Figure 5.7 illustrates the DSSS PLCP frame format that the 802.11 specification refers to as a
PLCP protocol data unit (PPDU). The preamble enables the receiver to synchronize to the
incoming signal properly before the actual content of the frame arrives. The header field provides
information about the frame, and the PSDU (PLCP service data unit) is the MPDU the
station is sending.
• Sync This field consists of alternating 0s and 1s, alerting the receiver that a potentially
receivable signal is present. A receiver will begin to synchronize with the incoming signal
after detecting the Sync.
• Start Frame Delimiter This field defines the beginning of a frame. The bit pattern for
this field is always 1111001110100000, which is unique for DSSS PLCPs.
• Signal This field identifies the type of modulation that the receiver must use to demodulate
the signal. The value of this field is equal to the data rate divided by 100Kbps. The
only two possible values for the June 1997 version of 802.11 are 00001010 for 1Mbps
DSSS and 00010100 for 2Mbps DSSS. The PLCP preamble and header are both always
sent at 1Mbps.
• Service The 802.11 specification reserves this field for future use; however, a value of
00000000 means 802.11 device compliance.
• Length The value of this field is an unsigned 16-bit integer indicating the number of
microseconds to transmit the MPDU. The receiver will use this information to determine
the end of the frame.
• Frame Check Sequence Similar to the FHSS Physical layer, this field contains a 16-
bit CRC result based on CCITT’s CRC-16 error detection algorithm. The generator polynomial
for CRC-16 is G(x)=x16+x12+x5+1. The CRC operation is done at the transmitting
station before scrambling.
The Physical layer does not determine whether errors are present within the PSDU. The
MAC layer will check for errors based on the FCS. CRC-16 detects all single and double-
bit errors and ensures detection of 99.998% of all possible errors. Most experts feel
CRC-16 is sufficient for data transmission blocks of 4 kilobytes or less. • PSDU The PSDU, which is actually the MPDU being sent by the MAC layer, can
range from zero bits to a maximum size that can be set by the aMPDUMaxLength parameter
in the MIB.