IR Physical Layer Convergence Procedure (PLCP) Sublayer

IR Physical Layer Convergence Procedure (PLCP) Sublayer
Figure 5.20 illustrates the infrared PLCP frame format that the 802.11 specification refers to as
a PLCP protocol data unit (PPDU). The preamble enables the receiver to synchronize properly
to the incoming signal 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 presence of a pulse in consecutive time slots.
The 802.11 standard specifies that the Sync field must have a minimum length of 57
time slots and a maximum length of 73 time slots. A receiver will begin to synchronize
with the incoming signal after first detecting the Sync.
• Start Frame Delimiter The content of this field defines the beginning of a frame. The
bit pattern for this field is always the 1001bit pattern, which is unique for infrared
PLCPs. One (1) represents the presence of a pulse, whereas zero (0) represents no pulse
for information conveyed in infrared PPDUs.
• Data Rate This field identifies the data rate at which the PMD will transmit the frame.
The only two possible values, based on the June 1997 version of 802.11, are 000 for
1Mbps and 001 for 2Mbps. The PLCP preamble and header are both always sent at
1Mbps.
• DC Level Adjustment This field consists of a bit pattern that enables the receiving
station to stabilize the DC level of the signal. The bit patterns for the two supported rates
are as follows:
1Mbps 00000000100000000000000010000000
2Mbps 00100010001000100010001000100010
• 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 performs the operation on the Length
field before transmitting the frame.
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 This is actually the MPDU being sent by the MAC layer, which can range from
zero to a maximum size of 2500 octets.