802.11g doesn’t tell you which preamble to use.

The most relevant piece of the standard (802.11-2007 clause 9.13; it was copied unchanged from clause 9.10 in 802.11g) says: “Protection mechanisms frames shall be sent using one of the mandatory Clause 15 or Clause 18 rates and using one of the mandatory Clause 15 or Clause 18 waveforms, so all STAs in the BSA will know the duration of the exchange even if they cannot detect the ERP-OFDM signals using their CCA function.”

In English, what that means is that you need to use either the original 1 Mbps and 2 Mbps data rates with direct sequence modulations (clause 15), or the 5.5 Mbps and 11 Mbps 802.11b higher rate direct sequence. Either is acceptable, according to the standard.

When I wrote the original article in 2003, it was somewhat common to find pre-11b gear that would only understand long preamble. If you wanted protection to work, you needed to use long preamble. In the past four years, long preamble-only equipment has essentially become extinct, so it is OK in practice to use the short preamble. You may, very occasionally, run into an ancient pre-11b piece of equipment that needs long preamble, but it’s going to be a vanishingly small number of cases.

In 2003, the company I work for shipped APs with the default as long preamble. We made the decision for compatibility reasons, since there was a fair amount of pre-11b equipment available. As it became less common, we changed the default to short preamble, in large part for the performance reasons discussed in this post.

However, in the above blog, you include calculations with short preamble also. I am confused as to whether we can use both preamble modes of 11b in protection mode of mixed 11b/g operation ? Please guide in this regard, also kindly tell what the 802.11g Std. says .

Thanks in advance !!

This may be a bit repetitive to point out, but every 802.11g network that I have come across in the real world has been using protection. There are still a lot of 802.11b-only devices out there.