Is your box filled with gems or junk? - fiber optic network system box - Industry Trend or Event

The testing of the fiber optic components inside fiber optic data network system boxes is critical to the data network's overall performance.

To ensure that new fiber optic networks perform as quickly and efficiently as possible, IT managers conduct several tests on network components, including cables, wall outlets, breakout boxer, and connectors. For mission-critical, time-sensitive applications, they will also search high and low for the best-performing data network systems available.

Putting together these system is no simple task, and much of it depends on using the right components inside the box. There are several components that must perform optimally for the system to perform optimally in a network. These components are typically connectors, connector adapters, and transceivers.

Unlike copper technology, optical fiber technology does not have several decades of history behind it, which makes it difficult for IT managers to project how components will perform under different conditions. Instead, component suppliers establish their own labs for testing component performance and make the results available to equipment designers to help them make comparisons to other components.

To identify procedures and criteria for data network system components, these test labs begin with standards and suggested test procedures specified by national and international standards bodies such as the Telecommunications Industry Association/ Electronic Industries Association (TIA/ EIA) and the International Electrotechnical Commission (IEC, http://www.iec.ch). Bellcore (http://www.bellcore.com) suggests standards for service-provider and telecommunications networks.

Then, to ensure that the components meet or exceed more stringent demands made by both the market and customers, product engineers are often forced to create their own set of performance specifications.

In some cases, the lab also may test the components against specifications set by other industry forces such as leading data networking equipment providers. As a result, components in the industry are tested against different sets of specifications, and it can become difficult to make comparisons between components from different suppliers.

In addition, use of a component based on misleading performance results may negatively affect performance of a system, and may even render a system nonfunctional. It is critical for both the equipment manufacturer and the IT manager to have an understanding of test procedures to avoid varied or misleading performance results.

Single mode fiber systems are most commonly used for cable television and telecommunications. IT managers will find single-mode fiber in intrabuilding campus wide-area networks and even in backbones.

Because most of the network for these applications must endure outdoor conditions, single-model fiber components and equipment must be able to withstand harsh environments and mechanical stresses.

Multimode systems are most commonly used for data network equipment and short-haul communications, such as within an office building. Because these environments are more controlled and predictable, environmental and mechanical stress testing is not as strenuous. Terminating devices are less expensive and are usually simple in design. The larger core size of multimode fiber makes it easier to connect and gives it greater tolerance of components with less precision.

Test labs will run several tests for performance under different environmental and mechanical conditions. Environmental tests can include temperature cycling, thermal aging, thermal shock, and humidity. Mechanical testing can include stresses such as physical shock, vibration, tension, compression, durability, twist, and flexing.

While the performance of the component can be approximated, the conditions to which the components will be exposed is unpredictable. When systems are available throughout an industry in various applications, the test lab must guess the conditions to which the systems will be exposed. Factors such as temperatures, number of mating cycles, and other performance items may not be predictable, so testing of the component is difficult to plan.

Knowing the conditions under which the equipment will function is essential to choosing the proper components to go inside the box, and is therefore essential to choosing the proper box for use within a network.

Important criteria for fiber optic components are:

* Insertion loss. Insertion loss is the optical power lost due to the insertion of an optical component such as a connector or splice. It is taken as a baseline measurement to determine the performance of a component or cable.

Several factors can create different insertion loss results. One factor is fiber core size mismatch. If light is traveling from a smaller fiber core into a larger fiber core, the observed insertion loss may be very near zero because the wider receiving fiber acts as a "bucket" to capture the light from the smaller fiber core. However, light traveling in the opposite direction may yield a higher insertion loss because the smaller fiber core is not able to accommodate the light from the larger fiber core.

While this should not be a common problem, IT managers may want to be concerned with the potential for fiber core size mismatch in their networks. Using cables with fiber of the same core size will provide optimum performance.

The "system" light source is also a critical factor, especially in multimode systems. A light source that underfills a multimode fiber will often yield lower insertion loss (and give the perception of better performance) than a light source that overfills a fiber. Under certain conditions, it may be necessary to mimic the system source launch characteristics during testing using a device such as a mandrel wrap. To permit reasonably accurate comparisons among competitive products, follow industry-standard launching conditions.

* Change in optical transmittance. After insertion loss is measured, test labs run various environmental and mechanical tests on the components. After each test, and often during testing, the lab again measures the insertion loss and compares this with the baseline insertion loss to find the change in optical transmittance.

* Reflectance. When light traveling through a fiber hits a reflective surface, such as a connector endface that is open to air, some energy bounces back to the source. This energy is referred to as the reflectance and is especially troublesome to laser-based sources.

While reflectance has been an issue in single-mode systems for some time, it has not been greatly considered in multimode systems. This is not because reflectance is not an issue, but rather because reflectance in a multimode graded index medium does not lend itself easily to automated multichannel testing.

However, component suppliers and the TIA are working to create a technically sound measurement process for reflectance. Within five years, reflectance in multimode systems will likely be measured as regularly and rigorously as insertion loss and change in optical transmittance.

To begin to gain an understanding of issues involved, study the standards specified by the TIA, the IEC, and Bellcore. Be aware of the environmental and mechanical conditions to which your systems will be exposed to en , sure that the equipment manufacturers focused on factors that are important to your particular environment.

Be sure to keep your equipment manufacturer informed of how the systems perform in the field. Feedback from end users is the primary element in determining test procedures and specifications, and designers and test facilities need to be made aware of how well--or how poorly--their tests reflect real-world conditions.

David W. Fisher is the supervisor of quality control/design assurance for the global optical cable and accessories division of AMP Inc.

COPYRIGHT 1998 Nelson Publishing
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