[time-nuts] Best Chance GPS module

Bob Camp kb8tq at n1k.org
Fri Dec 2 07:02:48 EST 2016


> On Dec 1, 2016, at 10:54 PM, Gian-Paolo Musumeci <gp at pdti.net> wrote:
> On Thu, Dec 1, 2016, at 09:01 AM, Chris Albertson wrote:
>> The other thing you might look at is NOT using NTP but using PTP.   This
>> might be a better match to your needs but it requires that you replace
>> all your network gear with equipment that can make hardware time stamps
>> on the network packets.
> You don't actually need to have PTP-capable network gear to make PTP
> work
> reasonably well.
> I have a small test environment with a Symmetricom S300-Rb PTP
> grandmaster
> distributing time to six Cisco UCS blades running FreeBSD. The S300 does
> hardware timestamping, but the UCS blades do not. The network has a
> Cisco
> UCS fabric switch plus an Arista 7124S, neither of which support PTP
> transparency. Works fine; I haven't measured it exhaustively, but
> preliminary
> data suggests that I am getting 2.5e-5 seconds of drift in the worst
> case.

NTP running on the same sort of network will deliver roughly the same performance.
There is very little asymmetry and the “source” is properly done. The net result
is roughly microsecond timing on a simple LAN. 

> I've heard reports that full hardware timestamping and transparent
> switching
> can easily get you into the 1e-7 range, but I haven't tried that yet.

With proper time stamping throughout and good hardware and a high enough packet 
rate you can get to the nanoseconds level with 1588. 


> /gp
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