From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: mail.toke.dk; spf=pass smtp.mailfrom=; dkim=fail; arc=none (Message is not ARC signed); dmarc=fail (Used From Domain Record) header.from=umbernetworks.com policy.dmarc=quarantine Received: from mail.umbernetworks.com (mail.umbernetworks.com [198.74.51.139]) by mail.toke.dk (Postfix) with ESMTPS id C08D611E01B9; Tue, 09 Jun 2026 05:40:53 +0200 (CEST) Received: from webmail.umbernetworks.com (files.umbernetwork.com [198.74.51.139]) by mail.umbernetworks.com (Postfix) with ESMTPA id 0459E213CF7; Tue, 09 Jun 2026 03:40:52 +0000 (UTC) MIME-Version: 1.0 Date: Mon, 08 Jun 2026 20:40:51 -0700 From: bob.mcmahon@umbernetworks.com To: David Collier-Brown Cc: codel@lists.bufferbloat.net, Make-Wifi-fast , bloat , Cake List , Jiml , Igor Aleinikov , "Dave.seddon Ca" , Shotaro Saito In-Reply-To: References: <8e14c6935753c6263351ad00ec59b9cb@umbernetworks.com> <055e42685cddfa4c1a4ff4da089996eb@umbernetworks.com> <7455E3B4-7FB4-4D40-A900-B31151D12F6F@gmx.de> <52CE3DA7-EC5A-4FF8-A88E-26A7A6661983@gmx.de> <8qr7qons-5sp9-o30o-49qr-02p67prss6rr@ynat.uz> Message-ID: <16114610c7c4b11aec555c9eee2cd31e@umbernetworks.com> X-Sender: bob.mcmahon@umbernetworks.com Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Message-ID-Hash: 65DLGYLX52NJM52ZKG2QAWNOHCCTZSQI X-Message-ID-Hash: 65DLGYLX52NJM52ZKG2QAWNOHCCTZSQI X-MailFrom: bob.mcmahon@umbernetworks.com X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; loop; banned-address; emergency; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header X-Mailman-Version: 3.3.10 Precedence: list Subject: [Make-wifi-fast] Re: [Codel] Re: [Bloat] Re: [Rpm] Re: Re: [Cake] "Fi-Wi is a new forwarding plane for wireless" - Bob McMahon List-Id: Lets make wifi fast again! Archived-At: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: > > I believe you've just described a load-dependent queuing center. I > don't /personally/ know what to do to mitigate them, but the > queuing-networks mathies do. > Dave, Queuing theory describes the symptom, but MCS selection is a feedback-control problem. Every AP is running a hidden controller with a small local state vector. When loss occurs, that controller cannot reliably distinguish weak signal, hidden-node collision, OBSS interference, aggregation cost, or airtime overload. Those are different failure modes, but they can collapse to the same local observation: retries, missed ACKs, and degraded service time. The wrong control action can make the system worse. Lowering MCS may be correct for weak signal, but it is harmful when the real problem is hidden-node collision or airtime saturation. Lower MCS increases airtime occupancy, longer airtime increases collision exposure, and more collisions drive more retries and more fallback. That is an unstable discrete-time control loop: a pole near or outside the unit circle in the z-plane. There are also two different control objectives. A capacity-optimized controller maximizes delivered bits per second over an averaging window. A latency-optimized controller minimizes service-time variance and tail delay under load. Those optima are not the same. Larger aggregates, aggressive MCS selection, and retry persistence can improve average throughput while worsening tail latency. The AP firmware may be doing exactly what it was designed to do, but it is optimizing the wrong cost function for user experience. So yes, there is a load-dependent service center, but the service function is being created by an opaque feedback controller inside the AP. Queueing math can characterize the result. Control theory explains why the service center becomes unstable. Fi-Wi changes the control problem by increasing observability and changing the objective function. The concentrator has a richer state vector: per-STA PHY state, per-RRH receive visibility, retry probability, SINR, service-time history, aggregation cost, and WAN/WLAN congestion state. That lets the controller distinguish the cause of service-time variance before choosing the control action. An autonomous AP cannot expand its state vector or change the building-wide cost function without a fronthaul. The observability constraint is architectural. Bob