From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Stafford Subject: Re: MCS field: RFA Date: Tue, 2 Feb 2010 17:36:17 +0000 (UTC) Message-ID: References: <40101cc31001260626g4a47b7c6gde6f99e477e69ac9@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Return-path: Sender: radiotap-owner-sUITvd46vNxg9hUCZPvPmw@public.gmane.org To: radiotap-sUITvd46vNxg9hUCZPvPmw@public.gmane.org List-Id: radiotap@radiotap.org Matteo Croce writes: > Before it's approved I would ask the crew what is the best > thing to do with the old rate field when the MCS one is set. > We can leave it set with a value of 0 or just omitting it when > the MCS is set. The latter makes more sense to me. Omitting the Rate field seems like the most logical, but does not help out legacy parsers. The Rate field can represent the 20 MHz HT rates up to MCS 14 (13 ShortGI), and some of the higher indexes. This means a legacy parser could show accurate rate for all but the highest phy rates if the Rate field was included. You mention the possibility of leaving the Rate field in with a value of 0. That would be a good obvious key for someone looking over a capture to know that the rate could not be represented. Another option would be to peg the rate value at the max (an odd looking 127.5 Mbps). On the downside, this might be confusing if someone looking over a capture did not know this was an invalid 11n data rate. But on the upside, it is helpful for wireshark filters like "radiotap.datarate < 48" when you are trying to see any traffic that falls below 24 Mbps for example. Even making the filter "radiotap.datarate < 48 && radiotap.datarate != 0" would not let you test for < 24 Mbps since some of the HT rates are less than 24 Mbps. Leaving the Rate field in place and setting to the max when needed seems like it would be more helpful to legacy parsers. New parsers can just ignore the Rate field if MCS is present and calculate the rate from the MCS and other transmit factors (GI, Bandwidth). Hope this is helpful in discussion. -Bill