Small form factors are critical solutions for connectivity

The latest generation of connectors promises significant improvements for premises applications. But how do you choose which one is best for your network?

This is the premier of a monthly column designed to address issues surrounding the use of fiber in premises local area networks (LANs). As chair of the Fiber Optics LAN Section (FOLS) of the Telecommunications Industry Association (TIA-Arlington, VA), I hope to shed some light on a variety of fiber-related issues regularly discussed in this publication.

Since this column will reflect the views of all FOLS members, we will do our best to discuss issues and present data in an objective and unbiased way. FOLS primarily focuses on the use of fiber in the horizontal portion of the LAN. How ever, this column will periodically deal with more general questions about the use of fiber in your network. When all is said and done, hopefully you will feel better prepared for the many challenges facing you as you design, install, and maintain a fiber network.

Small-form-factor (SFF) optical connectors provide a small fiber connectivity solution for premises networks. These connectors are designed to be less costly than traditional fiber connectors. They have a footprint similar in size to the copper RJ-45 style connectors. The benefits of the SFF connectors are clear: They increase density and decrease cost in outlets, panels, and LAN electronics. But questions arise when users must decide which of the SFF connector options is "best."

There is no universal answer to this question, as "best" is a relative term that's based on your specific needs. PN-3723-3, "Optical Fiber Cabling Components" (the ballot copy to be published as ANSI/TIA/EIA-568B.3), does not recommend any one SFF connector (as it does the SC). However, it does list a set of performance criteria that a connector (whether small or normal size) should meet in all environments. Many of the connectors exceed the TIA requirements, making them suitable for general premises environments. But special situations may prompt closer examination of performance. Careful evaluation of your requirements along with other factors will help you determine how to evaluate these connectors and make the best decision for your application.

The majority of premises networks are installed in environmentally controlled offices. In these applications, the biggest nemesis to networks is the end user. Frustrated computer users have been known to kick, disconnect, reconnect, wrap, pinch, and otherwise destroy network connections. It is obvious then that durability and serviceability are primary concerns for connectors in these environments.

In industrial or other non-office type networks, being user-proof is not the primary concern. Programmable-logic controllers are not known to interfere with the network connections! In these environments, therefore, reliability over a wide temperature and humidity range may be more important than how well the connector handles twisting.

In other networks, high-speed performance and maximum distances are the concern. In these cases, the basic parameters of attenuation and return loss become paramount. Today's high-speed networks operate with an ever-decreasing margin. A few tenths of a decibel in lower connector insertion loss can add more comfort margin to these designs.

While the TIA does not specify specific deployment requirements, it does take them into consideration in the proposed TIA-568B.3 specification. These performance characteristics are normative (required) in Annex A, titled "Optical Fiber Connector Performance Specifications."

The first section is "Visual and Mechanical Inspections." It can best be addressed by asking, "Does the connector have an approved FOCIS standard?" The answer should be yes. FOCIS (Fiber Optic Connector Intermateability Standard) specifies the physical characteristics of a specific connector type. These FOCIS documents are numbered as TIA/EIA 604-X and intended to provide interoperability between various manufacturers of the same style connector.

Attenuation is one of the most commonly specified parameters. It is also referred to as insertion loss and is given in decibels. All of the SFF connectors are designed to meet the minimum requirements of 0.75 dB. This loss will satisfy the vast majority of premises-network requirements. However, lower attenuation may be desirable especially when designing networks with very slim optical power margins. Structured cabling systems may have as many as five connections from the work area to the equipment room. Lower insertion loss can help increase the number of connections that may be reliably supported in the link.

Return loss is probably the most confusing parameter, as it is often rendered differently. The draft of TIA/EIA-568B.3 lists return loss as a positive number and is thus given as a minimum-the larger the number, the better. "Reflectance" is a term commonly applied to the same parameter, but this number is negative-the smaller (more negative), the better. To accurately evaluate return loss or reflectance, I was forced to remember my math and use the absolute value. Take the absolute value given in decibels; generally, the larger the number the better. It should be greater than 20 dB, but some will be 40 dB or greater.

The "Low Temperature," "Temperature Life," and "Humidity" sections of this draft specification list requirements for how well the connector must perform in extreme environmental conditions such as those found in an industrial-

automation plant. All three of these parameters are evaluated by their impact on attenuation and return loss. In other words, attenuation and return loss are evaluated before, during, and after the test. Generally speaking, the smaller the number (i.e., the less they change), the better the connector will perform.

In office environments, ruggedness and durability are important. The following six test parameters address this aspect of connector performance: impact testing, strength, durability, retention, flex, and twist. Impact testing measures the effect of dropping mated pairs of the connector. Strength measures the effects of pulling on the mated pairs. The effects of repeatedly connecting and disconnecting the pairs are measured and reported as durability. Cable retention measures the effect of pulling on the cable jacket coming out of the mated pairs. The effects of flex on the connection are reported in the flex measurement, and twist measures the effect of twisting the connection.

In all these cases, there is a final measurement made of both attenuation and return loss. The smaller the attenuation and the greater the return-loss figure, the better the connector will perform. What should also be noted is how much they change from the "nonstressed" measurements. The smaller the change, the better the connector performs. Pay special attention to these measurements when deploying in environments where the connector will be subject to tampering by end users.

While I can't give you the results of all tests for all connectors, manufacturers should be happy to provide you with all the information necessary to make an informed decision on which connector type is best for your application. Each of the various SFF connectors has attributes that may make it better suited for your particular installation. Hopefully, you have a better understanding now on what measurements are available and how they might be applicable to your application.

This column lists some of the technical issues in connector selection. Keep in mind that other parameters also are involved. These include easy and reliable field installation, multiple vendor support (both from a connector standpoint and as an equipment interface), the packaging of the connector into outlets and patch panels, and maybe most critical, the supplier's support, quality, and cost.

Steve Stange is chair of the Fiber Optics LAN Section (FOLS) of the Telecommunications Industry Association (TIA) and senior product manager for Transition Networks (Minneapolis, MN). He can be reached via e-mail at [email protected].