Fiber only in DataCenters?

George Herbert george.herbert at
Fri Dec 21 18:54:23 UTC 2012

On Thu, Dec 20, 2012 at 3:20 PM, Matthew Kaufman <matthew at> wrote:
> On 12/17/2012 9:22 AM, joel jaeggli wrote:
>> If the facility is big enough the utility of twisted pair becomes quite
>> limited, both due to distance and differing electrical potential,
>> multibuilding campuses in particular make this is a nonstarter.
> For twisted-pair Ethernet: Distance yes. Differing electrical potential no.
> It is a balanced pair, transformer coupled at both ends. As long as AC
> common-mode pickup doesn't saturate the transformer core, it just works.

...Up to certain limits of DC / ground differential between the ends,
at which one can cause sparks anyways.

Yes, the POTS telcos use 48V in the same or lower quality wire pairs,
and the various CatN wires should be able to take it, and the
connectors.  I'm not sure whether the sparks were from 110 or 220 V of
differential, but I saw sparks.

>> In one facility I'm in, I'm over 300 meters from each of the MMRs, with
>> the results that the OOB for the serial console server for out equipment
>> located out there in the MMR's being on serial over fiber transceivers
>> connected by om4 multimode.
> RS232 serial is another story. Here the potential difference between the
> ends is a big deal. (I've even seen burned-through PC boards from what
> happens when pin 7 has 220 VAC flowing from one device to the other) But you
> can just run Ethernet out to the console server and plug it in next to the
> gear with the serial port to fix this.
> Matthew Kaufman

Ah, yes, those magic smokes.

-george william herbert
george.herbert at

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