[ntp:questions] Using different timebase for ntpd
Patrick.Loschmidt at gmx.net
Wed Mar 25 11:47:58 UTC 2009
Thanks a lot for your detailed answer!
Greg Dowd schrieb:
> Most if not all commercial ntp appliance manufacturers have some sort of
> hardware support, either RTOS, custom clock, better oscillator or even
> modified Ethernet devices to support hardware time stamping.
Well I'm mostly interested in systems doing hardware time stamping on
the Ethernet interface. My intention to just use another timebase are
just an intermediate step to ease my development.
> I tried modifying one
> of our SyncServers and gave a presentation at ietf tictoc bof on
> precision frequency transfer over packet based networks. Dave Mills also
> did some experimentation with this. While I used mode 3/4 with a
> lagging timestamp for the followup function, Dave restricted that
> operation to symmetric modes. I have another presentation from a sync
> conference last year comparing the two protocols. Let me know if you
> are interested and I'll dig them up.
Yes, please, I'm very much interested, since I did an extensive search
via IEEE Explore, ACM Library, google, etc. and couldn't really find any
results or even products explicitly stating that they use hardware support.
> In your case, you are not talking about hardware timestamping but
> software timestamping using a hardware clock. It is a good first step
> but os/stack jitter is still likely to be dominant.
Thanks for all the further hints. I have to admit, that I'm working in
the IEEE 1588 community for quite a while. Since I'm currently compiling
my PhD, I asked myself the basic questions like, why didn't we just
enhance NTP with HW timestamping to reach the accuracy?
So I came up with that idea, searched for implementations or scientific
papers giving results, reasons, etc. but had not much luck.
I found "A brief history of NTP time: Confessions of an Internet Time
Keeper" by David Mills, mentioning the Nanokernel and some fancy
synchronization results with 50ns RMS but I couldn't dig out the cited
reference, since the cite is incorrect ... Also proceedings of the PTTI
didn't help much. :-(
So, what I still would like to have is either documentation of someone
who already did NTP with hardware timestamps on Ethernet or can I
somewhere find a documentation of the NTP-Daemon to kernel clock
interface, in order to replace all the necessary system calls?
Anyway, I'll dig through the code an all your hints. Thank's a lot for
all the helpful answers in this newsgroup.
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