From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-x231.google.com (mail-oi0-x231.google.com [IPv6:2607:f8b0:4003:c06::231]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.bufferbloat.net (Postfix) with ESMTPS id B9A3A3B45C for ; Thu, 21 Apr 2016 14:02:33 -0400 (EDT) Received: by mail-oi0-x231.google.com with SMTP id p188so107756651oih.2 for ; Thu, 21 Apr 2016 11:02:33 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-transfer-encoding; bh=BDec2+TkJIK0LKuSNzbN6a7aTkvNAtzsZ5xJHeopsec=; b=atyeR8K3dkw9TkIVMBH/msZO/K5jMs3SRmQyp8pDtoUC0LVdf27+7ZOgFS9UvXy/Uc Wdn8+BV2sCnGedHYvODcbflC0T2wWUJxS8wB6/MbOBWO0TRcRnelIsynx8uXLvWnk/Ui bO4jTlrMmb637dsYDtc08kcISW1nkwE347DtN25rA98b4MaGrxpNUgi92IJW6IoswEKT 4xk8CnkC5g0nKgm5w8JIGRXtLgJtyTjI42HkUx64DNm1J9YkzDgliT4ejl9fD4ID4wxn NXRxugYqxTOGhjvTPqYUk6kaZjXE0mDOeR+TK49ao6qU9y1hEq6THTypfmF7507mAGp5 MLUQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:date :message-id:subject:from:to:cc:content-transfer-encoding; bh=BDec2+TkJIK0LKuSNzbN6a7aTkvNAtzsZ5xJHeopsec=; b=Z3z1vRVMY6It4eANtXTYxvtx3W0bpY9+5rr0flUJdFNOfKGQu2VUf/kaRubgTKiHie FRGXSiYh1u7Hy03NsDt3m+4TgVkPOnnjr0Po0KF1L4gXO10B+R6usQfBzuFtRUqjUGLE pvg9pNV950A9xeCVcT3r84QHE1bzFJlfMYE5znDYdzwaBDtD3IKTrSJWNhIUvzJ580aL 0F35QPLUAK5QbCDYWh3wXrzBpSVUS6EIUm9zyOkj5/nEXyRQ0OvUGMTJ4aImK6qMHDWq f3qWYkrBHJAWyHUYYeMo2JaghkS/WaoYL0M469+4CXnlpRxurWI7J/xVUod9LMe8KiVU eJXw== X-Gm-Message-State: AOPr4FVw6zPN/JDDujPfMGOQS+4t7W7sSHagyjsNy/Z5wHmhl8vQa+vBQGOuETD4GE3oWdZsJ+3oL5IOaLnn9Q== MIME-Version: 1.0 X-Received: by 10.157.57.132 with SMTP id y4mr6063554otb.169.1461261753151; Thu, 21 Apr 2016 11:02:33 -0700 (PDT) Received: by 10.202.79.194 with HTTP; Thu, 21 Apr 2016 11:02:33 -0700 (PDT) In-Reply-To: References: Date: Thu, 21 Apr 2016 11:02:33 -0700 Message-ID: From: Dave Taht To: David Lang Cc: make-wifi-fast@lists.bufferbloat.net, Henning Rogge Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Subject: Re: [Make-wifi-fast] the hidden station problem X-BeenThere: make-wifi-fast@lists.bufferbloat.net X-Mailman-Version: 2.1.20 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 21 Apr 2016 18:02:33 -0000 the original minstrel paper is well worth reading. http://blog.cerowrt.org/post/minstrel/ On Thu, Apr 21, 2016 at 10:57 AM, David Lang wrote: > On Thu, 21 Apr 2016, Dave Taht wrote: > >> I was watching myself do then make-wifi-fast Q&A and henning mentioned >> the hidden station problem and it's interaction with minstrel... >> >> https://www.youtube.com/watch?v=3DRb-UnHDw02o >> >> Since we are doing up some better testbeds, I am curious as to what >> might be a good (simplified) setup (bench or air) for it, and/or if >> there has been a paper that shows the interaction problems with >> minstrel in particular. > > > the basic way to see this is to take two stations and move them far enoug= h > apart, or put shielding between them so that they cannot talk to each oth= er. > > Then position a third station so that it can see both of the first two. > > If you really turn the power down, you may be able to get away with them > fairly near each other with a metal sheet next to one of them. > > You will see that you can talk to either of them quite nicely if the othe= r > is pretty idle, but if you have them both sending a lot of data at the sa= me > time, disaster strikes. > > > > If you are writing a simulator, add a probability that a packet transmitt= ed > from an edge station to the central station doesn't get through. Ramp up > this probability and watch what happens. A better simulator would scale t= he > probability up based on the amount of airtime needed, so that as the send= er > slowes down, the probability goes up. > > > > > This is one of the hardest problems for wifi to deal with. It manifests a= s > massive amounts of lost packets when the first two are sending to the thi= rd > one, and no amount of backoff helps. Slowing down the transmit rate just > makes things worse as it takes longer to transmit each bundle and so it's > more likely to be stepped on. > > > > Reading up on Minstrel at > https://wireless.wiki.kernel.org/en/developers/documentation/mac80211/rat= econtrol/minstrel > there is a comment > >> Inspection of the code in different rate algorithms left us bewildered. >> Why did all the code bases we looked at contain the assumption that pack= ets >> sent at slow data rates are more likely to succeed than packets sent at >> higher datarates? The physics behind this assumption baffled us. A slow = data >> rate packet has the highest possibility of being =E2=80=9Cshot down=E2= =80=9D by some other >> node sending a packet. > > > the answer to this is that the higher data rates require a better signal = to > noise ratio, and so if the problem is that the stations are too far apart= , > or there is a wall between them that makes the signal weaker, or that the= re > is just a lot of low-volume noise in the area, the slower data rates are = far > more likely to be understandable than the faster data rates. Since Wifi w= as > designed long before anyone imagined how common it would become (I rememb= er > when the pcmcia cards were >$1000 each rather than the current <$10 for a > much faster USB adapter), they designed the protocol to fall back to lowe= r > rates if the packets don't get through. > > This works well if you are out in the boonies and trying for range. It fa= ils > horribly in very high density environments (this is why most conference w= ifi > is worthless for example) > > > This is why it's a good idea to disable the lowest data rates if you know > that you don't need them. > > > > reading the minsrel page, it seems intuitively obvious to me that this > random packet drop would really mess with their moving average and thus t= he > decisions they end up making. > > David Lang --=20 Dave T=C3=A4ht Let's go make home routers and wifi faster! With better software! http://blog.cerowrt.org