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From: D Yuniskis on 23 Dec 2009 12:08
Hi,

Modern network infrastructure uses *lots* of buffering;
memory is (now) cheap enough to embed throughout the network
fabric.

With that, fine-grained synchronization over (wired) networks
becomes problematic -- there's no deterministic way for a
processor in a particular node to have any idea of its
relative packet time wrt any other node in the network
(though it is pretty obvious that a packet arrives at
its destination some time *after* leaving its source! :> )

Sure, things like NTP *try* to quantify this skew. But,
its goals are much more long-term... if it is wrong on
the short term, there is no significant consequence.
(I also suspect the apparent precision and accuracy that
NTP provides is largely delusional :-/ )

So, how *do* you achieve fine-grained synchronization
nowadays? What is *practical*? And theoretically
*achievable* (without an a priori characterization
of the network infrastructure and topology)?
From: Spehro Pefhany on 23 Dec 2009 12:13
On Wed, 23 Dec 2009 10:08:53 -0700, D Yuniskis
<not.going.to.be(a)seen.com> wrote:

>Hi,
>
>Modern network infrastructure uses *lots* of buffering;
>memory is (now) cheap enough to embed throughout the network
>fabric.
>
>With that, fine-grained synchronization over (wired) networks
>becomes problematic -- there's no deterministic way for a
>processor in a particular node to have any idea of its
>relative packet time wrt any other node in the network
>(though it is pretty obvious that a packet arrives at
>its destination some time *after* leaving its source! :> )
>
>Sure, things like NTP *try* to quantify this skew. But,
>its goals are much more long-term... if it is wrong on
>the short term, there is no significant consequence.
>(I also suspect the apparent precision and accuracy that
>NTP provides is largely delusional :-/ )
>
>So, how *do* you achieve fine-grained synchronization
>nowadays? What is *practical*? And theoretically
>*achievable* (without an a priori characterization
>of the network infrastructure and topology)?

Hey, Don:-

PTP.. IEEE-1588.

From: Vladimir Vassilevsky on 23 Dec 2009 12:10

IEEE 1588.

Basically, it is about getting/setting send/receive time for packets at
the physical layer, as opposed to going through a whole networking
protocol stack.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com




D Yuniskis wrote:

> Hi,
>
> Modern network infrastructure uses *lots* of buffering;
> memory is (now) cheap enough to embed throughout the network
> fabric.
>
> With that, fine-grained synchronization over (wired) networks
> becomes problematic -- there's no deterministic way for a
> processor in a particular node to have any idea of its
> relative packet time wrt any other node in the network
> (though it is pretty obvious that a packet arrives at
> its destination some time *after* leaving its source! :> )
>
> Sure, things like NTP *try* to quantify this skew. But,
> its goals are much more long-term... if it is wrong on
> the short term, there is no significant consequence.
> (I also suspect the apparent precision and accuracy that
> NTP provides is largely delusional :-/ )
>
> So, how *do* you achieve fine-grained synchronization
> nowadays? What is *practical*? And theoretically
> *achievable* (without an a priori characterization
> of the network infrastructure and topology)?
From: Spehro Pefhany on 23 Dec 2009 13:54
On Wed, 23 Dec 2009 11:10:14 -0600, Vladimir Vassilevsky
<nospam(a)nowhere.com> wrote:

>
>IEEE 1588.
>
>Basically, it is about getting/setting send/receive time for packets at
>the physical layer, as opposed to going through a whole networking
>protocol stack.

And figuring out between the nodes which node has the best clock, and
synchronizing the other nodes to the best clock with an algorithm that
rejects network communication jitter but compensates for long term
drift between clocks.

From: Paul E. Bennett on 23 Dec 2009 13:51
D Yuniskis wrote:

> Hi,

[%X ---- Stuff about NTP ---- X%]

> So, how *do* you achieve fine-grained synchronization
> nowadays? What is *practical*? And theoretically
> *achievable* (without an a priori characterization
> of the network infrastructure and topology)?

As well at the IEEE 1588 standard that has already been mentioned, you
should look at LXI also. This builds on IEEE 1588 and is used for
instrumentation purposes across networks.

See <http://www.lxistandard.org/>

--
********************************************************************
Paul E. Bennett...............<email://Paul_E.Bennett(a)topmail.co.uk>
Forth based HIDECS Consultancy
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-510979
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
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