Bug 853

WifiRemoteStationManager::GetControlAnswerMode() contains the code responsible for selecting the PHY rate at which an automatic control response will be transmitted in response to receipt of a frame at a given rate.

Such selection should be performed in accordance with Section 9.6 of IEEE 802.11-2007 (or an earlier version - this text hasn't changed much).

This bit of the standard basically says (not quite verbatim) that the control response should be transmitted at the highest rate in the basic rate set that is less than or equal to the rate of the received frame and that is of the same modulation class and, if no suitable basic rate is found, that the response should be transmitted at the highest mandatory rate of the PHY that is less than or equal to the rate of the received frame and that is of the same modulation class.

There are two small (small enough that I thought a single bug was sufficient) problems with the current code in WifiRemoteStationManager::GetControlAnswerMode()...

Firstly, the current attempt to select a rate "of the same modulation class" is in fact testing the rate ModulationType. This is not correct - the modulation classes are defined in Section 9.6.1 and Table 9.2 of IEEE 802.11-2007, and (for example in contrast with the current approach) lump Clause 15 and Clause 18 rates together into one class, and Clause 17 rates together in another. The fix for this is probably to compare the WifiPhyStandard instead.

The second shortcoming is that the code deals with the search within the basic rate set, but doesn't fall back to consideration of mandatory rates. There is a comment there that says such a search is unnecessary because the basic rate set includes the mandatory rates, but this is incorrect.

A prime counter-example is the case of many "802.11g" APs currently available in their out-of-the-box configuration. These APs obviously support the PHYs described in IEEE 802.11 Clauses 15, 18, and 19, and thus the mandatory rates of 1, 2, 5.5, 6, 11, 12, and 24 Mbps, however their basic rate sets typically will comprise only the "802.11b" rates of 1, 2, 5.5, and 11 Mbps so as not to preclude an 802.11b-only STA from joining the BSS.