IEEE 802.11 High Rate Direct Sequence Spread Spectrum (HR-DSSS) Physical Layer

The IEEE 802.11 High Rate Direct Sequence Spread Spectrum (HR-DSSS) Physical layer is a
rate extension to the 802.11 DSSS standard. HR-DSSS, commonly referred to as 802.11b,
includes complementary code keying (CCK) and operates in the 2.4GHz band to achieve additional
data rates of 5.5Mbps and 11Mbps. The HR-DSSS is the most common wireless LAN
implementation today and is interoperable with 802.11 DSSS implementations. This section
describes the architecture and operation of 802.11 HR-DSSS.  The PLCP for the 802.11 HR-DSSS is the same as DSSS (refer to Figure 5.7) except for the
following:
• Alternate short SYNC field A shorter alternate SYNC field can be used in implementations
in order to minimize overhead and maximize throughput. The short SYNC field
consists of 56 scrambled bits.
• Signal field contents The signal field value multiplied by 100Kbps indicates the data
rate. The following are mandatory data rates specified by HR-DSSS:
Data Rate Signal Field Value
1Mbps 00001010
2Mbps 00010100
5.5Mbps 00110111
11Mbps 01101110
• Service field values Bit 2 of the Service field indicates whether the transmitter frequency
and the symbol clocks are derived from the same oscillator. Bit 7 is used in conjunction
with the Length field to eliminate ambiguity when converting the number of
octets to corresponding transmit time for data rates greater than 8Mbps. For example, at
11Mbps, the Length field value equals the number of octets times 8, divided by 11, and
rounded up to the next integer. Bit 7 of the Service field will be 0 if the rounding takes
less than 8/11 and 1 if the rounding is equal to or greater than 8/11.