he maximum data rate, access points will progressively fall Wireless LAN Standards Table 6-7: 802.11b DSSS Modulation Methods, Coding and Data Rate Modulation Code length Code type Symbol rate Data bits Data rate (Chips) (Msps) per symbol (Mbps) Barker Barker DQPSK 1.375 DQPSK 1.375 back to lower rates until reliable communication is restored. This strategy is based on the implications of Eq. (4-1), which showed that SNR is proportional to the transmitted energy per bit, SO that by falling back to a lower data rate, a higher SNR and lower BER can be achieved. Conversely, if a station moves back within range for a higher rate, or if interference is reduced, the link will shift to a higher rate. Rate shifting is implemented in the PHY layer and is transparent to the upper layers of the protocol stack. The 802.11 standard specifies the division of the 2.4 GHz ISM band into a number of overlapping 22 MHz channels, as shown in Figure 4-9. The FCC in the US and the ETSI in Europe have both authorised the use of spectrum from 2.400 to 2.4835 GHz, with 11 channels approved in the US and 13 in (most of) Europe. In Japan, channel 14 at 2.484 GHz is also authorised by the ARIB. Some countries in Europe have more restrictive channel allocations, notably France where only four channels (10 through 13) are approved. The available channels for 802.11b operation are summarised in Table 6-8. The 802.11b standard also includes a second, optional modulation and coding method, packet binary convolutional coding (PBCCTT-Texas Instruments), which offers improved performance at 5.5 and 11 Mbps by achieving an additional 3 dB processing gain. Rather than the 2 or 4 phase states or phase shifts used by BPSK/DQSK, PBCC uses 8-PSK (8 phase states) giving a higher chip per symbol rate. This can be translated into either a higher data rate for a given chipping code length, or a higher processing gain for a given data rate, by using a longer chipping code. Chapter Six Table 6-8: International Channel Availability for 802.11b Networks in the 2