[ntp:questions] Re: NTP precision
Richard B. Gilbert
rgilbert88 at comcast.net
Fri Sep 23 19:34:04 UTC 2005
Tom Smith wrote:
> Leandro Pfleger de Aguiar wrote:
>
>> Hi
>> I´m using NTP to timestamp digital events in a specifit
>> application. On my application i need to know what is the exact
>> estimated error that my timestamp marks should consider. Some peaple
>> frequently use offset to mean what is the real estimated difference
>> between local and ref clock. Should i consider other parameters like
>> jitter, precision and accuracy manually ? Remember: i need to say
>> how mutch can my timestamp be wrong.
>>
>> Tanks Again !
>
>
> Interesting question. I think the data needed, taking into account
> all of the servers, is in the results of "ntpq -c rl 0":
>
> $ ntpq -c rl 0
> status=06f4 leap_none, sync_ntp, 15 events, event_peer/strat_chg,
> version="ntpd 4.2.0 at 1.1161-r Tue Feb 8 16:04:28 EST 2005 (1)",
> processor="alpha", system="OSF1/V5.1", leap=00, stratum=2,
> precision=-20, rootdelay=88.901, rootdispersion=39.233, peer=33113,
> ^^^^^^^^^^^^^^^^^^^^^
> refid=[IP address],
> reftime=c6de8d00.31a79fec Fri, Sep 23 2005 10:17:36.193, poll=10,
> clock=c6de8f3b.4f435265 Fri, Sep 23 2005 10:27:07.309, state=4,
> offset=0.231, frequency=-1.572, jitter=0.280, stability=0.001
> ^^^^^^^^^^^^ ^^^^^^^^^^^^
>
> However, I'm not sure of the proper way to put these together.
>
> Using "estimated error":
> "true" time = systemtime + offset +- jitter
> or
> "true" time = systemtime +- jitter
>
If I understand the definition of jitter, it has units of frequency
(1/T) and cannot meaningfully be added to or subtracted from time (T).
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