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g/networking • Posted by u/cat6_carlos •

Certified Cat6A that passes but fails in service — alien crosstalk in a tight bundle Structured Cabling

Hospital job, 340 drops of Cat6A. Every drop certified and passed on a DSX-8000. Customer stood up 10GBASE-T and about thirty ports in one area are negotiating at 1G or throwing CRC errors.

The drops that fail are all in one bundle that runs about 190 feet through a corridor. All of them passed permanent link certification individually.

Certification passing and service failing on the same cable is the thing I do not have a good explanation for.

💬 6 replies

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u/admin_jake ADMIN •
✓ Accepted Solution

The reply above has the mechanism exactly. Adding the practical part.

Alien crosstalk (PSANEXT and PSAACRF) is measured between cables, requires a disturber and a disturbed cable, and is not part of a normal cert. The DSX-8000 can do it with the right adapters and it takes a long time, which is why nobody does it as standard practice.

You do not need to measure it to fix it, though. The fix is mechanical:

  • Loosen the bundle. Tight-bundled Cat6A at 190 feet is worst case. Cables in sustained parallel contact over long distances is what couples them. Break the 30-cable bundle into groups of 6 to 8 with air between the groups.
  • Do not cinch. Velcro, dressed loose, never a zip tie pulled tight. A tight tie deforms the jacket and pulls conductors closer between adjacent cables.
  • Avoid long parallel runs of the same pair geometry. Some manufacturers' Cat6A uses jacket geometry or a shield specifically to control this. If you are re-pulling anyway, shielded F/UTP eliminates alien crosstalk essentially entirely.

Before you re-pull anything: go untie that bundle and separate it by hand in the worst area, then retest in service. On a 190 foot run in a corridor you can usually get enough separation to fix it without pulling a single new cable. I have seen this exact fix work more than once.

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✓ Solution
u/cat6_carlos OP •

Went and opened the tray this morning. It is one solid bundle of thirty, zip tied every three feet, pulled tight enough to flatten the jackets visibly at each tie.

Cut the ties, hand separated into four groups with maybe two inches between, re-dressed with velcro. Twenty-two of the thirty ports came up at 10G immediately. Working on the rest.

No new cable. Three hours of work on something I had already certified as passing.

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u/rack_and_stack_rob •

The flattened jackets are the tell and they are visible from across the room once you know to look. A zip tie tight enough to deform the jacket has changed the cable's electrical characteristics. It is no longer the cable that was tested and listed.

Velcro only. There is no application in structured cabling where a tight zip tie is the right answer.

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u/fiber_splicer_faye •

Standard permanent link certification does not test alien crosstalk. It tests the cable against itself. Alien crosstalk is coupling between adjacent cables in a bundle and it is the specific failure mode 10GBASE-T is sensitive to.

Your bundle is the problem, not any individual cable.

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u/fiber_fred •

Or price fiber. At 190 feet with 340 drops in a hospital, the delta between Cat6A done properly and OM4 with media converters at the edge is smaller than people assume, and it does not have this failure mode at all.

Not always the right answer. Worth running the number though.

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u/wireless_wendy •

Worth knowing for anyone quoting 10G work: if the customer specifies 10GBASE-T over copper at long lengths in dense bundles, price alien crosstalk testing or price F/UTP. The certification everybody assumes covers this does not, and finding out in service is the expensive version.

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