On Sun, 15 Feb 2009 10:48:48 -0800 (PST), Dave Hart wrote: > On Feb 15, 6:04 pm, Unruh <[email protected]> wrote: >> Hans =?iso-8859-1?Q?J=F8rgen?= Jakobsen <[email protected]> writes: >> >There ARE asymetric delays even for ntp packets. >> >For my 14M/1.5M VDSL line I see an offset of 7-800 microseconds: >> > ntpq -p >> > remote refid st t when poll reach delay offset >> > jitter >> >============================================================================== >> >-ns.tele.dk .GPS. 1 u 48 64 377 22.450 0.891 >> >0.096 >> >-tix.ns.tele.dk .GPS. 1 u 7 64 377 22.338 0.698 >> >0.183 >> >-puma.tele.dk .GPS. 1 u 19 64 377 27.048 0.784 >> >0.145 >> >+GPS_HP(0) .GPS. 0 l 10 16 377 0.000 -0.776 >> >6.230 >> >oPPS(0) .1PPS. 0 l 10 16 377 0.000 0.012 >> >0.015 >> >+GPS_ONCORE(0) .GPS. 0 l 8 16 377 0.000 0.035 >> >0.006 >> >> Something is really weird here. .776ms from a GPS refclock is horrible. >> That is about a factor of 300 worse than you would expect. > > You misread. -0.776 represents the timestamp of the line feed at the > end of a GPS timecode line on the serial port. The PPS refclock on > the next line is likely associated with the GPS_HP above it. In other > words, ntp is configured to treat the HP GPS as two distinct > refclocks, a serial GPS timecode producer which will get you to the > right second, and a PPS source which will only become influential once > the time is within, uh, a half second I believe. On the other hand, > the oncore GPS refclock on the last line is a monolithic driver > providing serial timecode and PPS in one refclock, based on the > offset. > >> In fact all of >> your GPS refclocks show bad time What are you doing to your clocks? > > There are only two GPSes represented by the last three lines. All the > positive 700-900 usec offsets are nearby stratum 1 NTP servers across > the DSL divide, confirming the point that now matter now teeny your > packet is, it still takes 10 times longer to ride upstream 1.5M than > it does to ride downstream 14M. The difference are not that big for NTP packets over DSL for what reason I have not dived into. Assumming roundtrip to DSLAM are 20.6 ms and offset .75 ms gives up 11.05 and down 9.55 (roundtrip/2 +-offset) /hjj
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