[ntp:questions] How to keep Linux server in Chicago and Mumbai in sync to within 5 microseconds
Richard B. Gilbert
rgilbert88 at comcast.net
Wed Jan 12 22:44:46 UTC 2011
On 1/12/2011 2:57 AM, unruh wrote:
> On 2011-01-12, Chris Albertson<albertson.chris at gmail.com> wrote:
>>> Two GPS receivers ( Garmin GPS 18X for about $70 each and a bit of time
>>> soldering on an RS232 plug.
>>> )
>>> Certainly cheaper and much more accurate. And if you can see the cell
>>> tower to get the CDMA you should be able to see the GPS sattelites.
>>
>> Yes the GPS can be even cheaper than $70. But the CDMA signal really
>> can go through walls that the GPS signal can't. So I'd say the way to
>> deside is to bring a hnd held GPS to the proposed antenna ocation and
>> see what hapens. Can it get a good view of the sky. Most GPSes have
>> a display screen that will answer that question.
>>
>> Also the OP is asking for +- 5 microseconds. That is moderately hard.
>
> No way you will get that out of CDMA AFAIK. The time from the tower is much
> longer than that.
> GPS will give that to you.
>
>> It means each box can only have a worst case 2.5 uS error. To get to
>> that level some care is required.) The specs on the GPS 18X say the
>> pps is "+- 1 microsecond" which is not very good. It uses to much of
>> his error budget for my taste, For the same price you can have at
>> least 10X better. and maybe 100X better with a $200 Thunderbolt GPS.
>> Then in effect none of his error budget is used up outside of the
>> computer.
>
> The problem is that the entry to the computer (interrupt handling) takes
> a lot more time than 1usec. Ie, unless he has very special hardware he
> will be hard pressed to get his 2us, and there is no way with CDMA
> AFAIK.
>
>>
>> Then we have to know a little about the computer. Hopefully it has a
>> real serial port and is running BSD or Linuix and has the pps kernel
>> API.
>
> Probably a special parallel port interrupt handler would be better.
>
>>
>> This is that kind of thing that will suck up the OP's time. There are
>> a million little things that he was to learn and each takes 10 minutes
>> or an hour. To me reading this stuff is "entertainment" and a good
>> way to avoid the TV and learn something. But to another person it is
>> just a huge "time sink" (pun intended)
>>
>> Bottom line: This is just the classic "build or buy" business decision.
>>
>> =====
>> Chris Albertson
>> Redondo Beach, California
There are a few companies that specialize in GPS timing. Symmetricomm
is one such. They will sell you a GPS clock for big bucks. The last
time I looked, the bottom of the line cost about $1200 US. There are
other companies though I can't think of their names at the moment.
At the other end of the scale, you can get a GPS timing receiver for
around $100 US. This is a "no frills" deal; you will need to know how
to use a soldering iron. You will also need a five volt DC power supply.
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