Q: When testing a Category 5 cable run we read a star pass on the remote near-end crosstalk. The margin was 1.7. Is this acceptable? The cable manufacturer said that even if the margin were 0, the test would be satisfactory.
Harold Hoffman
Hoffman Electric
Lake Ariel, PA
A: The cable manufacturer is correct in that the cable passes the telecommunications systems bulletin tsb-67 criterion, even down to zero margin.
In the years since tsb-67 was first published, many Category 5 cable constructions have been brought to the market. Variations in jacketing material, conductor insulation, and physical shape have created performance inconsistencies between different Category 5 cables.
"Meets or exceeds Category 5" can be seen in ad after ad--for cables that just pass Category 5 to cables that exceed two times the Category 5 requirements. So how can you tell? Attenuation-to-crosstalk ratio (acr) has become the benchmark. acr is determined by the difference between near-end crosstalk and attenuation at specific frequencies. As an example, let`s use the worst-case numbers from tia/eia-568a; a cable with 32-decibel crosstalk and 22-dB attenuation at 100 megahertz would have an acr value of 10 dB (32 - 22 = 10) at 100 MHz.
Currently the Gigabit Ethernet Alliance of the Institute of Electrical and Electronics Engineers (ieee--Washington, DC) is developing a gigabit standard titled "1000Base-T for utp," targeted for completion in September 1998. The emerging gigabit standard for Ethernet will be based upon generic Category 5 cabling. The ieee standards committee evaluated two proposals: (1) to support horizontal distances of 50 to 60 meters with inexpensive electronic hubs or (2) to support 100 meters with expensive and complex electronic hubs. They decided to support proposal (2). Studies have shown that Category 5 performance is inadequate for robust 100-meter solutions using inexpensive electronic hub technology.
Donna Ballast is a communications analyst at The University of Texas at Austin and a BISCI registered communications distribution designer (RCDD). Questions can be sent to her at:
Cabling Installation & Maintenance or at PO Drawer 7580,
The University of Texas,
Austin, TX 78713;
tel: (512) 471-0112,
fax: (512) 471-8883,
e-mail: ballast@utexas.edu.
