Up for Discussion

Before the last meeting of the TIA TR-42 committees that write cabling standards, a major fiber optic test equipment company asked if I would petition one of the committees to reconsider OFSTP-14, a new standard for testing installed multimode cable plants. The original version of this standard, dating to the late 1980s, was replaced two years ago by an ISO/IEC international standard that changed practically everything about it.

I was asked to do this because I was one of the original authors of OFSTP-14 and had already presented data at earlier TIA meetings questioning the appropriateness of this new standard.

The new OFSTP-14 standard allows optical time-domain reflectometer testing in addition to insertion loss testing and adds a new metric for measuring modal distribution, dropping the old method of using a mandrel wrap. It was also organized in an unusual fashion. The block diagrams of how you performed the tests were hidden in the back in the appendices, while the majority of the standard itself was devoted to trying to explain the very complex issue of encircled flux.
I agreed to take their request back to the committee but said I would query other fiber optic test equipment manufacturers about their understanding and support of this new standard. I talked to seven test equipment manufacturers: two supported the new standard, four disagreed with it, and one didn’t really understand it at all. I also talked to three fiber optic component manufacturers that were unconvinced it was possible to implement in the field.

When I reported back, well, let’s just say my comments were not well received. It turns out that standards are sometimes like bills in Congress, where it seems practically nobody reads them before they vote on them, only to deal with the repercussions afterward. That was certainly the case here.

However, my comments stimulated a lot of discussion that made obvious why standards may require some interpretation before practical use. Several discussion participants were involved in the original international (ISO/IEC) document that was approved some years ago. This document was aimed at a new generation of multimode cable plants that were being designed for 10G Ethernet (10 Gbps) and faster.

Multimode cable plants for 10G Ethernet are unique. They are probably the final generation of multimode cable plants that will ever be used for networking. At 10 Gbps, multimode fiber’s bandwidth limitation limits data links to no more than a 2-decibel (db) loss and a 300-meter length. Because of this limitation, companies, such as Google, are already moving to single-mode fiber for data centers.

However, testing multimode fiber for these short, low-loss links is tricky. The uncertainty of the measurement can be high unless the source characteristics are well controlled. Thus, this new standard focused on defining the source characteristics for these short, low-loss links.

If one stands back and examines this process, some problems with standards become apparent. Most important to note, manufacturers write standards for themselves to provide consistent specifications for their future products, and they need to ensure products from various manufacturers work together. Remember standards for Category 6 unshielded twisted-pair copper cables had to be negotiated for years before one could assume interoperability for different manufacturers’ products. The same process is going on right now for bend-insensitive multimode fiber.

But what happens to older products—networks made from products covered by earlier generations of standards or new networks that operate at speeds well below 10 Gbps that have higher losses? There are a lot of fiber optic networks in use today—and new ones being installed all the time—still using 62.5/125 multimode fiber that have loss budgets of 5–10 dB or more. This legacy fiber is even being removed from some newer standards because it will not support the high-speed systems for which these standards are being written.

Yet, some people are saying all networks must be tested using this new standard, which was not intended to apply to them. That makes no sense.

Perhaps it is time to reconsider what standards mean and for whom they are written. Manufacturers do not write standards for contractors, installers or users, at least according to a former head of the TIA standards committees. It’s the manufacturers’ responsibility to interpret the standards for their customers.

Other organizations—such as the National Electrical Contractors Association and The Fiber Optic Association—create standards with contractors and users in mind. Next month, we will look more at standards written for you.

HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.

About the Author

Jim Hayes

Fiber Optics Columnist and Contributing Editor

Jim Hayes is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.JimHayes.com.

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