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RE: [802.3af] clocking over Ethernet

The topic of accurate clock distribution is one that has not been covered
well by 802, in my impression. While I firmly agree that it is beyond the
scope of the P802.3af charter, I believe the discussion was useful for
helping to identify the problem and brainstorm on possible solutions.
The censureship seemed somewhat premature, as the possibilities
had not been fully explored. I know that P1394 also considered sending
clock-sync over the power, in some meetings. In their case, I think
that particular technical solution was abandoned for technical reasons,
but the thought process was productive and spawned further useful
David V. James, PhD
Chief Architect
Network Processing Solutions
Data Communications Division
Cypress Semiconductor
110 Nortech Parkway
San Jose, CA 95134
Work: +1.408.942.2010
Fax:  +1.408.942.2099
Work: davidj@xxxxxxxxxxxxxxxx
Base: dvj@xxxxxxxxxxxx
-----Original Message-----
From: owner-stds-802-3-pwrviamdi@xxxxxxxxxxxxxxxxxx [mailto:owner-stds-802-3-pwrviamdi@xxxxxxxxxxxxxxxxxx]On Behalf Of Geoff Thompson
Sent: Tuesday, July 31, 2001 10:01 AM
To: Tony Bayley; Raymond.gass; 'WM'
Cc: stds-802-3-pwrviamdi@xxxxxxxx; Tony Jeffree; bob.grow@xxxxxxxxx; davel@xxxxxxxxxxxx; David_Law@xxxxxxxxxxxxxx
Subject: 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 to
connected Data Terminal Equipment with 802.3 interfaces. The amount of power
will be limited by cabling physics and regulatory considerations. Compatiblity
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 by
progress in silicon technology. These devices are characterized by low power
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 IEEE 802.3.

Please move this discussion to another venue.


Geoff Thompson

| Geoffrey O. Thompson                           |
| Chair IEEE 802.3                               |
| Nortel Networks, Inc.  M/S SC5-02              |
| 4401 Great America Parkway                     |
| P. O. Box 58185                                |
| Santa Clara, CA 95052-8185  USA                |
| Phone: +1 408 495 1339                         |
| Fax:   +1 408 495 5615                         |
| E-Mail: thompson@xxxxxxxx                      |
| Please see the IEEE 802.3 web page at          |
| To download your FREE copy of Std. IEEE 802.3  |

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 MDI
is not a good way to distribute timing information.  Typically only the
devices at the very edge of the network will be powered via MDI.  The
hubs, switches, etc nearer the network core are unlikely to be powered
via MDI, so the network-edge powered-devices will be in discrete groups
clustered around their own PSE, with no universal power over MDI
connectivity between them.  Timing information distributed along with
power over MDI, in this scenario, would only allow synchronisation of
the PDs attached to a single PSE, and would not be scalable to larger

Tony Bayley
Principal Hardware Engineer

Bluesocket Limited
Jellicoe House
Botleigh Grange Office Campus
Grange Drive
Hedge End
SO30 2AF
United Kingdom

email:  tbayley@xxxxxxxxxxxxxx
Tel:  +44 (0) 1489 773983
Fax:  +44 (0) 1489 773966

-----Original Message-----
From: owner-stds-802-3-pwrviamdi@xxxxxxxxxxxxxxxxxx
[mailto:owner-stds-802-3-pwrviamdi@xxxxxxxxxxxxxxxxxx] On Behalf Of
Raymond Gass
Sent: 31 July 2001 08:47
To: Steve Jackson; WM
Cc: stds-802-3-pwrviamdi@xxxxxxxx
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



-----Message d'origine-----
De : owner-stds-802-3-pwrviamdi@xxxxxxxxxxxxxxxxxx
[mailto:owner-stds-802-3-pwrviamdi@xxxxxxxxxxxxxxxxxx]De 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 Protocol

It is fairly inexpensive to implement a small NTP client in Ethernet
devices, and getting cheaper.

You may wish to look at this comprehensive URL:

and also read the archives at the Usenet newsgroup:


Steve Jackson
* * *

WM wrote:
> 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 802.3af.
> Power over MDI is what we are looking for for several years and it is
> 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. An
> 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 that
> 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
> Joerg
> --------------------------------------------------
> Wilhelm Mikroelektronik GmbH
>    Sueggelstr. 31  -  44532 Luenen  -  Germany
>   phone ++49 2306 928280  fax: ++49 2306 928289
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