 2.467 2 Mbps. Both rates use the same chip rate of 1Mbps and have essentially the same bandwidth. The difference in 2.472 data rate is achieved by using differing modulations. 2.484 Packets at the basic rate of 1 Mbps, and the preambles of packets that otherwise use the extended rate of 2 Mbps, use differential BPSK (DBPSK), as shown in Figure 5.1-10. The data portion of extended-rate packets (Note that the arrows in Figure 5.1-10 showing the dif- uses differential quaternary phase-shift keying ferences in phase are indicative only of the phase change (DQPSK). Recall that the bandwidth of a signal is mainly between the moments at which the signal is sampled, not determined by the symbol rate, not by the nature of the of the path the signal takes between sample times. This individual symbols; because QPSK carries 2 bits per apparently subtle point has significant consequences in symbol, it is possible to roughly double the data rate terms of the requirements on the radio. A moment's without expanding the bandwidth, at a modest cost in reflection will clarify why such a change might be desir- required (S/N). In each case, the data bits are scrambled able. Imagine that we wish to accurately state the abso- in order before transmission; this procedure avoids the lute phase of a received QPSK signal by, for example, transmission of long sequences of 1s or Os that might be comparing it with a local oscillator that at the beginning present in the source data, which would give rise to of the packet is locked to the phase of the carrier. A spectral artifacts. 1500-byte (12,000 bit) packet at 2 Mbps lasts 6000 usec. The 802.11 modulations are differential variants of During this time, the total phase change of the carrier is the BPSK and QPSK schemes, that is, the phase of each 2n (2.4 GHz) (6000 X 10 sec) = 9 X 107 radians. If we symbol is defined only with respect to the symbol that wish the phase of the local oscillator to remain accurate preceded it. Thus, if the phase of a BPSK signal is the to, for example, /12 radi