[ntp:questions] SNTP server + ntpd 4.2.4 client

Noob root at localhost.invalid
Fri Mar 14 15:46:34 UTC 2008

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?


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