A look at 802.11a, b, and g throughput with short preambles

Three and a half years ago, I used a simple model to calculate the maximum throughput of 802.11 networks. Recently, a reader wrote to me and asked me how the use of the short preamble would affect the throughput numbers. The answer is that using the short preamble increases throughput by more than 20%.

The re-calculations aren’t particularly complicated. The short preamble is only 96 microseconds instead of 192. The initial slow component of the preamble is shorter, and the second and final component is transmitted at a faster data rate. This affects the model by making any frame transmitted with 802.11b data rates take 96 fewer microseconds in the air.

In the 802.11b model, that savings exists on four frames (the initial data frame with the TCP payload, the 802.11 ACK, the second data frame with the TCP ACK, and the final 802.11 ACK). Therefore, the simple TCP+ACK transaction the model is based on is 384 microseconds shorter.

In the 802.11g with CTS-to-self protection, the short preamble saves on just two frames. In CTS-to-self protection, the frames transmitted at 802.11b data rates are the two CTS frames used to lock up access to the medium. Therefore, the TCP+ACK transaction unit in the model is 192 microseconds shorter.

Finally, in the 802.11g with RTS-CTS protection, there are once again four frames with a preamble saving. Both of the two 802.11 data frames carrying a TCP payload are protected by an RTS and a CTS frame, so the TCP+ACK transaction unit is once again 384 microseconds shorter.

Throw it into a spreadsheet, and you get the following table, where the short preamble rows are new.

Technology Transaction time (μs) Transactions/ second TCP payload throughput, Mbps Improvement over long preamble
802.11b, long preamble 2084 479 5.6 -
802.11b, short preamble 1700 588 6.9 +23%
802.11g, CTS-to-self, long preamble 898 1113 13.0 -
802.11g, CTS-to-self, short preamble 706 1416 16.5 +27%
802.11g, RTS-CTS, long preamble 1285 778 9.1 -
802.11g, RTS-CTS, short preamble 948 1054 12.3 +35%

It’s also important to note that the equipment on the market rarely uses the full RTS-CTS protection. It is an option on most equipment, but it is rarely used.

4 Responses to “A look at 802.11a, b, and g throughput with short preambles”

  1. Chris says:

    So the preamble issue doesn’t apply to Wireless G (“native” mode for lack of a better description) or Wireless A?

  2. matthew says:

    The preamble does not enter into either of those cases. 802.11a doesn’t have backwards compatibilty to worry about, since it was the first 802.11 PHY developed for the 5 GHz band. What you refer to as the “native” mode in 802.11g is called ERP-OFDM by the standard. ERP-OFDM looks almost exactly like 802.11a. (In fact, large parts of the 802.11g specification refer to the clause for 802.11a.) The only time that you need to worry about the long versus short preamble is on 802.11 frames transmitted at 1, 2, 5.5, or 11 Mbps.

    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.

  3. Rishabh says:

    In your previous article, you mention that frames involved in protection operation (self-CTS or RTS/CTS) would compulsorily use the long preamble mode, which also gets reflected in your calculations as 192us is used.
    Article: http://www.oreillynet.com/pub/a/wireless/2003/08/08/wireless_throughput.html?page=2
    Under Protection 1: CTS-to-self, you write
    Protection dramatically reduces the maximum theoretical throughput because the additional CTS transmission is required with its “long” 802.11b headers.

    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 !!

  4. matthew says:

    Rishabh,

    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.

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