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21 Wireless Local Area Networks
FHSS PHY
In FHSS PHY, data transmission over media is
controlled by the FHSS PMD
sublayer as directed by the FHSS PLCP sublayer. The FHSS PMD takes the binary
information bits from the whitened PSDU and converts them into RF signals by
using carrier modulation and FHSS techniques (see Figure 21.10).
The format of the PPDU is shown in Figure 21.11. It consists of the PLCP
preamble,
PLCP header, and PLCP service data unit (PSDU). The PLCP preamble
is used to acquire the incoming signal and synchronize the receiver’s demodulator.
The PLCP header contains information about PSDU from the sending physical
layer. The PLCP preamble and header are transmitted at 1 Mbps.
The
sync fi eld
contains a string of alternating 0s and 1s
pattern and is used by
the receiver to synchronize the receiver’s packet timing and correct for frequency
offsets.
The
SFD fi eld
contains information marking the start of a PSDU frame.
FHSS radios use a 0 0 0 0 1 1 0 0 1 0 1 1 1 1 0 1 bit pattern. The leftmost bit is
transmitted fi rst.
The
PLCP length word
(
PLW
)
fi eld
specifi es the length of the PSDU in octets
and is used by the MAC layer to detect the end of a PPDU frame.
The
PLCP signaling fi eld
(
PSF
) identifi es the data rate of the whitened PSDU
ranging from 1 to 4.5 Mbps in increments of 0.5 Mbps.
The
header error check fi eld
contains the results
of a calculated frame check
sequence from the sending station. The ITU CRC-16 error detection algorithm is
used to protect the PSF and PLW fi elds.
Data whitening
is used for the PSDU before
transmission to minimize DC
bias on the data if long strings of 1s or 0s are contained in the PSDU. The PHY
stuffs a special symbol every 4 octets of the PDSU in a PPDU frame. A 127-bit
PSDU
Data Clock
Data
Whitener
Data Dewhitener
Symbol
Mapping
Gaussian
Shaping
Filter
GFSK
Modulator
GFSK
Demodulator
Hop
Timing
Recovery
PSDU
