eral options are possible. First, in the better (S/N) than the old QPSK chips. This implies that presence of mixed traffic, "g" stations can use the RTS/ higher rate communications will have a shorter range CTS approach to reserve the medium using 802.11b than the lower rates. The use of OFDM signals gives rise packets and then use the reserved time to send faster to higher peak-to-average power ratios, allowing less 802.11g packets. This approach is simple but adds transmission power from the same amplifiers to avoid overhead to the "g" packet exchanges. Alternately, a sta- distortion. The benefit of coexistence is achieved at the tion can send a CTS packet with itself as the destination: cost of reduced actual data rates in the presence of legacy this approach, known as a CTS-to-self, is obviously more stations due to the overhead of any of the coexistence appropriate for an access point, which expects that every protocols discussed above. These problems are fairly associated station can hear it, than a client. minor bumps in the road, however: it seems likely that A "g" station can also use a mixed frame format, in 802.11g products will dominate the WLAN marketplace which the preamble is sent in a conventional 802.11b until crowding in the ISM band forces wider adoption of form and then the appended data are transferred as 802.11a/dual-band devices. Wireless local area networks CHAPTER 5. 5.1.3.8 802.11 (In) )Security implement and would not incur SO much overhead as to reduce the WLAN data rate by an objectionable Most participants in wireless networking, or indeed net- amount. working of any kind, are at least vaguely aware that 802.11 The resulting Wired Equivalent Privacy (WEP) secu- presents security problems. One can hardly have a discus- rity system was made optional in the classic standard but sion of 802.11 without dealing with encryption and secu- was widely implemented and extended. The system is rity; however, as whole books have already been written on a symmetric key system, in which the same key