The 802.11g standard supported a number of different PPDU (preamble and header) formats. The different PPDU formats could be classified under 3 categories and are provided below
- 11b Long Preamble format and the short Preamble format for 802.11b which was optional in 802.11b
- The Optional DSSS-OFDM at all OFDM rates and optional ERP-PBCC at all PBCC rates
- The ERP-OFDM format
The Long Preamble and the Short Preamble formats are the same as the 802.11b specification. They are shown below
The Short Preamble format supported 1 and 2 Mbps Rates.
The Long Preamble Format supports 1, 2, 5.5 and 11 Mbps data rate.
The Long Preamble/short Preamble format fields are similar to 802.11b Long Preamble/short Preamble format respectively. They are described below
- 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
- for the long preamble, it is a series of scrambled ‘1’s sent to synchronize the receiver followed by SFD
- for the short preamble, it is a series of scrambled ‘0’s sent to synchronize the receiver followed by SFD
- The SFD – (Start of Frame De-limiter) contains information regarding the start of a PPDU frame
- The SFD is 0xF3A0 (1111 0011 1010 0000 in binary) for the long preamble and
- The bit-reversed value of 0xF3A0 is used in the short format – that is 0x05CF (0000 0101 1100 1111 in binary)
- 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 standard committee)
- Bit 7 – bit 7 (Length Extension bit) 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
- The long preamble/short preamble format for 802.11g only varies from the 802.11b long preamble/short preamble format in the following manner
- The use of one bit in the SERVICE field to indicate when the optional ERP-PBCC mode is being used.
- The use of two additional bits in the SERVICE field to resolve the length ambiguity when the optional ERP-PBCC-22 and ERP-PBCC-33 modes are being used.
- Three additional optional rates given by the following SIGNAL field octets where the LSB is transmitted first in time:
- X’DC’ (MSB to LSB) for 22 Mb/s ERP-PBCC
- X’21’ (MSB to LSB) for 33 Mb/s ERP-PBCC
- X’1E’ (MSB to LSB) for all DSSS-OFDM rates
- The Service Field parameter changes are shown pictorially below
