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I feel comfortable with the "general" knowledge about the rates of frames for preambles (long/short) and the headers, etc.

What is getting me is specifics like the frame rate for probe requests, probe responses. In reading the study guide, I just want to presume the answer is that they (probe requests/responses) are always sent at the same speed. In looking at the diagrams in the book, it seems like that is the correct answer...but.....

I guess my question is:

when are frames transmitted at the lowest supported rate versus the lowest basic rate, and is it based on the AP or the client.

I was looking at a 802.11B sniffer trace and saw that probe requests were sent at 1Mb and probe responses sent at 2Mbs... so I thought that was the answer..... BUT...then looking at another 802.11G, I saw the probe requests at 2Mb and responses at 2Mb, so I am puzzled as to if they are supposed to be at certain rate, or is the rates determined by the lowest supported rate, based on the AP and/or the client...

Does any of this make sense?

This will depend on the TYPE of frames you're talking about and what standards the equipment adheres to on each end of the wireless link. You can download the 802.11, 802.11b, and 802.11a standards for free at: http://standards.ieee.org/wireless/

802.11b and 802.11g, section 9.6 "Multi-rate Support" defines what you're looking for.

In the scenario you described, the probe requests were being sent at the lowest supported rate (1 Mbps), and in those probe requests, the supported rates of the client are listed. 802.11b and 802.11g both say that the station must transmit management frames to a broadcast address at one of the rates listed in the Basic rate set (basic rates list). Apparently, your card chose the lowest supported rate, which is a smart thing to do to get EVERY AP in the area to respond.

Since probe response frames are unicast frames, they can be sent at any rate supported by the receiving station (which the AP would know since the Probe Request frame from the station had that information in it). When you see the probe requests go out at 2 Mbps, it means that the station chose that data rate out of the options it had among its supported rate set. This could've been 2, 5.5, and 11, or even could've been 1, and 2. There's no rule in the standard saying that Mgmt frames, such as probe requests, have to be sent at the MINIMUM supported or MINIMUM basic rate, though probe requests in particular are usually sent at the minimum supported rate.