Click here to enlarge imageBecause Gigabit Ethernet (GbE) was originally designed by IEEE 802.3ab to work over the installed base of Category 5. The IEEE (Institute of Electrical and Electronics Engineers) wanted to take advantage of the huge installed base of Category 5. "Hello Mr./Ms end-user, I have GbE and it will not require you to re-cable. All you'll need to do is buy my new electronics." Oops-well, maybe. At least it should have worked that way if everything was done correctly-five, six, seven or more years ago when the cable was originally installed.
The problem is, however, that there are no perfect installations; Murphy's Law is everywhere, especially on the job site during an installation. So, IEEE went back to the TIA/EIA and said, "Help! We've got GbE, which we thought would work over 'legacy' Category 5, but we have some concerns. 'Legacy' Category 5 is the term used to denote existing Category 5 cabling plant, which could have been installed last week or nine years ago." Enter TIA/EIA TSB-95," Additional Transmission Performance Guidelines for 4-pair 100-ohm Category 5 Cabling (October 1999). Testing of legacy Category 5 for GbE compliance added parameters for return loss, ELFEXT, and PSELFEXT.
Now, imagine you're standing on the edge of a cliff with a 1,000-foot drop (i.e., 1,000 Mbits/sec). Even if everything was done correctly with your legacy Category 5 install and it passes TSB-95, you're still standing with your toes literally hanging over the edge of the Gigabit cliff. There is no margin for error. Does anyone really want to install a system with zero margin?
Enter TIA/EIA 568-A-5 Category 5E (Addendum 5). This document was released February 2000 and included the TSB-95 parameters and some additional margin, increasing cable NEXT by 3 dB-from 32 dB to 35 dB at 100 MHz. Other parameters, such as PSNEXT, were also improved. The net result: instead of your toes hanging off the edge of the Gigabit cliff, you were now 3 feet back (i.e., 3 dB) from the edge. The system bandwidth, however, did not change; you still only had 100 MHz of frequency range-the same as Category 5.
Over the edge
With Category 5E, then, you are only cabling for today, supporting systems up to GbE and up to a maximum 100 MHz of bandwidth. Enter Category 6. The TIA/EIA goal was to double the bandwidth of Category 5 from 100 MHz to 200 MHz, specifically as it relates to power sum attenuation to crosstalk (PSACR). Additionally, as requested by the IEEE 802.3 committee, the TIA agreed to specify system and components to 250 MHz.
Today, there are discussions of 1.2, 2.4 and even 4.8 Gbits/sec protocols over copper, but it won't be over Category 5E. The medium will be Category 6 for a copper solution, or fiber. To go along with this, IEEE is developing a 10-Gbit backbone standard to support the existing 1 GbE horizontal application.
The reason to move to Category 6 is the same as why people installed Category 5 up to nine years ago. Ethernet 10Base-T was designed to run over Category 3, which was the first data grade UTP cable out of TIA/EIA 568 Commercial Building Telecommunications Wiring Standard (July 1991). As soon as Category 5 became available, the market shifted to it-except for those few individuals who put in Category 4.
Why choose Category 6 when all you need is Category 5E? The answer is the same as why put in Category 5 when Category 3 was all you needed. First, to deliver margin in the key transmission parameters of attenuation and NEXT. Second, to cable for emerging technologies and future applications. But you don't need to wait for the next killer application to justify the move to Category 6. At a recent presentation, a speaker pointed out that network use today-with shared files on a server or e-mails with attachments-already eats up too much bandwidth and speed.
His point was obvious: If you are cabling only for today, put in Category 5E. But if you are cabling for tomorrow's growing demands that are already having an impact on your network, and the inevitable emerging applications that will follow, then put in Category 6.
Gregory Niemiera, RCDD, is technical sales and marketing manager at Mohawk/ CDT (Leominster, MA), and a voting member of TIA/EIA-568 Commercial Building Telecommunications Cabling Standard. You can contact him via e-mail at [email protected].
