Richard,
Really and truly do believe interpreting stratum as quality of service
is a bright red herring. It was never intended for that purpose. Its
primary purpose is avoiding timing loops. There is an absolutely
wonderful metric with which to interpret quality, a combination of
maximum error (synchronization distance) and estimated error (system
jitter). There are explicit provisions in the reference clock interface
that allow the driver to adjust these values with respect to whatever
statistics the clock provides. It would seem the PDoP commonly provided
by the GPS receiver firmware would be a prime candidate.
Dave
Richard B. Gilbert wrote:
Wolfgang S. Rupprecht wrote:
"Lee Sailer" <[EMAIL PROTECTED]> 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.
-wolfgang
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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