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Re: [gpsd-dev] PPS over USB
From: |
Gary E. Miller |
Subject: |
Re: [gpsd-dev] PPS over USB |
Date: |
Mon, 7 May 2012 11:11:19 -0700 |
Yo Ed!
On Mon, 07 May 2012 15:52:07 +0100
Ed W <address@hidden> wrote:
> > The S1 is my personal, at-home, FreeBSD+Garmin18 ntpd server.
>
> Aha! Then in that case I definitely agree you are sorted! I would
> presume you can achieve low few 100s micro second sync (or better?)
> quite easily?
My experience is that a host adjacent to S1 on a GigE link can sync
to better than 100 microSec jitter. Variable depending a lot on
the load on the hosts and the net.
> Problem seems to be that unless you all have the same
> reference (eg gps) then you could all be microsecond "accurate", just
> all at an offset to each other?
Exactly, thus the preference for a single time source for all units
in the test.
> I might guess that this is the goal of thumbgps - get the offset to
> zero?
Plus or minus a handfull of milliSec.
> However, it appears in principle that you might achieve almost
> the same simply syncing to a carefully chosen pool of stratum 1
> servers?
Chicken and egg problem. How can you get good time over a network
link of unknown (and asyn) delays? Thus the observed data that it
can be pretty hard to sync to an S1 over a cable modem to even
100 milliSec. 100 times worse than the GR-601W locally.
> So I'm still a touch unsure what the ultimate reason is for
> this?
To measure and understand the effects of bufferbloat on network
latency. Since this is off topic for gpsd I suggesst you check out
the bufferbloat project html pages and mailing lists.
RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
address@hidden Tel:+1(541)382-8588
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- Re: [gpsd-dev] PPS over USB, (continued)
- Re: [gpsd-dev] PPS over USB, Eric S. Raymond, 2012/05/06
- Re: [gpsd-dev] PPS over USB, Eric S. Raymond, 2012/05/06
- Re: [gpsd-dev] PPS over USB, Terje Mathisen, 2012/05/06
- Re: [gpsd-dev] PPS over USB, Gary E. Miller, 2012/05/06
- Re: [gpsd-dev] PPS over USB, Dave Taht, 2012/05/06
- Re: [gpsd-dev] PPS over USB, Ed W, 2012/05/07
- Re: [gpsd-dev] PPS over USB, Terje Mathisen, 2012/05/07
- Re: [gpsd-dev] PPS over USB, Ed W, 2012/05/07
- Re: [gpsd-dev] PPS over USB,
Gary E. Miller <=
- Re: [gpsd-dev] PPS over USB, Hal Murray, 2012/05/07
- Re: [gpsd-dev] PPS over USB, Eric S. Raymond, 2012/05/07
- Re: [gpsd-dev] PPS over USB, Dave Taht, 2012/05/07
- Re: [gpsd-dev] PPS over USB, Eric S. Raymond, 2012/05/07
- Re: [gpsd-dev] PPS over USB, Hal Murray, 2012/05/08
- Re: [gpsd-dev] PPS over USB, Eric S. Raymond, 2012/05/08
- Re: [gpsd-dev] PPS over USB, Ed W, 2012/05/08
- Re: [gpsd-dev] PPS over USB, Dave Taht, 2012/05/08
- Re: [gpsd-dev] PPS over USB, Eric S. Raymond, 2012/05/08
- Re: [gpsd-dev] PPS over USB, Ed W, 2012/05/08