What is Insertion Loss?

Understand Insertion Loss

Insertion loss is the amount of energy that a signal loses as it travels along with a cable link. It is a natural phenomenon that occurs for any type of transmission—whether it's electricity or data. This reduction of signal, also called attenuation, is directly related to the length of a cable—the longer the cable, the greater the insertion loss. Insertion loss is also caused by any connection points along with a cable link (i.e., connectors and splices).

Insertion Loss Formula

A key performance parameter for both copper and fiber applications, insertion loss is measured in decibels (dB). It is typically a positive number that is calculated by comparing the input power of the signal at the source to the output power at the far end. The lower the insertion loss, the better the performance. If insertion loss is too high, it can prevent the signal from properly being received and interpreted by active equipment at the far end of a link. Because insertion loss is directly related to distance and the number of connection points, industry standards call out insertion loss limits and specify the number of connections and distance limitations for specific applications.

Insertion Loss vs. Return Loss vs. Reflectance

Like insertion loss, return loss is another parameter that is important in both copper and fiber systems. Rather than measuring the amount of loss over a link, return loss measures the amount of power injected from the source compared to the amount reflected toward the source. Like insertion loss, return loss is also a positive number. However, unlike insertion loss, the higher the number, the better the performance. Decreased reflections result in a higher return loss. In other words, if none of the signals was reflected, there would be an infinite return loss. A higher return loss also generally correlates to a lower insertion loss. It’s important to note that in optical fiber applications, the inverse of return loss is reflectance, which measures the amount of back reflection created by a reflective event (i.e., connector) compared to the amount of light injected. Also expressed in dB, reflectance is a negative number.

Insertion Loss in Optical Fiber

Insertion loss in optical fiber cabling systems is much less than copper, which is why fiber supports much greater distances and long-haul backbone applications. For example, multimode fiber loses only about 3% (0.3 dB) of its original signal strength over a 100-meter distance while a Category 6A copper cable loses about 94% (12 dB) of its signal strength over the same distance. Still, there are limits on the amount of insertion loss that specific fiber applications can handle, and higher bandwidth applications have more stringent loss requirements. For example, the 10 Gb/s application 10GBASE-SR over 400 meters of multimode fiber allows a maximum channel insertion loss of 2.9 dB, while the 100 Gb/s application 100GBASE-SR4 allows a maximum of just 1.5 dB.

Fiber Insertion Loss Budgets

Based on the maximum insertion loss values published by industry standards for specific applications, loss budgets are determined early in the design phase to ensure that the cable plant does not exceed the maximum specification. Based on manufacturer specifications for the fiber and connectors, as well as the maximum specified loss of any splices or splitters, fiber insertion loss budgets are calculated by adding the insertion loss for the length of the fiber and each planned connection point in the channel. The active equipment also needs to be considered per the equipment manufacturer’s specifications based on any differences between transmitters and receivers, as well as some margin to account for the loss of power over time that can occur due to transmitter age.

Causes of Insertion Loss in Copper Cabling Systems

In copper cabling, insertion loss is largely dependent upon the gauge of wire—23 AWG wires will have less insertion loss than the same length 24 AWG (thinner) wires. Wire gauges have therefore increased for higher frequency applications with Category 5e typically at 24 AWG and Category 6A at 22 or 23 AWG. That's also why some of the new popular thinner 28 AWG cables are limited to shorter distances to compensate for the increased loss. Also, stranded copper cabling exhibits 20-50% more insertion loss 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. This is also a concern in copper cables carrying DC power via remote powering technologies like power over Ethernet (PoE) that can cause the temperature to further increase, especially in cables located in or near the middle of a cable bundle that can’t properly dissipate heat.

In addition, the use of lubricant on cables to facilitate installation can cause an insertion loss failure—even when everything else passes. Lubricant is highly conductive, which causes electrons to disappear from the cable and not be picked up by the tester. Over time, as lubricant cures and becomes less conductive, insertion loss will improve.

What Makes Good Insertion Loss Testing Equipment?

Whether you’re testing fiber or copper, the key to a good insertion loss tester is accuracy. For fiber certification testing, that means you need an Encircled Flux-compliant tester with the ability to test multimode and single-mode fiber links at multiple wavelengths and advanced automatic Pass/Fail analysis to industry standards or custom test limits. Additionally, the ability to set up a tester easily and accurately can go a long way in reducing time and preventing testing errors.

For copper certification testing, it’s important to select a tester with standards-based Level V accuracy that has undergone rigorous evaluation by an independent and technically qualified laboratory. The tester should have the ability to certify the performance of all categories of cable and current applications. It should show results for all parameters on all four pairs of a cable, including insertion loss. This is especially important since insertion loss being higher on only one or two pairs can be an indication of a bad connection. In addition, a tester with diagnostic capabilities can reduce the time required to fix cabling faults.