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21 Wireless Local Area Networks
procedure (PLCP) sublayer; (2) use of signal carrier and spread spectrum (SS)
modulation to transmit data frames over the media under the control of the physical
medium dependent (PMD) sublayer; and (3) providing a carrier sense indication
back to the MAC to verify activity on the media (see Figure 21.6).
Each of the physical layers is unique in terms of the modulation type,
designed to coexist with each other and operate with the MAC. The specifi cations
for IEEE 802.11 meet the RF emissions guidelines of FCC, ETSI, and the Ministry
of Telecommunications.
DSSS PHY
In the DSSS PHY, data transmission over the media is controlled by the PMD sub-
layer as directed by the PLCP sublayer. The PMD sublayer takes the binary infor-
mation bits from the PLCP protocol data unit (PPDU) and converts them into RF
signals by using modulation and DSSS techniques (see Figure 21.7). Figure 21.8
shows the PPDU frame, which consists of a PLCP preamble, PLCP header, and
MAC protocol data unit (MPDU). The PLCP preamble and PLCP header are
always transmitted at 1 Mbps, and the MPDU can be sent at 1 or 2 Mbps.
The start of frame delimiter (SFD) contains information that marks the start of
the PPDU frame. The SFD specifi ed is common for all IEEE 802.11 DSSS radios.
The
signal fi eld
indicates which modulation scheme should be used to receive
the incoming MPDU. The binary value in this fi eld is equal to the data rate mul-
tiplied by 100 kbps. Two modulation schemes, differential binary phase shift
keying (DBPSK) — for 1 Mbps — and differential quadrature phase shift keying
(DQPSK) — for 2 Mbps—are available.
The
service fi eld
is reserved for future use. The
length fi eld
indicates the
number of microseconds necessary to transmit the MPDU. The MAC layer uses
this fi eld to determine the end of a PPDU frame.
The
CRC 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 signal, service, and length fi eld.
Management
Physical Layer
PHY Convergence
Procedure (PCLP)
MAC Layer
PHY Medium
Dependent (PMD)
