analog signal that represents the the receiver. degree of correlation. The following differential decoder performs the opposite operation of the differential en- coder described above and outputs the 1 or 2 Mbps data. 802.11b The process of despreading the input signal by corre- The "b" supplement to the original 802.11 specification lating it with the stored spreading sequence requires supports a higher rate physical layer for the 2.4 GHz synchronization of the receiver with transmitter timing band. It is this 802.11b version that provided the impe- and frequency. To facilitate this, the transmitted frame tus for Wi-Fi proliferation. With it, data rates of 5.5 starts with a synchronization field (SYNC), shown at the Mbps and 11 Mbps are enabled, while retaining down- beginning of the physical layer protocol data unit in ward compatibility with the original 1 and 2 Mbps rates. Figure 5.2-11. Then a start frame delimiter (SFD) marks The slower rates may be used not only for compatibility out the commencement of the following information with devices that aren't capable of the extended rates, bearing fields. All bits in the indicated preamble are but also for fall back when interference or range condi- transmitted at a rate of 1 Mbps, no matter what the tions don't provide the required signal-to-noise ratio for subsequent data rate will be. The signal field specifies communication using the higher rates. the data rate of the following fields in the frame so that As previously stated, the increased data rates provided the receiver can adjust itself accordingly. The next field, for in 802. 11b do not entail a larger channel bandwidth. SERVICE, contains all zeros for devices that are only Also, the narrow-band interference rejection, or jammer compliant with the basic version of 802.11, but some of resisting qualities of direct sequence spread-spectrum its bits are used in devices conforming with updated are retained. The classical definition of processing gain versions. The value of the length field is the length, in