[time-nuts] wifi with time sync
kb8tq at n1k.org
Sat Jan 14 10:46:16 EST 2017
Ok, what I see is that every few hours, I get a “rogue delay” on a single ping. How
would NTP help me spot a single transit with a 250 ms round trip and identify the
time it occured? Keep in mind that NTP is going to throttle back to a very low level
of “chat” quite quickly…..
While this *is* getting far more into my WiFi (which I had no real intention of doing) it
does apply to timing and running audio over WiFi as well. The basic transport as it
runs up through the various layers is *not* very good time wise. There is indeed a
real need for some sort of overlay to take care of that issue. I’d still love to know if
this magic protocol is simply giant buffers and some sort of tagging or if they do
something more interesting.
> On Jan 14, 2017, at 12:32 AM, Chris Albertson <albertson.chris at gmail.com> wrote:
> 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
>>> 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:
>>>> 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?
>>>>> 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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> Chris Albertson
> Redondo Beach, California
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