[rescue] Rescued Sun servers, now what?!
mouse at Rodents-Montreal.ORG
Mon Feb 20 15:57:29 CST 2012
>> (That's assuming it's a twisted-pair-Ethernet cable at all, of
>> course; while it's overwhelmingly likely it is, especially in view
>> of the markings you quote, there are a few other things RJ45-style
>> 8P8C connectors are used for.)
> PBX? ISDN?
Serial, in some cases; for example, a lot of cisco routers/switches use
an 8P8C serial console connector, and I've seen multi-port serial cards
that use 8P8C for their ports, presumably for physical space reasons.
And real copper PRIs are, in my (quite limited) experience, delivered
over the same connector, but with a different pinout (and vastly
different signal characteristics).
There probably are others, but those are the ones I've run into often
enough to remember.
>> A straight-through cable connects pin 1 on one end to pin 1 on the
>> other end, 2 to 2, etc, all the way up to 8 to 8; a crossover cable
>> connects 1 to 3, 2 to 6, 3 to 1, 4 to 4, 5 to 5, 6 to 2, 7 to 7, and
>> 8 to 8. (The peculiar-looking choice of wires to swap is of
>> historical origin [...])
> I'm thinking that the phrase 'twisted pair' explains it: the pairs
> are swapped, but the common wires are not.
That's a reasonable guess, though it happens to be false.
10- and 100-Mbit twisted pair use two pairs. One pair is on pins 1 and
2, the other on pins 3 and 6. The reason for this is historical.
Originally (back in the days when RJ still meant Registered Jack :),
the plan was to use the pairs in order from the inside out: first pair
on the middle pins, 4 and 5, second pair on 3 and 6, then 2 and 7, then
1 and 8. This works fine for POTS telephony. But it turned out that
the 2-&-7 and 1-&-8 pairs violated the constraint on how much you could
untwist the wires for Ethernet, so for twisted-pair Ethernet use those
pairs got put on 1-&-2 and 7-&-8 instead. 10- and 100-Mbit use the
1-&-2 and 3-&-6 pairs, and a crossed cable just swaps the one pair with
the other - hence the DC connection characteristic I described. THe
4-&-5 and 7-&-8 pairs are completely unused for 10 and 100; they
occasionally get used for DC power, or for a second Ethernet (you can
carry two 10 or 100 links over a single run of CatV, though it requires
care to get the connectors right on the ends), but in most cases they
just sit there unused. I've seen a few particularly cheap cables that
don't even have the contacts for pins 4/5/7/8. Gigabit depends on
having all four pairs (and has stricter constraints on the twisted
It is important to get the pairs right, though a lot of equipment is
surprisingly tolerant. A friend of mine told me of a cable he had in
his house which worked fine at 10 but produced lots of errors at 100;
turns out he had wired it up with the pairs on 1-&-2, 3-&-4, 5-&-6, and
7-&-8, so one of the pairs he was using was not a proper twisted pair,
instead being one wire from each of two pairs. 10Mbit was tolerant
enough to let him get away with it for the length of cable he had, but
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