Discussions surrounding whether or not to test a fiber channel with the patch cords have been unclear. The answer to this long-standing question is “it depends.” Let’s take a closer a look.
In reality, the main deciding factor for whether to test the channel with the patch cords included or to just test the permanent link depends on the specification provided by the end user or their consultant. If the spec calls for it, you need to include them. It’s that simple.
If after checking the spec the question still remains as to whether or not patch cords should be included in channel testing, consider the fact that patch cords are factory terminated and offer a lower risk of defect and errors. Installation of the permanent link typically has much more impact on the performance and insertion loss of the channel.
Sometimes it is also not logistically feasible to test the channel with the patch cords because they are often not in place during initial testing, before active equipment is installed and up and running. They are also regularly moved or changed during reconfigurations without necessarily re-testing the channel.
While most should not have to worry about testing the channel with the patch cords (unless it’s specified), the fact remains that sometimes things can go wrong. Fiber patch cords can have dirty end faces or they could be damaged—especially since they are a moveable component.
Another key consideration is whether or not typical insertion loss values or maximum insertion loss values are being used to design the channel. If using typical insertion loss values, it is important to know how the patch cords impact the channel. Even a negligible loss on the patch cord can impact the design and ultimately the channel performance. However, when using maximum insertion loss values, patch cords will not likely have an impact.
While not necessary for achieving certification on an end-to-end Belden FiberExpress channel, we recommend using maximum insertion loss values when designing channels and testing the channel with the patch cords whenever possible. Testing the channel with the patch cords doesn’t take any extra time, and it’s better to be safe than sorry.
And if at first, you only tested the permanent link and then experienced higher-than-normal bit error rates once the active equipment was up and running, it might be time to retest the channel with the patch cords in place.
Dwayne Crawford has more than 20 years of experience in the datacomm industry. He has served on several international standards committees to advance high-performance/low-latency protocols (such as IEEE-1394, GigE Vision and CameraLink) used in real-time image processing and utilizing high-performance computing platforms.