RE: [802.3af] clocking over Ethernet
The work of this group is limited by the "Scope" and
"Purpose" of the PAR for P802.3af:
6. Scope of Proposed Project
Define methodology for the provision of power via balanced cabling
connected Data Terminal Equipment with 802.3 interfaces. The amount of
will be limited by cabling physics and regulatory considerations.
with existing equipment will be considered.
7. Purpose of Proposed Project:
To provide power for a new class of devices with 802.3 interfaces enabled
progress in silicon technology. These devices are characterized by low
requirements and LAN connectivity.
Your proposals go well beyond these limits and, rather than being a
benign add-on to existing twisted-pair Ethernet, would be a radical
change to the entire asynchronous nature of Ethernet as it currently
exists. It is very clearly not an appropriate addition to the current
project. for P802.3af which is about power.
If you feel that trying to add synchronous timing to Ethernet networks is
an appropriate proposal for an amendment to the IEEE 802.3 Standard then
the proper method for pursuing this is found at:
See section 7.1
At best, such a project would only give you timing control within
Ethernet links and across Ethernet MACs. The transit time of information
across Ethernet switches and routers is completely outside the scope of
Please move this discussion to another venue.
| Geoffrey O.
| Chair IEEE
| Nortel Networks, Inc. M/S
| 4401 Great America
| P. O. Box
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| Please see the IEEE 802.3 web page
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At 02:56 PM 7/31/01 +0100, Tony Bayley wrote:
I would also like a method of achieving
synchronisation to a few
microseconds via ethernet. However The power supply in power over
is not a good way to distribute timing information. Typically only
devices at the very edge of the network will be powered via MDI.
hubs, switches, etc nearer the network core are unlikely to be
via MDI, so the network-edge powered-devices will be in discrete
clustered around their own PSE, with no universal power over MDI
connectivity between them. Timing information distributed along
power over MDI, in this scenario, would only allow synchronisation
the PDs attached to a single PSE, and would not be scalable to
Principal Hardware Engineer
Botleigh Grange Office Campus
Tel: +44 (0) 1489 773983
Fax: +44 (0) 1489 773966
On Behalf Of
Sent: 31 July 2001 08:47
To: Steve Jackson; WM
Subject: RE: [802.3af] clocking over ethernet
the problem I see with NTP is jitter and wander that makes
it not really appropriate to generate a sync pulse
I like the proposal from Joerg; should be investigated
De : owner-stds-802-3-pwrviamdi@xxxxxxxxxxxxxxxxxx
la part de
Steve Jackson Envoyé : lundi 30 juillet 2001 14:49 À : WM Cc :
Objet : Re: [802.3af] clocking over ethernet
Your application may be best served by using the Network Time
It is fairly inexpensive to implement a small NTP client in
devices, and getting cheaper.
You may wish to look at this comprehensive URL:
and also read the archives at the Usenet newsgroup:
* * *
> Hello together,
> sorry for dissturbing. I know that this is maybe not the right place
> to ask
> for it but maybe here are the right people to give me a helpful
> answer. In principle it could be also a nice add on for
> Power over MDI is what we are looking for for several years and it
> a must for several applications like VoIP phones etc. but to fully
> utilize and replace ATM, ISDN or other media by ethernet I think at
> least for migration a way to synchronize devices would be usefull.
> independent clock source like GPS is not usefull in our application.
> Is there any way to clock or at least sync devices on the ethernet?
> One way
> would be to use e.g. the link pulses but switches don´t guarantee
> all ports are in sync. The power supply in power over MDI would also
> be a way to add a sync pulse.
> Many thanks in advance for your comments.
> Best regards
> Wilhelm Mikroelektronik GmbH
> Sueggelstr. 31 - 44532 Luenen
> phone ++49 2306 928280 fax: ++49 2306 928289