Re: Using different timebase for ntpd

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. Truetime
was doing hardware gated ntp packets in the 90s. 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. Also, commercial product that does
hardware timestamping is available from us (Symmetricom). I'm sure
there are others out there. Brilliant Telecom has a ntp box that must
use hardware timestamping. Maybe Meinberg as well. Of course, where
the rubber really meets the road is recovering a signal. Most of the
products I mentioned above are optimized for generating a synch signal.
Their performance as client devices varies widely.

Symmetricom TimeHub NTP Blade
Symmetricom SSU-2000 NTP Blade
These are expensive solutions but they are both accurate and stable.

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. Also, ntp puts a
few more constraints on protocol operation and clock recovery than ptp.
While this lets it operate in more widely varying network environments,
it often restricts accurate clock recovery. If you get to hardware
timestamping, there are a few tricky things that arise. Depending upon
which 1588 profile you use (I use annex D), both ntp and ptp are layer 3
protocols. That means a hardware timestamp is really a layer violation.
PTP defines the timestamp as the beginning of the first bit following
the preamble. NTP doesn't define it (I've tried) but Dave has a page on
his website that talks about timestamping. He uses a different approach
(which corresponds to ietf ippm and ITU.T Y.1540) where the timestamps
are at the start of transmission and the end of receipt. These two
approaches have different strengths and weaknesses.

BTW, IEEE has just in the process of creating a study group to define
Ethernet timestamping. This is being driven due to the 802.1AS
community request.




-----Original Message-----
From: questions-bounces+gdowd=symmetricom.com@xxxxxxxxxxxxx
[mailto:questions-bounces+gdowd=symmetricom.com@xxxxxxxxxxxxx] On Behalf
Of Patrick Loschmidt
Sent: Tuesday, March 24, 2009 3:47 AM
To: questions@xxxxxxxxxxxxx
Subject: Re: Using different timebase for ntpd

Hi!

Thanks all for the answers so far.

David Woolley wrote:
As I understand it, he is replacing the kernel clock's role, so he
needs to replace all the system calls that read and discipline the
kernel clocks by ones that read and discipline the special hardware.

Correct. I have a PCI network card which runs it's own internal clock. I

made a literature research and couldn't find anyone implementing NTP
with hardware support for timestamping. Since I can modify the FPGA
based card, my first step is to get NTPd discipline the clock on the
card, that's why I have to replace all the system calls to the kernel
clock ...

Richard B. Gilbert wrote:
It might be useful to know exactly what hardware is in use. Some of
the
fancier clocks have provision for GPS input. I believe that both
Meinberg and Symmetricomm offer such. I believe that both companies
offer driver software for these clocks allowing you to read and set
them
and, perhaps, offering other functions as well.

Normally, I would just use a normal mainboard with a special oscillator,

but that doesn't solve my problem. As far as I could find out, nobody so

far modified the clock to be disciplined in NTP ...

Regards,
Patrick
.

Relevant Pages

  • Re: high precision tracking: trying to understand sudden jumps
    ... clients and the server support hardware timestamping of sent/received PTP ... To timestamp you have to read the computer's clock. ...
    (comp.protocols.time.ntp)
  • Re: ref clock for network POP locations
    ... I was referring to sub-microsecond accuracy over the network, it seems that the OP has no chance to use a hardware ref clock. ... PTP uses hardware timestamping at the MII/PHY level, this will not help when you use the Internet, but maybe the OP has his own nice WAN with low-jitter connections available. ... The mentioned LANTIME/NDT is basically an oscillator that is disciplined by NTP. ... I question whether it can maintain "sub microsecond accuracy" without the GPS discipline. ...
    (comp.protocols.time.ntp)
  • Re: Using different timebase for ntpd
    ... Interleaved broadcast mode with hardware derived timestamps and the NTP ... the NIC clock, while NTP disciplines a rotten quality computer clock. ... Dave Mills also ...
    (comp.protocols.time.ntp)
  • Re: 500ppm - is it too small?
    ... So machine running other than Windows don't suspend? ... broken hardware clock with software, ... Also, NTP (and possibly operating ... machines that use variable clock frequencies. ...
    (comp.protocols.time.ntp)
  • Re: NTP phase lock loop inputs and outputs?
    ... hardware can be up to 50 Hz off in normal conditions. ... NTP protocol and I'm trying to adapt some of the NTP algorithms to ... discipline an audio clock. ... NTP exchange says your clock is -20Hz off (and that's the only data I ...
    (comp.protocols.time.ntp)