User`s guide

Aironet Wireless Communications, Inc. 3-3 Confidential and Proprietary
802.11 Direct Sequence Frame Format
Figure 3.2 depicts the frame format used for each packet transmission.
PLCP Preamble
MPDU
144 bits
SYNC
SFD
16 bits
SIGNAL
8 bits
8 bits
16 bits
LENGTH
CRC
128bits
SERVICE
PPDU
PLCP Header
48 bits
16 bits
Figure 3.2 - PHY Header Description
Each frame consists of a PHY header which is always transmitted at the 1 Mbit/s data rate. The PHY
header (which is protected by a CRC-16) indicates the data rate and length of the data portion of the
payload. The PHY header is made up of the following components:
• A PLCP synchronization field which consists of 128 bits of scrambled ones. This portion of the
signal is used to detect the presence of a signal, provide a waveform on which to perform antenna
diversity as well as provide a constant pattern useful for determining accurate symbol timing.
• A Start Frame Delimiter (SFD) is a 16 bit pattern used to provide symbol level synchronization.
• An 8 bit 802.11 Signal Field indicates to the PHY the modulation which shall be used for
transmission (and reception) of the MPDU. The data rate shall be equal to the Signal Field value
multiplied by 100kbit/s. The DSSS PHY currently supports two mandatory modulation services
given by the following 8 bit words, where the LSB shall be transmitted first in time:
a) 0Ah for 1 Mbit/s DBPSK
b) 14h for 2 Mbit/s DQPSK
c) 37h for 5.5 Mbits/s CCK (PC4800 only)
d) 6Eh for 11 Mbits/s CCK (PC4800 only)
• The 8 bit 802.11 service field shall be reserved for future use. The value of 00h signifies 802.11
device compliance.
• An unsigned 16 bit integer Length field used to indicate the number of microseconds (16 to 2
16
-1)
required to transmit the MPDU octets contained in the data portion of the frame.
• A Header Error Check field which is a 16 bit CRC applied over the length and signaling fields to
ensure data integrity.
The PSDU portion of the frame contains the MAC data and control information. Each PSDU frame
consists of the following basic components:
a) A MAC Header, which comprises frame control, duration, address and sequence control
information.
b) A variable length Frame Body, which contains information specific to the frame type.
c) A Frame Check Sequence (FCS) which contains an IEEE 32-bit CRC.
The MAC Frame formats differ depending on the frame type (control, management or data). The basic
format is shown in Figure 3.3. The fields Address 2, Address 3, Sequence Control, Address 4 and Frame
Body are only present in certain frame types. Each field is defined in section 7.1.3 of the IEEE 802.11