the ISM band. used as the phase reference for the first OFDM symbol. This trick is possible because the bandwidth of the Finally, "g" also defines a PHY using an extension of 802.1 11a symbols (Figure 5.1-20) is about 16 MHz, just as the PBCC coding system; in the highest-rate variant, the classic symbols were. 8PSK is substituted for QPSK modulation. It is perhaps The problem presented by this radical reinvention is too early in the history of 802. 11g to draw confident that older systems have no ability to receive and interpret conclusions about its evolution, but in view of the limited the complex OFDM symbols and simply see them as success of PBCC in 802.11b deployment, and successful noise. Because the CSMA/CA MAC layer is heavily if limited deployment of OFDM-based 802. chipsets dependent on all stations being able to hear (at least) the by major manufacturers, it seems plausible that the access point, invisible preambles are a serious obstacle: OFDM variant will dominate 802.11g devices. receiving stations cannot set their NAV to reserve the Trade-offs are necessarily encountered in the en- medium (Figure 5.1-6) if they can't read the packet hancement of the PHY. Higher data rates use higher preambles. In order for "g" stations to coexist with older modulation states of the subcarriers and thus require "b" stations, sev