Hi,

Yes. There's nothing really stopping you from doing this. You can just
associate in both directions and then only send data out one link.

Now, having the nic be _transmit only_ and the other nic be _receive
only_ is a little tricky, as there's a bunch of 802.11 negotiation
stuff that's going on that really assumes it's happening on a single
VAP. The net80211 code could be hacked to transmit things out a
different device - ie, one VAP has a transmit device and a receive
device. You'd have to turn off things like hardware ACK, hardware
RTS/CTS, etc. But it's doable.




-a


On 21 May 2014 11:39, Richard Yao <r...@gentoo.org> wrote:
> Dear Everyone,
>
> I recently read about Ubiquiti’s AirFiber hardware and noticed that its link 
> efficiency is remarkable. Air Fiber’s link speed is about 770Mbps up and 
> 770Mbps down (a 1:1 split). People are reporting benchmarks that show 700Mbps 
> throughput over miles. The link efficiency is therefore in the range of wired 
> ethernet, which typically obtains iperf results in the range of 85% to 95%. 
> So far, all benchmarks of WiFi that I have seen never touch 1/e or roughly 
> 37% efficiency.  A few have come rather close to 1/e though.
>
> The 1/e number is significant because I am told that it is the theoretical 
> limit on the efficiency of wired ethernet when a there is a shared collision 
> domain on a coaxial cable. After reading about how the Air Fiber hardware 
> works, I hav suspicion that its link efficiency can be replicated between two 
> computers with off the shelf Wi-Fi hardware by abusing the radios via the 
> kernel driver. In specific, you would have two systems, each with two radios 
> on different frequencies. I will call each system A and B and refer to their 
> radios as indices into an array. e.g. A[0] and B[1].
>
> Much like the AirFiber, I envision node A as having A[0] be transmit-only on 
> the frequency that B[0] uses (frequency 0) with B[0] being receive only. 
> Similarly, I envision node B as having B[1] be transmit-only on the frequency 
> that A[1] uses (frequency 1) with A[1] be receive only. The kernel driver is 
> to instruct the WiFi hardware to ignore everything about the 802.11 protocol 
> possible (e.g. RTS is to be ignored), send frames when given a packet (in 
> send mode) and receive forward frames when hearing a packet (in receive 
> mode). No radio in send mode is to listen to packets and no radio in listen 
> mode is to send packets. The radios would be attached to directional antennas 
> and frequency 0 != frequency 1.
>
> I asked Adrian Chadd about this in IRC. He replied that it is possible to 
> hack the driver to obtain tight control over when 802.11 frames are 
> received/sent, but doing something like this would require oscillator 
> isolation and baseband RF isolation. He also asked that I send my question to 
> the list, so here it is.
>
> How doable is this with off the shelf hardware? Could simultaneous dual-band 
> equipment be abused to obtain the proper isolation (where 2.4GHz is 1 
> direction and 5GHz is another)? Would it be reasonable to expect wireless 
> throughput to achieve 90% of the link speed in this configuration?
>
> Yours truly,
> Richard Yao
>
> P.S. I am not on the mailing list, so please include me on CC.
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