[time-nuts] GPS for Spirent Smartbits
edu at kender.es
edu at kender.es
Thu Feb 15 16:01:43 EST 2007
At a good price a few ms would be very fine. And you have given me an
idea. We already have a NTP server at the labo with a M12 (not the timing
version) now the question is to make it mobile... I already need a
computer for controlling the Smartbit... so I only need a switch and to
cables to make a mini-mobile-intranet... I'll check this befor jumping in
the big bucks wagon. Best regards and thank you.
>> If you have an Internet access you can get a time reference with NTP
>> and even do timed captures.
>> We would like to get 2 GPS timing systems in order to test influence
>> of handovers in Wimax on moving vehicles.
> What sort of timing accuracy do you need?
> Assuming a few ms rather than a few microseconds is good enough...
> I didn't see much about timing in that blurb. It said it provides NTP
> service, but I didn't see anything fancy about where it got the time to
> "serve". The ntp code is normally both a server and client, so I assume
> can get the time from the net. You can test that by pointing it at your
> local NTP server or one out on the net. That should get you off the
> but the accuracy may not be very good.
> The favorite low cost GPS unit in the NTP community is the Garmin
> (There are two other models of the GPS-18, but they don't have the PPS
> signal.) You can get them for under $100. It requires some
> The usual trick is to steal power from a USB port. For more info, start
> or feed >garmin GPS-18 ntp< to google.
> GPS may not work in a lab full of computers. (You can test that with a
> normal hiking GPS unit.) The clean solution is to put the antenna up on
> roof. Mine works most of the time inside my house, but only if it's up
> near the ceiling. You may have to locate the server near a window and/or
> kludge up some longer cables or ...
> I'm assuming you can find two old PCs to dedicate for an NTP server on
> end. It would be simpler if you could get identical hardware and software
> both ends. It might work if you run it on a personal machine but that
> probably add more jitter if the user is doing anything. You might have to
> experiment. NTP doesn't need much CPU or memory.
> What I would use as a sanity check would be to slowly send short packets
> A to B and measure the delay. There will be two types of delay. One is
> speed of light (and silicon processing). The other is queuing. The
> reason I
> said slowly is to make sure the test is not generating enough traffic to
> contribute to the queuing delays.
> If you make a histogram of the delays, the left edge should be the
> case. If the histogram in one direction matches the histogram in the
> direction your clocks are probably good.
> 1] "Some assembly required" may be an American joke. It refers to
> where the father has to assemble the new toys for the kid. Sometimes it's
> not easy. They usually print a warning on the box but it doesn't tell you
> how much work it will be.
> These are my opinions, not necessarily my employer's. I hate spam.
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