[ntp:questions] is USB inherently evil, or only if something else is on the bus?

Steve Kostecke kostecke at ntp.org
Wed Aug 8 22:59:17 UTC 2012


On 2012-08-08, Rick Jones <rick.jones2 at hp.com> wrote:

> I've scrounged a couple old Pentium M laptops (Hewlett-Packard nc6000s
> for the trivia-minded) with serial ports while entertaining the
> possibility of undertaking a project to do what so many others have
> done - connect-up a GPS receiver with PPS support.  That has me
> wondering about some of the previous discussion about USB and how it
> is perhaps not "evil" but considered quite sub-par for serving-up the
> PPS signal.
>
> Is that unsuitability inherent in USB, so it matters not whether there
> is anything else on the USB, or is it more a case of being "bad"
> generally only when other things are on the same USB?  I'm still
> looking to go serial, but was wondering.

USB is packetized.

> Also, speaking of things considered "bad" and drifting - fudging the
> LOCAL(0) is definitely frowned upon right?

The Undisciplined Local Clock (127.127.0.x aka "LOCAL") has a default
stratum of 5. It is often recommended to fudge the Undisciplined Local
Clock farther away from possible strata at which real time sources
usually operate (e.g. to stratum 10).

> If I happen to have say four servers in a location which might loose
> its connectivity to the outside world I probably don't want those
> servers to fall-back on LOCAL(0) right?

This is an issue of systems architecture. Setting up all of your servers
with the Undisciplined Local Clock at the _same_ stratum is a recipe for
disaster because there will be no local master. You really only need
one system in that cluster to be your "Time Island Master". The system
with the most stable clock should be the only one configured with the
Undisciplined Local Clock driver.

> Would configuring each to have the other three as "peer" entities be
> the way to go?

I would configure the other three systems to be clients of the "Time
Island Master". 

A more elegant solution is to set up your systems in an Orphan Mode
"Mesh" (i.e. each node is a Server and a Client for whichever mode you
choose.) Orphan Mode allows your group of servers to autonomously select
a leader in the event no real time sources are reachable.

Orphan Mode can be used with any association mode; broadcast, multicast,
and manycast have the advantage of allowing you to use a common
configuration file across all nodes and make adding/removing nodes a
no-brainer.

Automatic Server Discovery
http://doc.ntp.org/4.2.6p5/manyopt.html

Association Management
http://doc.ntp.org/4.2.6p5/assoc.html

Miscellaneous Options
http://doc.ntp.org/4.2.6p5/miscopt.html

-- 
Steve Kostecke <kostecke at ntp.org>
NTP Public Services Project - http://support.ntp.org/



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