FAQ: Building-Cabling Practices

FAQ Building Cabling Practices

Question: How are most new office buildings cabled?

Answer: In North America each work area has at least two outlets. All outlets are cabled with category 5e four-pair 100- W UTP. Most users install a mix of backbone cables, including category 5e cables for voice and local data services plus multimode and single mode fiber.

Question: Do customers ever mix different categories of cabling in the same building?

Answer: Confucian philosophers say, "Everything that can happen has happened ." That doesn't make it a good idea. When different categories are mixed in the same link, the guaranteed performance supposedly reverts to the lowest category. For example, if you install category 3 connectors (guaranteed to 16 MHz) with category 5e cabling (guaranteed to 100 MHz), the performance of the whole mess is only guaranteed to 16 MHz. If that bandwidth is adequate, it doesn't hurt anything. There are certain combinations, however, that don't mix. Category 6 and 7 connectors contain certain parasitic-coupling compensation features that may not interoperate properly with their lower-category counterparts. Plugging a category 6 jack into a category 5e receptacle, for instance, may degrade performance below the category 5e level.

When buildings are purposely wired with different categories of cabling to different offices, your installation people probably won't keep good enough records to remember which is which. It's best to keep the whole building at the same level to the greatest extent practicable.

Question: The customer doesn't have cable records. How can I find out what cable is installed?

Answer: Ask an installer to test the cable to see if it complies with any recognized category of cabling. A good installer should have equipment to verify compliance. If he or she does not have the equipment, find another installer.

Question: Why bother supporting older categories of cabling?

Answer: In certain ranges of speed (20 to 100 Mb/s, for example), the design of a category 3 transceiver can be challenging. Restricting your transceiver to operation only on cabling with category 5 or better performance would definitely simplify such a design. Regrettably, requiring category 5 or better cabling may also lock you out of that precentage of customer sites that are unwilling to upgrade their existing cabling plant from older category 3 cabling to the new standards. Your choice is therefore one of design difficulty versus the size of your total available market.

At present, customers installing new cabling choose category 5e (or higher). The proportion of significant users that retain category 3 cabling is dwindling; however, the practical life expectancy of building data cabling is 5 to 20 years , so it could take quite a while to change over the entire installed base.

Conventional wisdom among system developers holds that many, but not all, users who need bandwidth of 100 Mb/s or greater understand the need for category 5e cabling and will pay to have it installed. If your application demands the extra bandwidth, go ahead and specify good cabling. On the other hand, if your application needs to cover 99% of the installed base, and high bandwidth is not your first consideration, you are better off sticking with the older category 3.

The same considerations will apply at each successive transition as you work your way up from DIW through categories 3, 5, 5e, 6, and the new IEC category 7.