From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-we0-f171.google.com (mail-we0-f171.google.com [74.125.82.171]) (using TLSv1 with cipher RC4-SHA (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority" (verified OK)) by huchra.bufferbloat.net (Postfix) with ESMTPS id 6CDBB20069D for ; Tue, 13 Mar 2012 20:42:59 -0700 (PDT) Received: by werm1 with SMTP id m1so2181429wer.16 for ; Tue, 13 Mar 2012 20:42:57 -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-type:content-transfer-encoding; bh=rL5dSlTNwAwxwA/GM48HeJMhdGso+P4BgGuPAxKTsjY=; b=IBEcEuTWdHLa5GytkLrrjN6yDzVaTRWFxEKXl6yugdrjvyvrtWiCyGnxEVbPkdMa9R JGhafrLSDPy7UrFPznVG3RdlUgwQy1z/VaTKGmkESue/oQX7nZMbtT++UMRTFo4W63zE AspGkROqeSL/zv929tt6cktwky3Jq5FjfDU+b+dhBRdyhSDFwx9SkwWD8rrfjNLtCeRB Lb20E0xL6+J9AHXAjeTgjPUW5diD6CUQruA0D8eQisgeMnAYbj0aJzlQnPM5iMJX7aXX J9ITUPBHtqo2JDkzoY5K8JMdivRtSwEGFQpCZ5uH1863YuQ+pbYcXHY67B+w5wS6/a71 HoHw== MIME-Version: 1.0 Received: by 10.180.80.104 with SMTP id q8mr2330178wix.14.1331696577239; Tue, 13 Mar 2012 20:42:57 -0700 (PDT) Received: by 10.223.151.8 with HTTP; Tue, 13 Mar 2012 20:42:57 -0700 (PDT) In-Reply-To: References: <20120313230612.GA24800@thyrsus.com> <4F5FF42B.9060900@c3energy.com> Date: Tue, 13 Mar 2012 20:42:57 -0700 Message-ID: From: Dave Taht To: "Ron Frazier (NTP)" Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable 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 List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 14 Mar 2012 03:42:59 -0000 On Tue, Mar 13, 2012 at 8:08 PM, Dave Taht wrote: > On Tue, Mar 13, 2012 at 6:28 PM, Ron Frazier (NTP) > wrote: >> On 3/13/2012 9:10 PM, Dave Taht wrote: >>> >>> On Tue, Mar 13, 2012 at 4:06 PM, Eric S. Raymond =A0wr= ote: >>> >>>> >>>> Mike Hord: > >> I'd like to get a further minor clarification on the project goal. =A0Do= you >> want your time reading to be within plus or minus 1 ms of UTC, for a tot= al >> range of 2 ms? =A0Or do you want it to be within plus or minus 500 us of= UTC, >> for a total range of 1 ms? > > Heh. Good question. > > As good as it is possible to get without having to have rubidium > clocks embedded in the stick? Ooh! OOh! I found an americium clock for only 13.99! http://www.cafepress.com/randumbshirts.327160457 > Better than 10ms is desirable, and the 1ms is something of an > arbitrary goal. Nyquist theorem applies, and then there's slop/noise > elsewhere in the system that will start cropping up (interrupt > latency, etc) I kind of realized I wasn't being as clear as I might like. right now the jitter and delay is well over 10ms on all the gps devices we've got our hands on, so it's the most noisy source of 'stable' time there is, and the internal system clock, as corrected by ntp, does better. Take the blox gps chip that I'm half in love with. It can - with tweaking, do a 15ns pulse - but everything after that you attach to that signal - microcontroller, usb bus, basic interrupt latency in the os, userspace response time, response to load, etc - will add additional jitter. And all that has to be measured. So starting with the least jitter possible and fixing it at every step of the way out of the chip is what would happen, which is why it's hard to specify (right now, without having a stable time source to play with at all - which I'm trying to aquire btw) whether .5 or 1ms jitter will affect things all that much. I did a lot of work with linux-rt back in the day, and with threaded interrupts it was possible to do really intense audio work with about 2.8ms sample period on high end (for the day) hardware. While much of the linux-rt code has made it into the mainline, that feature hasn't, and the box I'm mostly playing with runs at 680Mhz. I don't know what it's interrupt latency actually is... > > Trying to clearly identify problem hops from multiple sides is > dependent on the 'distance' inherent in the underlying technology. If > we wanted to identify problems on a single 10GigE ethernet segment, > that's measured in *ns*. We don't, thankfully. > > First hop out of a router is generally 1ms, same for wireless. Cable > is in the 2ms-8ms range. DSL, up to 60ms, same for wimax... > > So < 5ms would be the outside goal, and anything less than that - so > long as it can be thoroughly catagorized - is a bonus. > > Continental US distances are 60-100ms... > > > > >> >> Sincerely, >> >> Ron >> >> >> -- >> >> (PS - If you email me and don't get a quick response, don't be concerned= . >> I get about 300 emails per day from alternate energy mailing lists and >> such. =A0I don't always see new messages very quickly. =A0If you need a >> reply and have not heard from me in 1 - 2 weeks, send your message again= .) >> >> Ron Frazier >> timekeepingdude AT c3energy.com >> >> >> _______________________________________________ >> Thumbgps-devel mailing list >> Thumbgps-devel@lists.bufferbloat.net >> https://lists.bufferbloat.net/listinfo/thumbgps-devel > > > > -- > Dave T=E4ht > SKYPE: davetaht > US Tel: 1-239-829-5608 > http://www.bufferbloat.net --=20 Dave T=E4ht SKYPE: davetaht US Tel: 1-239-829-5608 http://www.bufferbloat.net