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I was recently installing some servers, and making the cables the right length so that everything in the rack was tidy.

2 out of quite a few links failed to operate at gigabit, degrading to 100mb.

All cables had been tested, the thing that these 2 cables had in common, was that they were 60cm long, shorter or longer ones worked fine. Thinking about it, 60cm is the wavelength of a 125MHz gigabit signal in copper, so the 60cm cable may be something of a resonator.

Are there specific cable lengths that we are supposed to shy away from using?

camelccc
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3 Answers3

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1000Base-T has no minimum cable length requirement.

60cm Gigabit cables are widely available. There's no particular reason a cable of that length would cause a problem. I suspect that your mapping of pins to wire pairs is incorrect. Most cable testers only confirm that each pin is wired to the corresponding pin on the other side. They don't check whether signal pairs map to wire pairs which is required for the cable to work reliably.

David Schwartz
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Interesting question (and the resonance idea reminds me of the tesla coil I built many years ago).

Same cable length may suggest that the two devices are similar to one another. Is it possible that a device or switch port setting is hard-set to 100mpbs? (swap in a 'working' gigabit cable and see if the link speed increases).

Other ideas:

10BASE-T and 100BASE-TX use two pairs (4 wires of the 8 available) and 1000BASE-T/1000BASE-TX use three pairs (6 wires of the 8 available). Are you positive that you tested the cables for three corresponding pairs?

I believe CAT5 wire spec calls for a minimum bend radius of about an inch; if these cables are shorter, perhaps they are making some tight corners, in which case interference is more likely.

Tony
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Gigabit Ethernet work on cables with any length, witch is less then 100 meters. There are two ways to spot the reason of your trouble. 1. Two cables are bad and you must renew them. Because Ethernet 100 Mbit/s works on two pairs and Ethernet 1000 Mbit/s works only on 4 pairs. 2. There is a network problem. It may be port speed settings, link aggregation, STP or something else.

Mikhail Khirgiy
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