[ntp:questions] SNTP server + ntpd 4.2.4 client

Richard B. Gilbert rgilbert88 at comcast.net
Fri Mar 14 17:44:02 UTC 2008


Noob wrote:
> Richard B. Gilbert wrote:
> 
>> Noob wrote:
>>
>>> I've been running ntpd 4.2.4 to synchronize my system clock using 
>>> remote stratum 2 servers as a reference. (The RTT to these servers is 
>>> in the 30-50 ms range.) The accuracy is in the 1-2 ms range, based on 
>>> the reported offset.
>>>
>>> I've been asked to evaluate the following time server, in order to 
>>> reach a better accuracy than what the current setup provides.
>>>
>>> http://www.heoldesign.com/index.php?id=58
>>
>>
>> That link takes me to a page advertising THREE products!  Which one 
>> did you have in mind?
> 
> 
> The HEOL-T101 (with a Fast Ethernet port).
> 
>>> Is it a problem that the time server only implements SNTP?
>>
>>
>> It should not be a problem.  The largest difference between NTP and 
>> SNTP is the effort to account for the vagaries of the internet!
> 
> 
> Cool. (I'll give RFC 4330 a look.)
> 
>>> What kind of accuracy may I expect?
>>
>>
>> These devices should be accurate to within the range of 25 to 100 
>> nanoseconds.
> 
> 
> The spec seems to mention +/- 40 ns.
> 
>> The limiting factor will be the jitter introduced while getting the 
>> time into your computer.
> 
> 
> I plan to connect my box to the time server using a cross-over cable. 
> (My box has 4 Ethernet ports, I will devote one to NTP traffic.) The RTT 
> is very stable at 80-85 µs.
> 
>> A SWAG (Scientific Wild Ass Guess) would be 500 microseconds.  By 
>> spending a lot of time and effort you might be able to get something 
>> better than that.  The chief difficulty would be measuring and 
>> controlling the delays within the computer.
> 
> 
> I thought the error was on the order of half the RTT, i.e. I could hope 
> for 40-50 µs in my situation?
> 
> Regards.

Given the above, you are correct, 40-50 microseconds.  I had assumed 
that you were using a serial port where the latencies are greater.




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