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Sorting Out Cabling Standards February 7, 2000 By Joel Conover Premise wiring is easily the longest-lasting investment you'll make for your infrastructure. Switches, routers and hubs have an average life expectancy of five years, but building wiring lasts 15 to 20 years. Cable manufacturers leverage this statistic when they claim that cabling costs encompass less than 5 percent of the total cost of your LAN. Does that mean you should run out and buy the latest and greatest Category 7-rated wiring system for your new or newly retooled building? Absolutely not. There is a lot of hype about cabling systems that offer superior frequency range (MHz), cross talk, attenuation and delay parameters, but 95 percent of it is just that--hype. While we could write a book about twisted-pair parameters and their effects on data transmission, that really wouldn't solve your basic problem. Instead, we offer this practical advice: Stick to the standards. On December 13, 1999, the Category 5E specification (TIA/EIA-568A-5) was officially ratified as a standard. This specification provides "extended" parameters for Category 5 wiring, including an extra three dB (decibels) of headroom (the amount of theoretical bandwidth your installation can support across the entire frequency domain up to 100 MHz) in several critical cross-talk parameters. The specification also enforces several attributes that were optional in the original Category 5 specification. The Category 5E specification does not provide for additional frequency bandwidth, thus it is likely that the highest data rate allowable for 100-meter applications will be 1 Gbps in full-duplex operation. In short, you should use Category 5E cabling in all new installations. It is absolutely critical that you use certified Category 5E equipment from end to end. Don't hesitate to ask whether your supplier sells TIA-568A-5-compliant hardware. Likewise, vigorously question your test-equipment manufacturer regarding its hardware's compliance with the specification: Just because a product claims to test Category 5E equipment doesn't mean the test equipment is up to specification. And finally, have your LAN professionally installed and tested. Remember, you should expect to be using this wiring for the next 15 to 20 years. It would be a damn shame to have to rip it all out and rewire in five years because your installer wasn't certified to install Category 5E cabling. Insist on headroom reports on all cabling installed in your facility (more on headroom below); pass/fail simply is not good enough.
Do You Need To Upgrade? Category 5E was developed to handle high-speed data-transmission technologies, such as Gigabit Ethernet over copper. If you use Category 5 to deliver bandwidth to the edges of your network, you might be a candidate for Category 5E. However, this is not always the case. The TIA (Telecommunications Industry Association) TSB-95 bulletin contains recommendations for run- ning Gigabit Ethernet over existing Category 5 installations. If your cable plant is relatively new (or if you are particularly lucky), you may not need to make any changes to your infrastructure to deliver Gigabit Ethernet over copper.
Maximum Headroom Category 5 cables have electrical properties that can be measured in the frequency domain. This is what any number of $5,000 cable testers can do for you in a matter of seconds. These testers minimally measure attenuation, near-end cross talk (NEXT), attenuation-to-cross-talk ratio (ACR), delay and delay skew. All the values shown in the chart "Wiring Terminology" contribute to the amount of headroom in your cables. The important fact here is that you can measure these values and compare them to the recommendations in TIA TSB-95. If your cabling plant falls within the recommendations, you're good to go for Gigabit Ethernet. If it does not, there could be several reasons: Your cabling may be of poor construction or old enough that these parameters weren't a concern. Your cross-connects could be non-Category 5-compliant. Or your installation may have been poorly pulled. Because so many of these parameters depend on the exact spacing and number of twists per inch in a cable, an improper installation--one that was pulled using too much force, for example--may have stretched your cables to their limits, literally. The TIA has drafted new standards for Category 5 cabling that include more stringent measurements of ELFEXT (equal-level far-end cross talk), powersum measurements and return loss. These new standards are likely to become the de facto standards for Category 5 installations, and many cable vendors meet and exceed these standards today.
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