From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from snark.thyrsus.com (static-71-162-243-5.phlapa.fios.verizon.net [71.162.243.5]) by huchra.bufferbloat.net (Postfix) with ESMTP id 5259F20069D for ; Tue, 13 Mar 2012 18:53:26 -0700 (PDT) Received: by snark.thyrsus.com (Postfix, from userid 1000) id 0A63E40617; Tue, 13 Mar 2012 21:53:06 -0400 (EDT) Date: Tue, 13 Mar 2012 21:53:06 -0400 From: "Eric S. Raymond" To: "Ron Frazier (NTP)" Message-ID: <20120314015306.GA25681@thyrsus.com> References: <20120313230612.GA24800@thyrsus.com> <4F5FF42B.9060900@c3energy.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4F5FF42B.9060900@c3energy.com> Organization: Eric Conspiracy Secret Labs X-Eric-Conspiracy: There is no conspiracy User-Agent: Mutt/1.5.21 (2010-09-15) Cc: thumbgps-devel@lists.bufferbloat.net Subject: Re: [Thumbgps-devel] Project clarification X-BeenThere: thumbgps-devel@lists.bufferbloat.net X-Mailman-Version: 2.1.13 Precedence: list Reply-To: esr@thyrsus.com List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 14 Mar 2012 01:53:26 -0000 Ron Frazier (NTP) : > I'd like to get a further minor clarification on the project goal. > Do you want your time reading to be within plus or minus 1 ms of > UTC, for a total range of 2 ms? Or do you want it to be within plus > or minus 500 us of UTC, for a total range of 1 ms? The latter. Total range of 2ms. This is believed to be achievable because the data sheet for the ZTI Z050 (the only existing USB thumb GPS with PPS) promises 1ms in the way you seem to mean it. Their claim is credible given that PPS is guaranteed accurate within 50ns. The default USB polling rate of 125Hz implies polling latency of 8msec, but it's possible to jack that up by a factor of 8 within the USB spec, and I'm certain that's what ZTI's timing software is doing. That would produce a polling interval of 1ms, neatly matching their accuracy claim. The additional 50ns jitter (6 magnitudes down) and the handful of us required for processing on the host side (3 magnitudes down) would vanish compared to this. If I'm misunderstanding ZTI's accuracy claim, it's possible we might get the tighter 500us range. We'll test. Either figure will do for Dave's intended use, which is network delay tomography where the feature size is likely to be about 10 milliseconds. -- Eric S. Raymond