> between 2 fully optimized AC routers with 3 streams (3x3 antenna config) at 975 Mbit/s (40Mhz band) negotiated speed.
Doesn't seem to add up. The max rate for AC 3x3 40 MHz is 600 Mbps PHY rate not 975 Mbps. This is with GI at 400 ns and the maximum signal quality. 975 would only be reachable on AC via 80 MHz channel 3x3 with a GI of 400 ns and a less than optimal signal quality.
> Which was really disappointing, I was thinking AC would be faster than N, which gets around the same real throughput of ~1/4 of the negotiated speed (i.e. 450 Mbps 3x3 = ~120 Mbps real bandwidth).
That's a ~2.16x increase for a single generation jump!
> So, the only real way to get a higher speed is to use more streams (on both ends) or 80 MHz of band width, which is only available on higher end chipsets and is really finnicky in most conditions.
Depending on your environment bumping to an 800 ns guard interval may help, or at least lower the jitter from retries. Also for P2P having a good directional antenna can be more important than having more spatial streams/spectrum. Both of the latter increase maximum bandwidth but decrease signal quality while good RF design like a good antenna and placement will serve to increase signal quality.
Alternatively for P2P there are purpose built solutions that tweak a lot of the standard Wi-Fi assumptions to deliver better performance. A typical go-to is Ubiquiti's airmax line which is on the lower end of the cost spectrum and has options in the 2.4/5/60 bands. On the higher end there is the airfiber line which adds in the 24 GHz band as well as dedicated send/receive options. This makes the connection full duplex rather than half duplex which solves a litany of slowdownsz in one swoop. Unfortunately it also comes at a premium price point. If you don't like Ubiquiti then Cambium also has decent P2P gear, I don't recall if they have any cheap options though.
On airfiber I was able to achieve gigabit speed over a couple miles between buildings. We actually used it as the primary link for the IT office to the rest of the network as it performed overal better than the 300 mbps fiber line.
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To your original question I've gotten real world >800 Mbps goodput on AC 2x2 80 MHz and real world >1 Gbps on AX 2x2 80 MHz. These are half duplex speeds of course. If the P2P use case is long distance I'd stay away from 6 GHz as there are some power limitations there.