Is IEEE802.11 preamble a null frame?

Question

I'd like to know if preamble is a null frame. One person has quoted such in this forum. But, in many other websites, it is given that it is some part of information needed to receive and transfer data. Could you kindly clarify me?

Answer 1

I've had a quick glance at the 802.11-2012 specification from the IEEE..

There's a few sections in that document referring to generating either a long or short preamble, and in no case is it null data.

Figure 17-1 shows the format for the interoperable (long) PPDU, including the High Rate PLCP preamble, the High Rate PLCP header, and the PSDU. The PLCP preamble contains the following fields: SYNC and SFD.

The PLCP header contains the following fields: signaling (SIGNAL), service (SERVICE), length (LENGTH), and CRC-16. Each of these fields is described in detail in 17.2.3. The format for the PPDU, including the long High Rate PLCP preamble, the long High Rate PLCP header, and the PSDU, does not differ from the format for 1 Mb/s and 2 Mb/s.

The only exceptions are: a) The encoding of the rate in the SIGNAL field; b) The use of a bit in the SERVICE field to resolve an ambiguity in PSDU length in octets, when the length is expressed in whole microseconds; c) The use of a bit in the SERVICE field to indicate if the optional PBCC mode is being used; d) The use of a bit in the SERVICE field to indicate that the transit frequency and bit clocks are locked.

So it looks very much like the preamble contains various bits of data about the PHY properties of the signal.

I doubt it's null, in any case.

The image shows that the preamble is comprised of the Sync (56 0s), And the SFD (Start Frame Delimiter), which is a 16 bit field, comprised of 0000110010111101 for a long preamble, and 1001 for a short preamble.

Have a read of the 802.11 standard, and this: http://www.cwins.wpi.edu/publications/pown/chapter_11.pdf page 453.