 in 802.11 is still useful since have been less expensive. However, the updated versions products based on it may be available. In FHSS WLAN, of 802.11-specifically 802.11a, 802.11b, and 802. transmissions occur on carrier frequencies that hop pe- 11g-have all based their methods of increasing data riodically in pseudo-random order over almost the rates on the broadband channel characteristics of DSSS complete span of the 2.4 GHz ISM band. This span in in 802.11, while being downward compatible with the Short-range wireless applications and technologies CHAPTER 5.2 1 and 2 Mbps DSSS modes (except for 802.11a which operates on a different frequency band). DSSS PHY 2412 MHz 2437 MHz 2462 MHz The channel characteristics of the direct sequence spread spectrum physical layer in 802.11 are retained in the high Figure 5.2-8 DSSS non-interfering channels. data rate updates of the specification. This is natural, since systems based on the newer versions of the speci- detected in a receiver by a matched filter or correlation fication must retain compatibility with the basic 1 and detector. Figure 5.2-9 is one possible implementation of 2 Mbps physical layer. The channel spectral mask is the modulator. The DSSS PHY specifies two possible shown in Figure 5.2-7, superimposed on the simulated data rates-1 and 2 Mbps. The differential encoder takes spectrum of a filtered 1 Mbps transmission. It is 22 MHz the data stream and produces two output streams at wide at the -30 dB points. Fourteen channels are allo- 1 Mbps that represent changes in data polarity from one cated in the 2.4 GHz ISM band, whose center frequen- symbol to the next. For a data rate of 1 Mbps, differential cies are 5 MHz apart, from 2.412 GHz to 2.484 GHz. binary phase shift keying is used. The input data rate of The highest channel, number fourteen, is designated for 1 Mbps results in two identical output data streams that Japan where the allowed band edges are 2.471 GHz and represent the changes between consecutive input bits. 2.497 GHz. In the US and Canada