Insertion loss - signal attenuation on the wire pairs

"Help, I'm failing insertion loss on my long runs!" An often-heard cry for help when performing acceptance measurements on structured copper cabling. To be able to provide professional support in such a case, one must know the basics of this electrical property.

Insertion loss (IL) indicates the attenuation of a signal by one or more components inserted into a signal path. The insertion loss describes the ratio between the injected and transmitted power.

With the insertion loss, the signal attenuation is evaluated, over the entire path from the beginning to the end of the line. The associated measurement is intended to evaluate whether the required limit values for the transmission of the data signals are complied with and whether the partner station can also clearly recognise the signals. If the measurement reveals problems with the insertion loss, several causes can be possible. Essentially, it is the length and the quality of the line, expressed by its high-frequency characteristics.

Do not pull in the cable!
The sum of all high-frequency properties gives a data cable the characteristic of a low-pass filter; it lets low frequencies through better than high frequencies. On the one hand, this low-pass characteristic limits the bandwidth of the transmission of the cable and forces the installation of amplifiers (repeaters) after a certain cable length in order to regenerate the signal again or, as with data cables, to limit the permissible cable length per limit value either directly via the length or indirectly via the maximum insertion loss.

The high-frequency properties are so important because the copper cables are not simply energy suppliers, but essential components of the transmission path of the now very high-frequency data signals. Particularly mechanical overstressing of a cable directly causes a deviation from its nominal characteristic values and leads to an immediate deterioration of the transmission properties up to the interruption of communication. Hence the urgent advice to all installers never to "pull in" data cables, but only to "insert" them.

Measuring parameters Insertion loss
The insertion loss of a cabling section is measured in the frequency range of the corresponding cabling class (e.g. class EA up to 500 MHz). For this purpose, not only a single frequency is measured as a reference point, but the entire spectrum from 1 MHz up to the maximum frequency of the respective standard is swept. The results are recorded at frequency points defined by the standard and used for evaluation. The measured values depend on the length of the cables and the wire cross-section of the cable used. The attenuation of the wire pairs is the logarithmic ratio of the signal that is fed into the pairs and the signal that arrives at the other end of the cable section. Of the four attenuations of a typical 8-core cable, the largest value is used to evaluate the cabling route. This has been defined in the corresponding standards.

Today's cabling certifiers also display the total measured values of all wire pairs graphically and also save them with all data points.

Since the achievement of the maximum acceptable insertion loss is not bound to a fixed cable length, the measurement parameter "length" is only determined as an informative value in the ISO/IEC standards and standards derived from them. In the American standards, on the other hand, there are fixed pass/fail limits for the length of a line.

Causes of errors
Errors in the insertion loss parameter can have various causes, as already mentioned. The main cause for a failure of this measurement is simply too long cables in the transmission path. After that, the causes are cables that are not of sufficient quality (too low power category) or are measured against too high standards or were simply overstressed during insertion.

For an example of two insertion loss errors in one (see Figure 1): cable too long, all pairs outside the limits (red line) and cable not sufficient for selected measurement standard, Cat 7 (600 MHz) cable vs. Cat 8 (2,000 MHz) measurement standard.

Too long!
The rule of thumb is that the installed length of an AWG 22/23 cable up to approx. 90m always has enough reserve to be able to transmit data signals without problems; lengths above this can, but no longer have to function. If the connection cords of the active components are already in the cable, the total length should not exceed 100m if you want to be on the safe side. However, there are always irritations when lengths of installed tracks are measured that are actually smaller than the magic 90m, but the measurement of the insertion loss still goes wrong. What people passionately like to do wrong is to specify the so-called NVP value or shortening factor of the installed cable. This value indicates how much slower than the speed of light signals travel through the cable.

It is needed to make the length measurement as precise as possible by determining the time of flight of reflected signals. This correction value is directly proportional to the measured value and if it is set too low, all lengths appear shorter than in reality and vice versa. This is where the question arises how it can be that the insertion loss fails although the cable is supposedly shorter than the 90m. If, however, the NVP value had been set correctly, not too small, one would have seen immediately that the length exceeded the value by far and thus the cause for the failure of the insertion loss is obvious.

Too thin!
Recently, however, there has also been an increase in enquiries about attenuation errors in cable sections that have lengths between 60m and 90m, the lengths have also been correctly determined, but still fail when measured. The cause is increasingly modern data cables that were once developed especially for data centres and are now marketed with new designations primarily for the home area. Characteristic of these cables is a smaller cross-section, typically solid core with AWG 26 diameter. The reduced diameter produces higher attenuation values and only allows lengths of up to approx. 60m. Sufficient for data centres and single-family homes, but often too short for office environments. Since these cables are also suitable up to category 7, they are often used in office environments without knowing their limitations.

Flexible cables are also often used for short-term or mobile cabling, but they have similar attenuation characteristics and should only be used within their length restrictions, otherwise there are the same problems as described above.

Too bad!
Like all components in a passive cabling system, data cables are also divided into performance categories (see Table 1). These are defined by the possible achievable high-frequency bandwidth. The common categories range from Cat5e with a bandwidth of 100 MHz up to Category 8 with a bandwidth of 2,000 MHz. This classification is also used for the connection components and thus the rule is that if, for example, you want to build a cabling route with a bandwidth of 500 MHz in order to transmit up to 10Gbit/s Ethernet, you must use at least category 6A components.

If a cable of a lower category, e.g. category 5e (100MHz) is used in this setup, there is a great risk of failing the measurement, not only with the insertion loss, but also with the crosstalk (NEXT) and the reflection (return loss). Transmission links are like the famous chain that is only as strong as its weakest link. A component of too low a category drags down the performance of the entire link. Rarely, but from time to time, the installer also catches laying cables that have already left the factory defective and have somehow survived the quality control there. A typical error occurs from time to time with the double-shielded S/FTP cables we usually use. Here it happens that during the production step of wrapping the core pairs with the inner foil, this foil does not wrap around the core pair, but turns backwards and thus this signal pair ends up on the drum over a certain distance without shielding. If you now install this piece of cable, you will notice when measuring that not all four wire pairs produce approximately the same attenuation curve as usual, but one wire pair breaks away and either causes the parameter of the insertion loss to fail for longer distances or causes the parameter ACR-F to fall, which evaluates precisely this synchronisation of the attenuations.

Broken!
As already mentioned, data cables react very allergic to installation errors. Since the geometry of a data cable is largely responsible for its high-frequency properties, installation errors that change the mechanical structure of the cable, e.g. overdrawing, kinking, crushing, immediately lead to changes in the basic electrical properties, such as the pair capacitance and thus the cable impedance and the associated parameters. Such damaged cables can often be recognised by their reflection behaviour. Errors in placing the cables on the termination components can also produce similar effects and cause measurements to fail.

Conclusion
The general recommendation to measurement teams on the road with wiring certification: Always be able to quickly isolate the main sources of error in your measurement problems. In the workshop, perhaps even build a reference section from the components and the selected cable and measure it with your measuring device if it is freshly calibrated and equipped with new(valuable) measuring cables or tips. In this way, make sure that your settings on the device are correct and that the installed materials meet the desired requirements and take this route with you to the construction site. When inserting the cables, make sure that the length restrictions are observed and lay, not pull(!), the cables. If, despite these precautions, your measurement results deteriorate, carry out another measurement on your sample section and compare the result with the original protocol. This way you can immediately see if your measuring system is starting to wear out or if the system is possibly causing trouble due to a batch problem or fluctuating processing quality. Always have a fresh set of measuring cables or tips with you, too, so that you are not left high and dry in case of failure. Most of the time, these things happen when your dealer is already closed! Save yourself time-consuming and thus cost-intensive troubleshooting by being well prepared and having spare material. Your profit margin will thank you!

Alfred Huber
Head of Technology
Softing IT Networks GmbH