From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ob0-x229.google.com (mail-ob0-x229.google.com [IPv6:2607:f8b0:4003:c01::229]) (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 9A9723B4BA for ; Fri, 22 Apr 2016 12:44:38 -0400 (EDT) Received: by mail-ob0-x229.google.com with SMTP id tz8so51724114obc.0 for ; Fri, 22 Apr 2016 09:44:38 -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=0b9KppkXUCwjJV+hXy/5EHkTMBr9lywRjC9168JbKH8=; b=auPEcWIaymUL5HDVxhHC7E/XZOg96NXOOmHjkLWjcBeVNYYBknEB47Hr3Yv5rtzXU8 U654P9Ese/YA0LlNUxumT4lEGgeIW7n4SOUsCQ7rzqwqMeT+z0l3IifOBfDL873sVHWj chvSdTohls1QHtrpGi3K/WzP1yi4rQ8yPyh92MpQN8gcQs59gIQ/AsRo/juAkNaEgHGV umILlsAFm23OUj/J+/OYdGuGVFnG77LMPx9Gn5I4n001qGDG+aZ5NLO8CAUNYb9cu/uz 7J7HVilXkJUhGfl/7yOyLmG6FC1bacIbT26O4MVphr4QWXM5rwvL2WlpK+mqOUn5Agc3 FaNQ== 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=0b9KppkXUCwjJV+hXy/5EHkTMBr9lywRjC9168JbKH8=; b=fHxgaztnrbPOJs8SUlm+lNgUBDSzUTP8opqK9byKvbAOAsuw3wJNOOY7khBEISVeTS TdKdm4KyJwPrD3BHFK6N8KeLovWnR0tUGr95x4QooC2VETRFpMUzyg8GxCold0N+FEVt xdLy39jEw7poyi8+Xt3E1NExn3BcwPG6I9ziXhxSPsnZb3qEGonJsA10M8qVcySvjLx3 RTSAW47WLzlsT8UcU4FpYgZrayJ1YmhmVh/x5Xoq8JO9wqtWss9FExsllt6xXzSZ44ws RyObqd9s4aQcU2TSlhyyxszS3T61K2uz5iTPdyDUA37FDTlDW9VYUA2qJdbTNGNzMbG+ sLQw== X-Gm-Message-State: AOPr4FWn8QYbVizy0VGXbph7SGf0Edkr5HqferP9Hho7Jo0WhRbt7Dlbdq7eZKjykCCnErGHI9Zg72K6KoZbGA== MIME-Version: 1.0 X-Received: by 10.60.52.177 with SMTP id u17mr1963443oeo.61.1461343478009; Fri, 22 Apr 2016 09:44:38 -0700 (PDT) Received: by 10.202.79.194 with HTTP; Fri, 22 Apr 2016 09:44:37 -0700 (PDT) In-Reply-To: References: Date: Fri, 22 Apr 2016 09:44:37 -0700 Message-ID: From: Dave Taht To: Henning Rogge Cc: David Lang , make-wifi-fast@lists.bufferbloat.net 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: Fri, 22 Apr 2016 16:44:38 -0000 I have just finished re-consuming the entire (2600+ pages) 802.11-2012 specification. For those of you into that level of personal abuse, or have insomnia, or want to make a dent in wifi's problems, I do recommend a close reading. As specs go, it is pretty darn readable. I made extensive notes on it, which I will publish at some point. Probably of most interest to debloaters was section 9 on ampdu frame formats (which sort of requires reading sec-3-8), various forms of scheduling, how a block ack works (and could be more often disabled via QosNoack), and so on. the spec is now free: http://standards.ieee.org/about/get/802/802.11.html and I would love it if everyone here did try to get through section 9, which is only about 50 pages. In rereading it I was struck by how much most of our problems were in areas that the standards viewed as out of scope! and if you would like some insight as to how the 802.11wg works, see: http://grouper.ieee.org/groups/802/11/Publicity/What%20is%20802.11%20doing= .pptx I started at the 802.11ac one and then went off to sleep on a bed of nails for relaxation. (you kind of need to read both if you care about ac, as it has extensive mods to the previous spec) Nowhere in either spec was any reference to the hidden node problem. AC pees on way more channels and does do some sort of dynamic backoff, but... agh... I have to read the whole thing I guess. On Thu, Apr 21, 2016 at 11:44 PM, Henning Rogge wrote: > On Thu, Apr 21, 2016 at 7:57 PM, 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 enou= gh >> apart, or put shielding between them so that they cannot talk to each ot= her. >> >> Then position a third station so that it can see both of the first two. > > Yes, three stations would be the minimum, but I would suggest trying > it with 4 stations (all of them in IBSS/Adhoc-Mode). > > With three stations you still have one who can see everyone, which > might change the effects. > > As soon as you have the chain of 3/4 stations, just setup IP > forwarding routes and send traffic over the chain in one or both > directions. > >> 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. > > Easy to do in an office building with 5 GHz devices... often you > cannot even reach offices through two walls. > >> You will see that you can talk to either of them quite nicely if the oth= er >> is pretty idle, but if you have them both sending a lot of data at the s= ame >> time, disaster strikes. > > Yes. > >> If you are writing a simulator, add a probability that a packet transmit= ted >> 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 = the >> probability up based on the amount of airtime needed, so that as the sen= der >> slowes down, the probability goes up. > > Not sure how well current simulators can handle this. > >> This is one of the hardest problems for wifi to deal with. It manifests = as >> massive amounts of lost packets when the first two are sending to the th= ird >> 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. > > I think reporting the "used airtime" in the beacons would help. This > might be a first step to detect the presence of hidden stations > because your own view of the airtime is different than the one of your > neighbor. > > It would also be possible to add a hash of all known neighbor mac > addresses to your beacon. This way your neighbors KNOW if there is the > potential of a hidden station. > >> Reading up on Minstrel at >> https://wireless.wiki.kernel.org/en/developers/documentation/mac80211/ra= tecontrol/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 pac= kets >>> 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 apar= t, >> or there is a wall between them that makes the signal weaker, or that th= ere >> 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 = was >> designed long before anyone imagined how common it would become (I remem= ber >> 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 low= er >> rates if the packets don't get through. > > Exactly... the "lower bits per airtime" should help the receiver to > decode the frames. QAM64 (for high data rates) needs a lot more SNR > (or SNIR, Signal to Noise and Interference Ratio) than QPSK. > >> This works well if you are out in the boonies and trying for range. It f= ails >> horribly in very high density environments (this is why most conference = wifi >> is worthless for example) >> >> This is why it's a good idea to disable the lowest data rates if you kno= w >> 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 = the >> decisions they end up making. > > I wonder if it would make sense to try the best data-rate again for a > second time if you detect you are in a hidden station environment with > a "low amount of airtime available". > > Henning --=20 Dave T=C3=A4ht Let's go make home routers and wifi faster! With better software! http://blog.cerowrt.org