Date: Wed, 21 May 2014 13:08:44 -0700 From: Adrian Chadd <adrian@freebsd.org> To: Richard Yao <ryao@gentoo.org> Cc: "freebsd-wireless@freebsd.org" <freebsd-wireless@freebsd.org> Subject: Re: Can we obtain higher wireless link throughput by abusing 802.11 radios to form unidirectional simplex pipes? Message-ID: <CAJ-VmomVZdsQzkXggKEv6u2V=qrRAf1FDH2aUes7Ueiz8k6-mg@mail.gmail.com> In-Reply-To: <F205F041-2D59-4E9D-830E-09AD46E0F09E@gentoo.org> References: <F205F041-2D59-4E9D-830E-09AD46E0F09E@gentoo.org>
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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 <ryao@gentoo.org> wrote: > Dear Everyone, > > I recently read about Ubiquiti=E2=80=99s AirFiber hardware and noticed th= at its link efficiency is remarkable. Air Fiber=E2=80=99s link speed is abo= ut 770Mbps up and 770Mbps down (a 1:1 split). People are reporting benchmar= ks that show 700Mbps throughput over miles. The link efficiency is therefor= e 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 ne= ver 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 theoretica= l limit on the efficiency of wired ethernet when a there is a shared collis= ion domain on a coaxial cable. After reading about how the Air Fiber hardwa= re works, I hav suspicion that its link efficiency can be replicated betwee= n 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 frequ= ency that A[1] uses (frequency 1) with A[1] be receive only. The kernel dri= ver 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 pac= ket (in send mode) and receive forward frames when hearing a packet (in rec= eive mode). No radio in send mode is to listen to packets and no radio in l= isten mode is to send packets. The radios would be attached to directional = antennas and frequency 0 !=3D 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 receiv= ed/sent, but doing something like this would require oscillator isolation a= nd 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-b= and equipment be abused to obtain the proper isolation (where 2.4GHz is 1 d= irection and 5GHz is another)? Would it be reasonable to expect wireless th= roughput 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. > _______________________________________________ > freebsd-wireless@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-wireless > To unsubscribe, send any mail to "freebsd-wireless-unsubscribe@freebsd.or= g"
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