Aug 1, 2001 12:00 PM
Cabling Distance Limit Longer
Just happened to read your article, “Microsoft's Multimedia Meeting Facility,” by Greg Doyle (May 2001). It is so rare that someone even mentions the wire and cable in these magazines. Funny, since more time is usually spent installing the wire and cable than anything else, and yet it is never acknowledged despite long lists of equipment chosen and installed.
There is one minor point I would like to mention, however. You say that the Belden 1694A is good for 100 meters at 1.5 GBps (HD). Our suggested distance, based on the SMPTE formula, is 370 feet (113 meters). This distance is, in fact, about half way to the digital cliff. So it is possible, given the state of the art in equipment at each end, that the distance might be close to double that, or 740 feet (226 meters). Once you are past our “safe” number, you just have to look at bit errors. If there aren't any, you're fine.
And, with the new generation of chips available today that can pull a perfect HD image out of a completely closed eye… well, I'm no longer sure what the distance limits are in the real world on any cable! Certainly, 100 meters is a very safe estimate for 1694A. I just wanted to be sure you knew you could actually go a lot farther.
Belden Electronics Division, via e-mail
Don't Overlook Fiber Optic Safety
On page 43 in “Going The Distance,” by John Lopinto (May 2001), the author asserts that there is no spark danger from a severed fiber optic cable. Essentially, this is true, but the broken ends can be extremely dangerous in other ways. Fiber optic systems that are intended to transmit data from floor to floor or building to building use lasers as the driving light source. Harm can easily occur from careless handling of severed or stripped fiber-optic cables.
Besides the laser beam, the glass fibers themselves are relatively fragile and may snap easily, offering a needle-like lance that may pierce the skin and cause chemical irritation and pain. Worse yet, that broken morsel of fiber is almost invisible, so it may ride home on a shirt collar and end up on a pillow ready to pierce an ear or eye. This is not an alarmist threat. I have talked with fiber workers who have had just that very mishap occur. The result was a visit to a hospital emergency room and a week off work. They were lucky that no permanent blindness resulted.
Not so fortunate are the individuals who, without pausing to think about the energy involved, wave that open end of fiber around (with or without a connector). Most splicing can be safely done on a hot, or energized, fiber, but proper precautions must be taken. A live laser can burn or blind at close range, and that is exactly what is coming out of the end of a broken or stripped fiber when the laser source is still energized. Remember, too, that the lasers in use are invisible to the eye, so a worker cannot see the beam exiting the end of the fiber, but it is present and “hot.”
I have seen communication lasers develop sufficient power through a single mode fiber to punch tiny holes in an aluminum chassis. Just think how much more fragile your skin or eye is — a pulse at the wrong time can cause permanent blindness. There are proper and safe ways to determine the activity and power of energized fibers. Perhaps the next time S&VC runs an article on handling fiber optics, you could ensure that the safety regulations and practices of the industry find their way into print.
S&VC welcomes your comments and ideas. Contact Nathaniel Hecht, 16842 Saticoy Street, Van Nuys, CA 91406; or firstname.lastname@example.org.