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Re: Cross or straight through [7:85929] posted 03/18/2004
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""W. Alan Robertson""  wrote in message
> I can't believe this thread is still alive!!
> Ok, for a bi-directional fiber connection, you need a pair of fibers
> (That is 2 strands).  One of the strands connects to the transmit port
> on the first device, and to the receive port on the second device...
> The other strand connects from the transmit port on the second device,
> and the the receive port on the first device...
> On so called "patch" cables, where the twp SC connectors (SC are the
> "push/pull", vs. the ST "push/turn" type) are clipped together, they
> generally cross the fiber strands for you.

Looking at the fiber patches connecting two 3550's via gbics - nope - not
crossed over. YMMV, I guess. :->

> All fiber connections behave in this manner.  We don't see a fiber
> "crossover" cable because with fiber, EVERY connection is a crossover.

perhaps if you sketched this out so challenged folks like myself can
understand. EG:

3550-------fiber patch panel----------building riser--------fiber patch
      patch cable                |------ single run - not effected -----|
patch cable

are you saying the installer reverses the fibers at each patch panel?

In my early experiences with fiber ports on Cisco boxes, I can recall having
to do hit or miss swapping the connectors to the patch to get an interface
up. It may be that I even had to "break" the connector, as I do recall that
the early SC connectors were snapped together.

On GBIC,s I don't see how the connecters can be snapped together there is a
good half inch between the centerpoints of each GBIC port ( the xmit and rx
holes )


> Copper networking is different.
> Switch/Hub ports are pinned out one way, and host ports (Where host is
> any non-switch/hub device, Servers, Workstations, Routers, Firewalls,
> etc.) are pinned the opposite way.  The "crossover" happens in the
> pinning of the ports themselves, so that with a straight through cable,
> the transmit pair of the connecting device maps to the receive pair of
> the infrastructure device, and vice versa.
> When connecting a Host to another Host, or a Switch to another Switch,
> we need to provide the crossover in the cable because similar devices
> are pinned in an identical manner. With a straight through cable, TX
> would match to TX, and RX would match to RX, leaving no joy.
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