Cable Testing 101: Measuring Insertion Loss for Fiber and Copper
August 18, 2016 / General
The amount of energy that a signal loses as it travels along a cable link used to be referred to as attenuation--the verb attenuate being defined by Merriam Webster as "to reduce the force, effect or value of something."
This reduction of signal happens along the length of any cable, and it is a natural phenomena that occurs for any type of transmission--whether it's electricity or data. The longer the cable, the greater the attenuation.
Since attenuation is directly related to the length of a cable, that would explain why there are specific distance limitations called out in industry standards. The key to these specifications is to ensure that the insertion loss isn't too high to prevent the signal or power from properly reaching the far end.
For network cabling, standards now more correctly use the term "insertion loss" which refers to the loss of the signal strength at the far end of a link and includes the attenuation caused by the cable and any connection points along the way (i.e., connectors and splices), as well as any signal lost to reflections of the signal. Despite this correction in terminology, the limits, test procedures and requirements have not changed.
Measured in decibels (dB), insertion loss is one of the performance parameters that is actually measured in both copper and fiber certification testing, but there are key differences between the two.
Let's take a closer look.
Fiber exhibits much less insertion loss than copper, which is why it is used for greater distances and long-haul backbone applications. For example, multimode fiber loses only about 3% of its original signal strength over a 100-meter distance while a Category 6A copper cable loses about 94% of its signal strength over the same distance.
Another key difference is that insertion loss over copper changers with the frequency of the signal--copper links exhibit more insertion loss for higher frequency signals. For example, the maximum allowed insertion loss for Category 5e specified to 100MHz is around 22dB at 100MHz while Category 6 specified to 250MHz is a little over 32dB.
In copper cabling, attenuation is also largely dependent upon the gauge of wire--23 AWG wires will have less attenuation than the same length 24 AWG (thinner) wires. Wire gauges have therefore increased for higher frequency applications with Category 5e typically at 24AWG and Category 6A at 22 or 23AWG. That's also why some of the new popular thinner 28AWG cable require shorter distances to compensate for the increased loss.
Also, stranded cabling exhibits 20-50% more attenuation than solid copper conductors, which is why solid conductors are used for the longer permanent link portion of a copper channel and stranded conductors are limited to shorter patch cords.
For copper cabling, attenuation can also be related to temperature. Higher temperatures cause more attenuation in all cables, which is why standards specify maximum operating temperatures for copper cabling or require length de-rating for hotter operating environments.
What's the Same?
While different factors impact insertion loss in copper and fiber, regardless of the media, excessive length and bad connections are the two main causes. This drives home the need to follow the standards requirements on length and ensure quality workmanship for both copper and fiber terminations.
For copper, the clue to a bad connection is if insertion loss is higher on only one or two pairs of a 4-pair connection. Fluke Networks' DSX-5000 CableAnalyzer shows insertion loss traces for all four pairs of cable and each should be below the limit line. If all pairs are virtually equal, check for excess length.
For fiber, poor alignment of fiber cores in connectors or splices can cause insertion loss, as well as air gaps and end face contamination. Again, an issue that is directly related to workmanship during the termination process.
Basic fiber testing, known as Tier 1 certification, measures insertion loss of the entire link. If the link fails insertion loss testing, the use of an Optical Time Domain Reflectometer (OTDR), known as Tier 2 certification, provides insight into the loss of specific connection points and length of the cable.