[ntp:questions] NTP tunning for OWD measurements

pret3nder at gmail.com pret3nder at gmail.com
Sat Oct 27 19:48:01 UTC 2012


Ok, thanks for clearing that up for me. Makes sense :-)

On Saturday, October 27, 2012 8:35:40 PM UTC+1, unruh wrote:
> On 2012-10-27, pret3nder at gmail.com <pret3nder at gmail.com> wrote:
> 
> > Yes, circuit switched networks don't have this problem,
> 
> > for sure. 
> 
> > If, for example, having a GPS on S1, and using it as a
> 
> > timeserver for X1, and considering the incoming and outgoing
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> > delay on that path are equal, when measuring the OWD 
> 
> > between S1 and X1, I would get a measurement with an accuracy
> 
> > equal to the offset reported by NTP, right?
> 
> 
> 
> It it really were equal, then the time on X1 would not have an offset,
> 
> since ntpd would drive them to equality. The problem is that they are
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> not equal. and thus you get fluctuations in the offset and ntpd tried to
> 
> drive the clock to eliminate that offset. If the two delays are equal
> 
> then you really have nothing to measure. The one way equals half the
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> delay,always. 
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> 
> 
> >
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> >
> 
> > On Saturday, October 27, 2012 7:34:46 PM UTC+1, unruh wrote:
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> >> On 2012-10-27, pret3nder at gmail.com <pret3nder at gmail.com> wrote:
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> >> 
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> >> > The accuracy requirement is not written on stone,
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> >> 
> 
> >> > but <1ms would be the goal we're aiming for.
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> >> 
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> >> >
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> >> 
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> >> > I think you made some confusions on the units 
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> >> 
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> >> > there (usec and sec, when I think those are in ms),
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> >> 
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> >> 
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> >> 
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> >> usec and msec I should have said.
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> >> 
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> >> 
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> >> 
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> >> > but I got your point. The way NTP works, it estimates 
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> >> 
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> >> > the one-way delays as RTT/2 and uses that to correct 
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> >> 
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> >> > the computer clock to UTC. So, without having an external
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> >> 
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> >> 
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> >> 
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> >> Yes.
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> >> 
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> >> 
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> >> 
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> >> > source on both ends that can correct the computer
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> >> 
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> >> > clock with a delay <1ms, I can't get that accuracy, right?
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> >> 
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> >> 
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> >> 
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> >> Yes. All you can say is that it lies between all of the delay being on
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> >> 
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> >> the outgoing trip and all the delay on the ingoing. Now, I agree that is
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> >> 
> 
> >> unlikely, but certainly if say the outgoing delay is .6 of the total
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> >> 
> 
> >> delay and the ingoing is .4, you could not tell that. For example the
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> >> 
> 
> >> outgoing path could go through different routers, switches etc, than the
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> >> 
> 
> >> ingoing path. Eg, Toronto to Vancouver could go through Dallas while
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> >> 
> 
> >> Vancouver to Toronto could go through chicago instead.  (and the
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> >> 
> 
> >> equivalent for shorter hops). Each packet is independent and is routed
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> >> 
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> >> independently of any prior packets passing either way. The internet is
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> >> 
> 
> >> set up to handle each packet independently on purpose to increase fault
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> >> 
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> >> tolerance. An internet connection does not establish a permanant two way
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> >> 
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> >> link between machine as old telephone calls did. 
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> >> 
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> >> >
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> >> >
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> >> 
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> >> > On Saturday, October 27, 2012 6:15:30 PM UTC+1, unruh wrote:
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> >> 
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> >> >> On 2012-10-27, pret3nder at gmail.com <pret3nder at gmail.com> wrote:
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> >> 
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> >> >> 
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> >> 
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> >> >> > OWAMP can refer to One-Way Active Measurement Protocol,
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> >> 
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> >> >> 
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> >> 
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> >> >> > see http://tools.ietf.org/html/rfc4656, or the tool that follows
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> >> 
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> >> >> 
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> >> 
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> >> >> > that protocol to measure one-way delay, 
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> >> 
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> >> >> 
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> >> 
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> >> >> > see http://www.internet2.edu/performance/owamp/
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> >> 
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> >> >> 
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> >> >> >
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> >> >> 
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> >> 
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> >> >> > NREN, as stated before, means National Research and Education Network.
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> >> >> 
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> >> >> >
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> >> >> 
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> >> >> > @unruh: yes, I was talking in the range of microseconds,
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> >> >> 
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> >> >> > not single microsecond accuracy!
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> >> >> 
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> >> >> 
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> >> >> 
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> >> >> Then why do you not say WHAT your accuracy requirement is? You still
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> >> >> 
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> >> >> have not done so. 
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> >> >> 
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> >> >> 
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> >> >> 
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> >> >> 
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> >> >> 
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> >> >> > This is the current output of ntpq -p on X1 server:
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> >> >> 
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> >> >> >
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> >> >> 
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> >> >> > [alias at x1 ntp]# ntpq -p
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> >> >> 
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> >> >> >      remote           refid      st t when poll reach   delay   offset  jitter
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> >> 
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> >> >> 
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> >> 
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> >> >> >=============================================
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> >> 
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> >> >> 
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> >> 
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> >> >> > *time01.nren.pt  .DCFa.           1 u    7   16  377    5.661    0.021   0.015
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> >> 
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> >> >> 
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> >> 
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> >> >> >  time02.nren.pt  .DCFa.           1 u    6   16  377    1.240    0.078   0.026
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> >> 
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> >> >> 
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> >> 
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> >> >> >  time04.nren.pt  .DCFa.           1 u    2   16  377    2.324    0.102   0.009
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> >> 
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> >> >> 
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> >> 
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> >> >> >  time03.nren.pt  .DCFa.           1 u    -   16  377    8.757    0.109   0.011
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> >> 
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> >> >> 
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> >> >> >
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> >> 
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> >> >> 
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> >> 
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> >> >> > If the delay, offset and jitter values are shown in milliseconds,
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> >> 
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> >> >> 
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> >> 
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> >> >> > I am led to believe X1 currently has an offset to time01 stratum
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> >> 
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> >> >> 
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> >> 
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> >> >> > 1 server of 21 microseconds, right? If S1 has the same offset,
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> >> 
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> >> >> 
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> >> 
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> >> >> > I can expect a maximum error of 42 microseconds on the owd 
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> >> 
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> >> >> 
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> >> 
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> >> >> > measurements, ignoring the errors caused by the delay asymmetry.
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> >> 
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> >> >> 
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> >> >> > Is this correct?
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> >> >> 
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> >> >> 
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> >> >> 
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> >> >> No. It means that the current offset -- the difference between the
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> >> 
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> >> >> 
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> >> >> measurement of the mean time between sent and received-- for time01 is
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> >> 
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> >> >> 
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> >> 
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> >> >> .021 us. That does NOT mean that your system is withing .021 sec on the
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> >> 
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> >> >> 
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> >> 
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> >> >> true time. Note that all four of the time01-4 are presumably equivalent
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> >> 
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> >> >> 
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> >> 
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> >> >> clocks and the scatter for the offset is more like .1ms. The delays are
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> >> 
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> >> >> 
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> >> 
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> >> >> 5ms, so the best you can say is that the real time lies between +- 2.6ms
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> >> 
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> >> >> 
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> >> 
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> >> >> since yo uhave no idea what the difference between outgoing and incoming
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> >> 
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> >> >> 
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> >> 
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> >> >> delay times is. THAT IS WHY YOU NEED GPS ON BOTH ENDS.
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> >> >> 
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> >> >> >
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> >> >> 
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> >> >> > Pedro
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> >> >> 
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> >> >> >
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> >> >> 
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> >> 
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> >> >> > On Saturday, October 27, 2012 5:19:09 PM UTC+1, Richard B. Gilbert wrote:
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> >> 
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> >> >> 
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> >> 
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> >> >> >> On 10/26/2012 5:56 PM, pret3nder at gmail.com wrote:
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> >> >> 
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> >> >> >> 
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> >> >> 
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> >> >> >> > Hi all, and thank you for your answers. I'm afraid I might not have
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> >> 
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> >> >> 
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> >> >> >> 
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> >> >> 
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> >> >> >> > been clear about my objectives, so I'll try to explain clearer.
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> >> >> >> 
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> >> >> >> > I'll also try and keep the lines smaller, and please, excuse me if
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> >> >> >> 
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> >> >> >> > I make any mistakes, as english is not my native language.
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> >> >> >> >
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> >> >> >> > This project involves a NREN, which interconnects several institutions.
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> >> >> >> I give up?  WHAT IS "NREN"?????
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> >> >> >> <snip>
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> >> 
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> >> >
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> >



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