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I would like to learn more about how clients and APs shift to various MCS values as the client moves around. Is there a utility (preferably Windows based) that shows which modulation and coding scheme is in use on a client?

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Rate adaptation, whether 802.11n or a/b/g, is ususally one of those "secret sauces", including roaming algorithms, for which the exact particulars might be hard to obtain.

A sniffer ,such as Omni-Peek, or Wireshark, etc. can read the beacons or packets and tell you exactly what is going on.  As Tom says, "Packets Don't Lie".

Short of that, the program InSSIDer will display what the AP is "advertizing" for its maximum rate.   From that, and an MCS Index table you can tell what it's top MCS Index is.

Airmagnet probably still has their 802.11n Fundamentals poster which has a very good chart of all the rates you are likely to see - all (31 rates) x (20 or 40 MHz) x (long or short guard interval) i.e. all 128 of them.. 

And by the way, you can't always trust what your client adapter says the connection speed is.   They often only show you the maximum speed possible - it looks more impressive.

Thanks for the reply! I have used OmniPeek to observe the MCS of other devices. I'm hoping to find some way to display the MCS that my PC is currently using. It would be great to watch that and see how it shifts as the PC moves around in relation to the AP. I have used the Xirrus Windows gadget with some success -- it displays the current data rate in use, which shifts in somewhat real-time. I guess I could work backwards and correlate the data rate to the MCS.

Howard, Dave,

I have that outstanding AirMagnet poster (now Fluke) on the wall in my office, but could not find it on the web exactly.

I did find, if you sign up for a free course on 802.11n, the poster comes free:  http://www.terra-wave.com/images/100808TesscoAM/100808_Tessco_Airmagnet.html

Also, here is the link for the AirMagnet pdf file which explains the poster in detail.  Very good white paper, and it helped me pass CWAP!  http://airmagnet.flukenetworks.com/assets/whitepaper/WP-802.11nPrimer.pdf