Contaminated cabling


I've been receiving inquiries from clients who are being urged by certain vendors to install Category 6 unshielded twisted pair (UTP) cabling. Of course, the vendors don't mention that there is no Category 6 standard right now -- the Telecommunications Industry Association and the International Organization for Standardization (ISO) are still in the proposal stage (ISO 11801) -- so whatever is installed will be proprietary.

If you're upgrading to Gigabit Ethernet, there will be an additional complication associated with Category 6. ITT Industries recently published a white paper that should be required reading for anyone planning Gigabit Ethernet deployment over Category 6 cabling. The white paper mentions a potential new source of noise, alien crosstalk, which is currently being discussed by the international standards committees. The presence of the noise would call into doubt Category 6 cabling's ability to deliver the enhanced bandwidth that it promises.

Category 6 cabling is four-pair cable in which each pair is wrapped with foil insulation. The four-pair bundle is then wrapped in more foil insulation, as well as a fire-retardant polymer jacket. With all that protection, perhaps we should call this "mummy cabling." In any case, twisted pairs are color coded for differentiation's sake.

Alien crosstalk is the interference caused when a wire pair in one cable induces noise in the wire pairs of adjacent "victim" cables. This interaction is always worst between pairs of the same color because these pairs' twist ratios are virtually identical.

Alien crosstalk has not been much of a problem yet because the cabling schemes have been designed to handle the data transmission rates with a minimum of interference. Crosstalk increases with higher throughput, though, which could prove to be a problem for Gigabit Ethernet because of the encoding method it uses. Gigabit Ethernet simultaneously transmits data on all four cable pairs and in both directions (full duplex mode). In addition, Category 6 UTP cabling systems are likely to be more susceptible to alien crosstalk noise because its cables are manufactured with tighter tolerances in order to handle traffic up to 250 MHz. That could increase the levels of induced noise between almost identical cable-pair twist ratios in adjacent cables.

ITT Industries reports that there are only three things that can control alien crosstalk:

  • A cable design that decreases the combined sources of noise and thus increases the overall signal-to-noise ratio. This would require changing the Category 6 draft specification limits.
  • Installation of cables in such a way as to preclude the use of bundled UTP cables, overfilling of conduits and trunking, and placement of parallel runs in close proximity. This is usually impractical.
  • The use of screened, balanced twisted-pair cables (such as those offered by ITT Industries -- imagine that). To be fair, vendors such as Anixter argue that their Category 6 cabling exceeds current specifications and can handle problems like crosstalk.

The third alternative makes it clear that the aforementioned white paper contains some bias, since ITT Industries reports that its own Category 6 cabling passed the alien-crosstalk noise tests. Still, alien crosstalk is an issue that the cabling standards committees are considering, and it is probably yet another reason to delay deployment of Category 6 cabling.

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