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The UTP FDDI transmits on pins 1&2, of the RJ45, and receives on pins
7&8. The crossover is in the cable plant (i.e. one of the patch cords).
10Base-T uses pins 1,2, 3 and 6, with the crossover usually in the
repeater. So the DTE is transmitting on 1&2, receiving on 3&6, and the
repeater is receiving on 1&2, and transmitting on 3&6. If the crossover
is in the cable plant (as it was with the early 10BASE-T repeaters)
both ends will transmit on 1&2, and receive on 3&6.
So, there isn't any danger of a completed DUPLEX connection being formed,
as the pins don't form the required connections. This means that if a
10BASE-T station is connected to an FDDI port, no connection will
be formed, and the ring is protected (because the FDDI CMT protocols
won't allow the connection).
If a UTP FDDI station plugs into a 10BASE-T repeater, it is
possible that the FDDI transmitter could be seen by the repeater's
receiver (i.e. the pins would connect to each other). But, the
10BASE-T repeater will need to see linkbeat pulses in order for the
link to wake up, and, since the FDDI does not produce linkbeat pulses,
the repeater will also not recognize the station, so the 10BASE-T
network is protected.
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| Well, I was almost right..........
It turns out that my answer in -.1 is wrong for the case where the
FDDI UTP station is connected to a 10BASE-T repeater with all straight
through cables. The other cases are correct.
Specifically, if pins 1&2 of the FDDI station connect to pins 1 and 2
of a repeater with an internal crossover, the FDDI can open-up the
10BASE-T repeater, and affect the 10BASE-T network. The level of
junk varies by repeater type.
The oversight was that I forgot how linkbeat really worked. Linkbeat
is really only qualifying against a squelch threshold, not a recived
waveform mask. If there is enough power in the cable, the linkbeat
detector will open up (its only required to reject +/- 50 mV. So,
a +/- 1 Volt UTP FDDI signal will cause the 10BASE-T linkbeat circuit
to open up. The receiver will then produce random garbage that gets
repeated onto the network.
For the 2 repeaters we looked at, it appears that the receiver
opens up and produces very short events, which the repeater
extends to be 96 bit runts. So, on the backbone there are a lot
of runt packets. They were running around 1000/second in the tests
we ran. This will probably vary by repeater. Some repeaters will
partition, others will just keep repeating the junk. It cannot be
predicted in advance.
Sorry for any confusion,
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