Saw your post ...
CPU speed is a big problem if you want 40Mbps throughput. As well, you need high signal levels, and low interference on channel-bonded "turbo" mode. Choose good antennas, the 23dBi's on MT get good write-ups.
RB-type boards with 266MHz CPUs (there are several about) offer about 25Mbps throughput flat out - 100% CPU utilisation.
We cracked it some weeks back using 1GHz CPUs, 5.3 or 5.8GHz using Turbo and Nstreme, and you can get 74Mbps TCP and 83Mbps UDP using BW tester in the lab - just a bit slower at 3km I think - I would love to hear what other users have achieved.
Another user was "a bit annoyed" to my last post helping others with exactly the same problem - we've developed a complete product that does all this, but despite lots of really interested people who mailed me, it's of course not ettiquette to advertise on another's forum ... so I won't repeat the contents - have search on this forum-
BTW, we make products, we're not an operator/provider, so that's where we fit in.
I personally agree with the support comment, some "out of the box" guidelines are needed for the most common configs. We hope to get these together shortly to support our products.
I'm just back from holiday, so need to catch up ...
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