[time-nuts] wifi with time sync
albertson.chris at gmail.com
Sat Jan 14 00:32:12 EST 2017
On Fri, Jan 13, 2017 at 1:11 PM, Bob Camp <kb8tq at n1k.org> wrote:
> Ok. so I bring up NTP on the laptop against a server on the other side of
> the country and install
> NTP on the laptop. I get all of the jitter and offset of my cable modem
> plus the network
> issues between here and who know where. If I want to know the specific
> delay issues just
> on the WiFi connection (like when it rotates keys), how do I separate that
Run an NTP server on your local network with a wired connection to the
router. Also in many cases the router itself can run NTP.
If you are looking for smaller delays than NTP's level of uncertainty which
is going to be some tens of milliseconds then you need a hardware back
channel. What I would do in that case is get s GPS with one pulse per
second output and feed that to BOTH the laptop and the wired NTP server.
Both servers will eventually sync to the 1PPS and have clocks running at
some tens of microseconds from each other. With clocks on both computers
sync's to that level you can trust time stamped log files. But this
requires a source of the 1PPS and some custom cables. If tens of
milliseconds is OK (that is 1,000 times worse) then software and one
Ethernet cable are enough
In short the best way is to have all the internal clocks of the computers
running UTC to some very close tolerance then when something happens you
log it and later process logs
Another idea; Connect the laptop to an NTP server with 100BaseT cable and
set up NTP to look ONLY over that interface. Then bring up wifi for all
other uses. The time sync will be maintained at millisecond level over
Ethernet then do your WiFi experiments. The 1PPS a couple orders of
magnitude better but more work too.
Actually your initial comment is right, you be measuring the uncertainty in
the WiFi delay added to the uncertainty in the Internet connection. But he
local WiFi would be 10x worse (at least) and dominate. If you used 5 or 7
NTP servers then NTP can figure out the uncertainty over the Internet by
comparing a large number of them and the effects of the local WiFi account
for most of it.
All that said, you can buy a good enough GPS receiver on eBay for about $10
now. One trouble is getting that 1PPS signal into a laptop that lacks a
serial port. Using a USB dongle serieould degrades the timing accuracy.
But still the BEST way to distribute time sync is via a hardware 1PPS
network. I use old RG58 coax salvaged from an old Ethernet to distribute
1PPS. The source of error is in the nanosecond range and mostly comes
from speed of light delays in the wire and not measuring the wire correctly
or not accounting for velocity factor correctly or noise. But even so NTP
using a 1PPS reference clock is going to keep the computer's system clock
accurate at close to the level off the system clock's resolution
> > On Jan 13, 2017, at 3:45 PM, Chris Albertson <albertson.chris at gmail.com>
> > Short answer: See man page for ntpq
> > Longer...
> > First run NTP then after some time (15 minute to an hour) at the command
> > line time type "ntpq -p"
> > "ntpq" will query NTP for timing statistics. It will report the average
> > delay between the local computer and the set of reference clocks (other
> > servers) that NTP is connected to. Along with the average delay you get
> > variation in that delay (std dev?) Note the if NTP can calculate the
> > delay, it has already compensated for it. It is only the uncertainty of
> > the compensation that matters, hence the need to report the variation.
> > The data shows the total delay and variation over the network and the
> > reference clocks might be thousands of miles away. So you might want to
> > run one on say your wifi router or a local computer with hardwire
> > connection to the router then you'd see the effect of only your wifi.
> > On Fri, Jan 13, 2017 at 12:35 PM, Bob Camp <kb8tq at n1k.org> wrote:
> >> Hi
> >> What standard protocol would you recommend I run from the command line
> >> my computer
> >> to get a quick estimate of the timing lag and variablilty on my
> >> particular WiFi connection?
> >> Bob
> >>> On Jan 13, 2017, at 3:25 PM, John Hawkinson <jhawk at MIT.EDU> wrote:
> >>> Can we please stop talking about pings?
> >>> Bob Camp <kb8tq at n1k.org> wrote on Fri, 13 Jan 2017
> >>> at 15:12:38 -0500 in <C88C78A6-A015-4DCC-9E23-394DC33A3470 at n1k.org>:
> >>>> I’m sure you are right about the response time. Right now the
> >>>> variation is running almost 3 ms at one sigma on a ping so there is
> >>>> a lot to do simply to get the accuracy anywhere near 1 us.
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> > --
> > Chris Albertson
> > Redondo Beach, California
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