From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.lang.hm (unknown [66.167.227.145]) (using TLSv1.2 with cipher ADH-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.bufferbloat.net (Postfix) with ESMTPS id A4F013B29D; Wed, 19 Oct 2022 17:37:43 -0400 (EDT) Received: from dlang-mobile (unknown [10.2.2.70]) by mail.lang.hm (Postfix) with ESMTP id D2F691513C0; Wed, 19 Oct 2022 14:37:42 -0700 (PDT) Date: Wed, 19 Oct 2022 14:37:42 -0700 (PDT) From: David Lang To: "David P. Reed" cc: David Lang , Bob McMahon , Cake List , Make-Wifi-fast , Bob McMahon via Rpm , bloat In-Reply-To: <1666214791.81584918@apps.rackspace.com> Message-ID: <308q0740-sro8-nn32-sn5o-337n631r86o0@ynat.uz> References: <938D9D45-DADA-4291-BD8A-84E4257CEE49@apple.com> <6710sq51-1151-s739-qq87-0r5264qrs9q8@ynat.uz> <6697ss38-s3nr-99n3-8q5o-p24q6q7923np@ynat.uz> <1666214791.81584918@apps.rackspace.com> MIME-Version: 1.0 Content-Type: text/plain; format=flowed; charset=US-ASCII Subject: Re: [Make-wifi-fast] [Cake] [Rpm] [Bloat] The most wonderful video ever about bufferbloat 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: Wed, 19 Oct 2022 21:37:44 -0000 Thanks, and how long does it take to transmit the wifi header (at 1Mb/s and at 11Mb/s)? That's also airtime that's not availalbe to transmit user data. And then compare that to the time it takes to transmit a 1500 byte ethernet packet worth of data over a 160MHz wide channel Going back to SM's question, there is per-transmission overhead that you want to amatorize across multiple ethernet packets, not pay for each packet. David Lang On Wed, 19 Oct 2022, David P. Reed wrote: > 4 microseconds! > > On Wednesday, October 19, 2022 3:23pm, "David Lang via Cake" said: > > > >> you have to listen and hear nothing for some timeframe before you transmit, that >> listening time is define in the standard. (isn't it??) >> >> David Lang >> >> On Wed, 19 Oct 2022, Bob McMahon wrote: >> >> > I'm not sure where the gap in milliseconds is coming from. EDCA gaps are >> > mostly driven by probabilities >> > . If >> > energy detect (ED) indicates the medium is available then the gap prior to >> > transmit, assuming no others competing & winning at that moment in time, is >> > driven by AIFS and the CWMIN - CWMAX back offs which are simple probability >> > distributions. Things change a bit with 802.11ax and trigger frames but the >> > gap is still determined by the backoff and should be less than milliseconds >> > per that. Things like NAVs will impact the gap too but that happens when >> > another is transmitting. >> > >> > >> > [image: image.png] >> > >> > Agreed that the PLCP preamble is at low MCS and the payload can be orders >> > of magnitude greater (per different QAM encodings and other signal >> > processing techniques.) >> > >> > Bob >> > >> > On Wed, Oct 19, 2022 at 12:09 AM David Lang wrote: >> > >> >> On Tue, 18 Oct 2022, Sebastian Moeller wrote: >> >>> Hi Bob, >> >>> >> >>>> Many network engineers typically, though incorrectly, perceive a >> >> transmit >> >>>> unit as one ethernet packet. With WiFi it's one Mu transmission >> or one >> >> Su >> >>>> transmission, with aggregation(s), which is a lot more than one >> ethernet >> >>>> packet but it depends on things like MCS, spatial stream powers, >> Mu >> >> peers, >> >>>> etc. and is variable. Some data center designs have optimized the >> >>>> forwarding plane for flow completion times so their equivalent >> transmit >> >>>> unit is a mouse flow. >> >>> >> >>> [SM] Is this driven more by the need to aggregate packets to amortize >> >> some cost over a larger payload or to reduce the scheduling overhead or >> to >> >> regularize things (as in fixed size DTUs used in DSL with G.INP >> >> retransmissions)? >> >> >> >> it's to amortize costs over a larger payload. >> >> >> >> the gap between transmissions is in ms, and the transmission header is >> >> transmitted at a slow data rate (both for backwards compatibility with >> >> older >> >> equipment that doesn't know about the higher data rate modulations) >> >> >> >> For a long time, the transmission header was transmitted at 1Mb (which is >> >> still >> >> the default in most equipment), but there is now an option to no longer >> >> support >> >> 802.11b equipment, which raises the header transmission time to 11Mb. >> >> >> >> These factors are so imbalanced compared to the top data rates available >> >> that >> >> you need to transmit several MB of data to have actual data use 50% of >> the >> >> airtime. >> >> >> >> David Lang >> >> >> > >> > >> _______________________________________________ >> Cake mailing list >> Cake@lists.bufferbloat.net >> https://lists.bufferbloat.net/listinfo/cake >>