[time-nuts] IEEE1588

[Bob Camp]

Hi

If you are running the newer version of 1588 *and* have good 1588 switches /
hubs, you can indeed run at sub microsecond levels. The same is true of
routers, again if they are designed with 1588 goodies inside them. Neither
box is a common item. The routers are even less common than the switches. 

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Chris Albertson
Sent: Wednesday, February 01, 2012 12:52 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] IEEE1588

On Wed, Feb 1, 2012 at 9:20 AM, Bob Camp <lists at rtty.us> wrote:
> Hi
>
> The "gotcha" with a public grandmaster is routing to it. Without 1588
> routers / hubs / switches / what ever, the result is compromised. You fall
> back into the same routing delay mess as NTP. Since public pretty much
means
> internet accessible, you would need to upgrade a lot of stuff. Since that
> includes the traditional "last mile" gear, it's not a simple proposition
at
> all. At least in my case, I get more delay from the ends of most
connections
> than I get from all the routing in-between.

I think except for testing you need the grand master to live on your
own Ethernet.   And I mean "Ethernet" in the most precise way.  Two
Ethernets with a router between them is not an Ethernet it is two
Ethernets with a router.     sub-uSec synchronization is very hard
over over any network if packets get stored and forwarded and that is
what routers do.

NTP is better if the network path is complex and you don't have
end-to-end control over all the equipment.

I think it's best to start from requirements rather then starting from
solutions.   But if your requiremt is to get time to within a few uSec
and all the machines are in the same lab PTP can work.

Next you have to figure out some way to TEST is the machines are all
in fact in sync at the uSec level and if yu are using PTP the test
should not also use PTP.   Testing, I think is the hardest part.


-- 

Chris Albertson
Redondo Beach, California

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