[ntp:questions] NTP tunning for OWD measurements

unruh unruh at invalid.ca
Sat Oct 27 17:15:30 UTC 2012


On 2012-10-27, pret3nder at gmail.com <pret3nder at gmail.com> wrote:
> OWAMP can refer to One-Way Active Measurement Protocol,
> see http://tools.ietf.org/html/rfc4656, or the tool that follows
> that protocol to measure one-way delay, 
> see http://www.internet2.edu/performance/owamp/
>
> NREN, as stated before, means National Research and Education Network.
>
> @unruh: yes, I was talking in the range of microseconds,
> not single microsecond accuracy!

Then why do you not say WHAT your accuracy requirement is? You still
have not done so. 


> This is the current output of ntpq -p on X1 server:
>
> [alias at x1 ntp]# ntpq -p
>      remote           refid      st t when poll reach   delay   offset  jitter
>=============================================
> *time01.nren.pt  .DCFa.           1 u    7   16  377    5.661    0.021   0.015
>  time02.nren.pt  .DCFa.           1 u    6   16  377    1.240    0.078   0.026
>  time04.nren.pt  .DCFa.           1 u    2   16  377    2.324    0.102   0.009
>  time03.nren.pt  .DCFa.           1 u    -   16  377    8.757    0.109   0.011
>
> If the delay, offset and jitter values are shown in milliseconds,
> I am led to believe X1 currently has an offset to time01 stratum
> 1 server of 21 microseconds, right? If S1 has the same offset,
> I can expect a maximum error of 42 microseconds on the owd 
> measurements, ignoring the errors caused by the delay asymmetry.
> Is this correct?

No. It means that the current offset -- the difference between the
measurement of the mean time between sent and received-- for time01 is
.021 us. That does NOT mean that your system is withing .021 sec on the
true time. Note that all four of the time01-4 are presumably equivalent
clocks and the scatter for the offset is more like .1ms. The delays are
5ms, so the best you can say is that the real time lies between +- 2.6ms
since yo uhave no idea what the difference between outgoing and incoming
delay times is. THAT IS WHY YOU NEED GPS ON BOTH ENDS.


>
> Pedro
>
> On Saturday, October 27, 2012 5:19:09 PM UTC+1, Richard B. Gilbert wrote:
>> On 10/26/2012 5:56 PM, pret3nder at gmail.com wrote:
>> 
>> > Hi all, and thank you for your answers. I'm afraid I might not have
>> 
>> > been clear about my objectives, so I'll try to explain clearer.
>> 
>> > I'll also try and keep the lines smaller, and please, excuse me if
>> 
>> > I make any mistakes, as english is not my native language.
>> 
>> >
>> 
>> > This project involves a NREN, which interconnects several institutions.
>> 
>> 
>> 
>> I give up?  WHAT IS "NREN"?????
>> 
>> 
>> 
>> <snip>



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