[ntp:questions] Re: Philosophical question about strata
Richard B. Gilbert
rgilbert88 at comcast.net
Fri Oct 14 14:04:34 UTC 2005
Wolfgang S. Rupprecht wrote:
>"Lee Sailer" <lee.sailer at direcway.com> writes:
>>1. If everything is working, stratum 1 is better than stratum 2.
>>2. If the stratum 1 hardware is suffering a slow degradation
>>breakdown, then the stratum 2 will be better.
>Well, this is a sad (but true) commentary on the state of consumer gps
>firmware. The gps (x,y,z,t) calculations, if done correctly, will
>tell you when things are going badly. One only needs to see 4
>satellites to work out the 4 unknowns. Usually one sees 8-12. There
>is an awful lot of redundant information in observing those extra
>satellites that one can use to determine error bars for the calculated
>x,y,z and time. There really isn't a good reason why a gps can't tell
>you "Hey, these numbers are out to lunch, please disregard."
>If the failing gps's in question put out EPE, HDOP or similar "quality
>of observation" numbers, it might be good to double-check that the ntp
>drivers really do ignore once the readings get bad.
>Question to the hard-core ntp jocks: Would anything break within the
>internal of ntp if the stratum of a reference clock were to be
>forcefully changed from the traditional stratum-1 for a correctly
>running reference to, say, stratum-5 for a gps with really crummy EPE
>that might be giving good time, but then again might not? (I assume
>this is within the spirit of "stratum" with stratum being an
>estimation of goodness, but please speak up if this isn't so.) I'm
>guessing that changing the stratum of an ntp reference clock should
>just work, because remote servers/peers can change their stratum and
>ntp automatically adapts to the new landscape.
I don't think of stratum as an "estimate" of goodness. I think it's
purely a designation of position in the hierarchy. A stratum one server
is stratum one because it gets its time from a primary standard; e.g. an
atomic clock. A server that gets its time from a WWV receiver is
technically stratum one and can be several milliseconds off because of
the vagaries of HF radio propagation. The "goodness" of a server also
depends on the path through which you receive time from it. A client
that is three thousand miles away from a stratum one server and
receiving time over a heavily used network is probably getting time
that is an order of magnitude poorer than a client three hundred feet away.
Fudging a server to a higher stratum than it would normally have should
make it appear less desirable to any client that has a choice of servers.
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