Date: Wed, 21 May 2014 14:39:26 -0400 From: Richard Yao <ryao@gentoo.org> To: freebsd-wireless@freebsd.org Subject: Can we obtain higher wireless link throughput by abusing 802.11 radios to form unidirectional simplex pipes? Message-ID: <F205F041-2D59-4E9D-830E-09AD46E0F09E@gentoo.org>
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Dear Everyone, I recently read about Ubiquiti=92s AirFiber hardware and noticed that = its link efficiency is remarkable. Air Fiber=92s 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 !=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 = 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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