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 (Î¼s)||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.