802.11b specified two PLCP formats – long and short PLCP. The PLCP formats are provided below
Fig Courtesy – 802.11 standards
As can be seen from the pictorial representation for long and short 802.11b plcp – the format is different in the number of sync preamble bits that are sent out.
The SFD/SIGNAL/SERVICE/LENGTH and CRC fields are the same in the two formats.
- SYNC – The SYNC field is used by the receiver to acquire the incoming signals and to synchronize the receiver’s carrier tracking and timing prior to receiving SFD
- The SFD – (Start of Frame De-limiter) contains information regarding the start of a PPDU frame
- The SFD is F3A0hex for the long preamble and the bit reversed value 0x05CF hex for the Short Preamble
- The signal field defines what type of modulation must be used to receive the incoming PSDU. The binary value determines the data rate multiplied with 100 kbit/s. The signal field is the below for the different rates
- 00001010 – 1Mbit/s
- 00010100 – 2 Mbit/s
- 00111110 – 5.5 Mbit/s
- 01101110 – 11 Mbit/s
- Three bits of the service field are used by 802.11b . The rest of the service field bits are zero
- Bit 2 – determines whether the transmit frequency and symbol clocks use the same oscillator
- Bit 3 – indicates whether CCK or PBCC is used (PBCC was a competing technology by TI to CCK – however it was rejected by the 802.11 standards committee)
- Bit 7 – bit 7 of the service field is used with the Length field to determine the time in microseconds
- Length Field – is an unsigned 16- bit integer that indicates the number of microseconds necessary to transmit the PSDU