802.11b PHY operation

802.11b PHY operation
As we discussed earlier, the 802.11b specification prescribes the high-rate extension
for the DSSS method (HR/DSSS) with an 8-chip CCK, which affords the technology
a maximum data rate of 11Mbit/s that degrades gracefully through 5.5Mbit/s,
2Mbit/s and 1Mbit/s, depending upon environmental and other factors.
During a transmit operation a PSDU is converted to PHY Protocol Data Unit
(PPDU) by appending the PLCP preamble and header information. The format of a
PLCP PPDU payload may differ in size, which is primarily due to the data rate being
supported. The default payload comprises a long preamble and header, as shown in
Figure 13.22; this would preclude the high-rate extensions (5.5Mbit/s and 11Mbit/s)
where an optional short preamble and header are used, as shown in Figure 13.23.
The PLCP preamble contains the synchronization (Sync) and start frame delimiter
(SFD) fields. The receiver uses the sync field as an early warning mechanism which
notifies it of a new payload (using a series of ones and zeros) and the receiver should synchronize prior to receiving the SFD. The SFD is used to indicate that the start of
a frame and the PHY parameters will follow. In the PLCP header an 8-bit Signal field
denotes the data rate that will be used for the transmission and reception of the
PPDU, as shown in Table 13.3. The values shown need to be multiplied by 100Kbit/s
to reveal the true data rate value.
The Service field is an 8-bit value where only bits 2, 3 and 7 are used; bit 7 is
used to supplement the Length field. Bit 3 is used to denote which modulation scheme
is supported, namely CCK or Packet Binary Convolutional Code (PBCC); whereas, bit
2 is used to denote the transmit frequency and the symbol clocks are derived from the
same oscillator and, as such, the PHY layer will set bit 2 to 1 (one) to indicate locked
clocks. The Length field is an unsigned 16-bit integer that contains the number of
microseconds that are required to transmit the PSDU. And finally, the CRC is used
to protect the integrity of the signal Service and Length fields.