Insertion Loss in Telecommunications Cabling

Written by Dave Harris, trueCABLE Technical Specialist, BICSI INST1, INSTC Certified

Every data transmission system experiences signal loss. Whether the signal is propagated by copper wire, optical fiber, Wi-Fi, or just yelling at the kids down the street, the signal is never as strong at the destination as it is at the source. 

In the case of physical voice communication, the kids will understand you if they are close-by. If they are farther away, they might hear that you are yelling, but they won’t be able to tell what you’re yelling. That means that with the addition of all that extra atmosphere in the signal path and with the strength of sound propagation falling rapidly with distance, the signal is no longer strong enough to transmit the data it is supposed to be carrying.

Signal Loss: A Simple Example

Let’s talk about another example, simple eyesight. I can call it “simple” because I am going to completely ignore anything that has to do with biology. (My go-to means of simplification.) In my interpretation of eyesight, data in the form of an image is transmitted along a reflected light path and is received and interpreted by a head containing eyes and a brain. (Sorry about the accidental biological content.) Assuming a head that is in good shape, this usually works pretty well. But not always. Sometimes the sun is so bright that it is overwhelming. This results in a situation where it is difficult to discern the data (the image) amidst all of the noise. My head usually responds to this by squinting my eyes and shading my brow to try to reduce the signal strength, and sometimes it works a little bit. 

Sunglasses work better. Insertion of a darkened lens (filter) into the light path reduces the intensity of the light so that the head is no longer overwhelmed, and the data can be received and interpreted. This reduction in signal strength is known as attenuation. The lens is an example of an attenuator. Further, we can say that the lens attenuates the original light signal because it acts to reduce the signal strength.

As the day continues, eventually the intensity of ambient sunlight will be reduced as day turns into night. As this happens, it becomes harder to see because the sunglasses are still attenuating the signal that is now much weaker. The sunglasses are now causing signal loss that is making the image difficult to see. So, removal of the attenuator from the data channel is needed for the signal to be strong enough for the data to be successfully transmitted. 

Signal loss is present in every data transmission system. When the data is electronic and is transmitted over conductive wire, optical fiber, or radio, the signal loss is known as “insertion loss.”

Insertion loss is really just what it sounds like. It’s the signal "loss” that happens as a result of “inserting” something into the physical path of the signal. Everything that is added to a signal path–every connector, coupler, RJ45 plug-jack connection, every kink and bend adds to the cumulative insertion loss in a cable channel. Even the addition of more cable adds to the loss, making distance an important consideration.

Insertion Loss in Coaxial Cable

One of the biggest producers of insertion loss in a coaxial cable signal pathway is a splitter. A splitter, as seen in Figure 1 below, is a device that splits the signal into two (or more) signals so that it can feed two or more devices, like when one antenna is shared between two TVs.

Figure 1. An example of a splitter for coaxial cable. (www.channelmaster.com)

This necessarily results in a large amount of signal loss, as each of those TVs are now only going to receive 50% of the signal it would have gotten if it had the antenna all to itself. Figure 2 shows how the splitter produces a signal loss of 50%. Signal strength (and signal loss) is expressed in units of decibels. Figure 2 also helps us to understand the decibel. After all the math is done, by somebody somewhere, it turns out that a 50% loss of signal strength is about equal to a loss of 3.5 dB.

Figure 2. Attenuation (signal loss) due to use of one splitter

That’s just for one splitter. Many homes have more than two TVs, some homes have quite a few TVs, and that means more splitters. As shown in Figure 3, addition of another splitter to the signal path doubles the attenuation to 7.0 dB, and cuts the signal strength in half again. So after the addition of the second splitter, we’re down to just 25% of the original signal strength.

All of those figures are for hypothetical splitters that operate at 100% efficiency which, of course, nothing really does. Also, each cable connection adds more attenuation due to signal leakage. These small signal losses are usually less than half a decibel, but they add up. Especially when you start including union fittings, wall outlets, a cable from the wall outlet to the set top box, the set top box itself, then a cable from the cable box to the TV. So as a rule of thumb, most installers expect each splitter to add between 3.6 dB and 4.0 dB to the total attenuation.

Figure 3. Attenuation (signal loss) due to use of two splitters

When coaxial cable is used to provide a home with both broadband internet and television service, the installer will usually route one of the outputs from the first splitter to the cable internet modem, and then use the other output to split further for multiple TVs. You can see why in Figure 3. This arrangement provides the strongest signal to the Internet modem, which is where most users need the greatest bandwidth.

More on Attenuation

Now that we understand a little bit about decibels from looking at the effect of splitting the signal in half, we can talk a little bit more about attenuation of the cable. Remember that a signal loss of 50% is about equal to 3.5 dB and a loss of about 7.0 dB means that about 75% of the original signal is lost. Sticking with the same pattern, does a loss of 10.5 dB mean that almost 90% of the original signal is just gone? I’m afraid so.

Insertion Loss vs. Attenuation

What is the difference between insertion loss and attenuation? Absolutely nothing. They are the same thing. For a particular system, they have the same value and the same units (decibels). You may use the two terms interchangeably, and most people do. If there is anything different about the two terms, it is just how the terms are used. For example, most people will say “attenuator” instead of saying “device for increasing insertion loss.” At the same time, when the loss is measured for an entire link, the value is reported by most instruments as “insertion loss.”

Insertion Loss in Twisted-Pair Ethernet Cable

Much of the insertion loss is inherent in Ethernet cable due to the electrical resistance of the copper conductors themselves. The greater the resistance, the greater the insertion loss. Variations in the resistance can be due to temperature, wire gauge and other factors. In addition to resistance, there are other causes of insertion loss.

Causes of Insertion Loss in Twisted-Pair Ethernet Cable

Resistance Due to Temperature
The conductivity of copper decreases with increasing temperature. That means when the temperature goes up, the resistance goes up too. And so does the insertion loss. This loss is so great that at high temperatures, the maximum distance for an Ethernet channel is shortened in a process called “derating.” You can learn more about temperature derating in our Cable Academy blog entry, “Temperature's Effect on Ethernet Cable Length.”

Resistance Due to Wire Thickness
The thicker the wire, the greater the conductivity, so the lower the resistance. So thicker wire can be expected to have lower return loss due to resistance. Additionally, stranded wire has a lower cross-sectional area when compared with the same gauge solid wire. So stranded wire has higher resistance and higher insertion loss. There's more on wire gauges at, ”Demystifying AWG: Essential Guide to Understanding Wire Sizes.”

Impedance Mismatch
Impedance can be thought of as the opposition to the flow of current through an electronic device or component. It is different to resistance in that it contains both resistance and reactance. If impedance is not matched throughout the channel, reflections can occur that increase insertion loss. Our component-rated keystone jacks and field term plugs contain circuitry to ensure impedance matching throughout the channel.

Termination Irregularities
Termination problems usually mean poor electrical connections. Poor connections are impedance mismatches and also add resistance, and that means more insertion loss. Faults in termination are often impossible to see without performing certification testing on the link.

Installation Irregularities
Ethernet cable must be installed carefully in order to prevent additional sources of signal loss. Just about any kind of physical damage to the cable can increase insertion loss. Riser cable is porous. If it gets wet, it needs to be replaced. Pay attention to minimum bend radii and maximum pull tensions. Cable lubricants, especially water-based varieties, are often conductive and can cause temporary (until the lubricant dries) insertion loss.

Insertion Loss in Optical Fiber

Fiber optic cable has much lower insertion loss compared to copper cable, and so optical transmissions operate at much greater distances and speeds. Multi mode fiber has greater insertion loss than single mode, so single mode fiber can transmit farther and faster than multi mode fiber. If you’d like to understand more about the differences, check out “Single mode vs Multimode Fiber Optic Cable.”

 Causes of Insertion Loss in Optical Fiber

The biggest cause of insertion loss in fiber optic cable is contamination of fiber mating surfaces. This is always the starting point if there are performance issues to track down in a fiber installation. Most installers carry specialized cleaning tools with them on the job, because they are constantly in use. For an in-depth look at fiber optic cleaning methods, take a look at “Cleaning Fiber Optic End Faces: Contamination Sources and Cleaning Methods.”

Another source of insertion loss in optical fiber is loss due to excessive bending of the fiber. Since this loss is not due to the intrinsic nature of the fiber, and rather how it is handled and installed, bending loss is an example of extrinsic loss. More about loss due to fiber bending can be read in our blog post, “Basic Principles of Fiber Optics Series: Micro and Macro Bending.”

Insertion loss in fiber can also be caused by scattering of the signal within the fiber. Scattering is a result of the interaction of the light signal with the molecular structure of the glass fiber. Absorption is loss caused by interactions of the light with impurities in the glass fiber. Scattering loss and absorption loss are due to the makeup of the fiber itself, and not by how the fiber is used or handled. Therefore, these losses are examples of intrinsic loss.

More about insertion loss in optical fiber can be found in our Cable Academy blog post, “Basic Principles of Fiber Optics Series: Attenuation.”

Conclusion

This article is meant for beginners who are just developing concepts for understanding signal loss. Accordingly, descriptions based on mathematics and advanced testing procedures have been avoided. Hopefully, this article helps underscore the importance of proper matching of cable to termination accessories, and the value of adhering to performance standards.

Our keystone jacks and field term plugs are all component rated, ensuring that impedance is matched throughout the cable channel, and minimizing insertion loss. Our patch cords are individually tested to ensure that insertion loss is well below levels accepted by standard for every single cord. Our cable is designed and tested to perform so well that we know it will easily pass certification testing when installed with component-rated accessories. And our guarantee is Forever.

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