[ntp:questions] Re: ntp & adsl

Folkert van Heusden folkert at vanheusden.com
Tue Jan 31 16:48:00 UTC 2006


> > I've heard that NTP acts funny when it tries to sync over an ADSL
> > line.
> > That seems to be the case indeed:
> []
> > And there it is: 20ms offset. From what I've heard this is caused by
> > ADSL being asymetric.
> > Now I was wondering, can't this be fixed by telling the NTP daemon how
> > much asynchronous the line is? I mean: it can be easily measured what
> > the down- and upload bandwidth is.
> 
> Have you measured and removed the delay for the DCF77 device?  There 
> receiver bandwidth will cause a delay, as will the propagation delay of 
> the radio signal.

Well if I look in the comments of my ntp.conf it seems I looked into
that a while ago:
# DCF77 receiver
# "Der DCF77 Sender steht in Mainflingen (50 0 Nord, 09 00 Ost), etwa 25
# km südöstlich von Frankfurt/Main."
# http://www.maporama.com/
# Mainflingen: 50 1' 54" , 9 1' 33" 50.0318 , 9.026
# Gouda/2804 RR: 52 1' 37" , 4 42' 15" 52.0272 , 4.7044
# http://www.fcc.gov/mb/audio/bickel/distance.html
# -> 374.766km
# speed of light: 299 782.458 km/s
# 1.25012652ms delay

so I need to add an offset of 1.25ms somehow? still less then this 17ms
offset I saw.

# mode 14, RAW DCF at 50baud, is 258ms (RAWDCF_BASEDELAY in refclock_parse.c)

Do I also need to add an extra offset of 258ms since it takes the first
bit at least 258ms to get in?


Folkert van Heusden

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