FHSS Physical Layer Convergence Procedure

FHSS Physical Layer Convergence Procedure
Figure 5.2 illustrates the format of a FHSS PPDU (also called a PLCP frame). In general, the
preamble enables the receiver to prepare clocking functions and antenna diversity before the
actual content of the frame arrives. The header field provides information about the frame, and
the whitened PSDU (PLCP service data unit) is the MPDU the station is sending.  The following describes each of the FHSS PLCP frame fields:
• SYNC This field consists of alternating zeros and ones, 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 The content of this field is always the 0000110010111101 bit
pattern, defining the beginning of a frame.
• PLW (PSDU Length Word) This field specifies the length of the PSDU in octets. The
receiver will use this information to determine the end of the frame.
• PSF (PLCP Signaling) This field identifies the data rate of the whitened PSDU portion
of the frame. The preamble and header of the PPDU are always sent at 1Mbps, but
the remaining portions of the frame can be sent at different data rates as indicated by this
field. The PMD, though, must support the data rate.
The leftmost bit of the PLCP Signaling field, bit 0, is always 0. The following table identifies
the data rate based on the value of bits 1, 2, and 3:
Bits 1–3 Data Rate
000 1.0Mbps
001 1.5Mbps
010 2.0Mbps
011 2.5Mbps
100 3.0Mbps
101 3.5Mbps
110 4.0Mbps
111 4.5Mbps
• Header Error Check 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 Physical layer does not determine whether errors are present
within the PSDU. The MAC layer will check for errors based on the Frame Check
Sequence (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.
• Whitened PSDU The PSDU can range from 0–4095 octets in length. Before transmission,
the Physical layer whitens the PSDU by stuffing special symbols every four octets
to minimize DC bias of the data signal. The PSDU whitening process involves the use of
a length-127 frame-synchronous scrambler and a 32/33 bias-suppression encoding
algorithm to randomize the data. Figure 5.3 illustrates the process of whitening the
PSDU.