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Note: The following technical article was current at the time it was published. However, due to changing technologies and standards updates, some of the information contained in this article may no longer be accurate or up to date.

All of the telecommunications standards referencing balanced cabling specify nominal impedance. Furthermore, these standards form the basis upon which equipment manufacturers design their products. From a system performance perspective, it makes better design sense to optimize system performance to 100 ohms rather than tune to some other component-specific nominal value. After all, the cabling system should be engineered to best support the networking equipment it is connected to.

The latest industry buzz centers around "tuned
system performance". It is important to
understand that some "tuned systems" or
impedance matching claims may be based on an
arbitrary component value. To ensure the best cabling
system performance, components should be
engineered and optimized to work together and with
the LAN equipment that it is connected to.

While cabling systems are designed to support specified transmission parameters (i.e., crosstalk, insertion loss, return loss etc.), the concept of "tuning" particularly relates to impedance performance. In particular, the measured impedance between the various cables and connecting hardware that comprise a cabling system often differ. These differences will result in a signal reflection, or return loss, at each connection point where an impedance mismatch occurs. Obviously, components with closely matched impedance values yield smaller reflected signal magnitudes and better return loss results. For the best performance, all components should be designed to a nominal impedance of 100 ohms to match that of the networking equipment.

At this time, TIA and ISO requirements are NOT harmonized. The key difference is that ISO does not allow "averaging" of data and TIA does. This cable meets the TIA requirements, but fails the ISO requirements. To ensure robust performance specify TIA and ISO compliant cables.

There are several ways to look at how well a component or cabling system is matched to a specific nominal impedance. For example, the same measured data can be reported a variety of ways:

  • Fitted Impedance (frequency domain): commonly used in association with "uniform" transmission lines such as cable. Provides frequency response of the cable's impedance, but factors out excursions caused by structural non-uniformity. (Specified by TIA/EIA-568-A)

  • Input Impedance (frequency domain): commonly used in association with "uniform" transmission lines such as cable provides frequency response of the cable's impedance, including excursions caused by structural non-uniformity. (Specified by ISO/IEC 11801)

  • Return Loss (frequency domain): commonly used in association with "non-uniform" transmission lines and elements such as connectors, or cabling links and channels provides frequency response of reflected signal energy as caused by a combination of impedance discontinuities caused by both connectors and cable, as well as deviations from a nominal impedance by one or more segments of cable.

  • Reflection Coefficient (time domain): commonly used in association with cabling links and channels to quantify discontinuities along a transmission line. Results can be displayed as a function of time or length based on nominal velocity of propagation. Can be used to locate faults in a transmission line or to identify specific source contributions that degrade return loss performance.

By using vector measurement techniques (e.g., magnitude and phase in the frequency domain or magnitude and distance in the time domain), it is possible to computationally convert between the frequency domain and time domain by way of Fourier transform.

Figure 1

To demonstrate the transformation of data from the frequency to the time-domain, a 4-connector, 100 meter long channel was constructed. Return loss for the channel is shown in figure 1 and demonstrates a worst case margin of +2.8 dB when compared to the proposed category 6 TIA and ISO requirements. Although not a standard's requirement, the impedance performance of the channel was plotted versus frequency as shown in figure 2. The time-domain equivalent of the impedance performance is plotted versus length in figure 3. Note the linear appearance of the time-domain data. The frequency domain data (figure 2) looks much worse than the time-domain data (figure 3), even though these plots depict the exact same information simply presented in two different formats.

Figure 2

When evaluating "tuned" and "matched" performance claims, especially in the time-domain, it is necessary to pay particular attention to the configuration under test. Often, time domain impedance data is presented for cables only. This information does not provide a direct indication of the impedance performance of the cabling system. Furthermore, it is difficult to make an assessment of the impact of different components on channel performance without close examination of their corresponding signatures in the time domain. It is also important to note that time domain results will vary depending on the test instrument's output waveform. Because impulse signal strength decreases as distance from the transmitter end of the TDR increase, often only the first 50 feet of time domain data provides an accurate indication of component interaction. An exploded view of the channel under test as recorded from the work area orientation is shown in figure 4. Note that the points of connectorization are visible, however, the impedance variance attributable to each component insertion is comparatively small. Because of the inherent variances in the frequency response of impedance over long lengths of cable, it is impossible to design a cabling system that is precisely 100 ohms across the entire frequency range.

Figure 3

Figure 4

The Siemon Company has long understood the importance of designing headroom into cabling systems, as well as optimizing impedance performance to 100 ohms. In fact, since 1994, all Siemon cabling systems and components have exceeded both the TIA and ISO requirements for all transmission parameters including impedance. When evaluating system claims, be sure to look at similar data formats (such as frequency-domain) that support apples-to-apples performance comparisons. To ensure the best system performance, specify cabling solutions that are optimized to the requirements of the industry standards and the network equipment that will be operating over it.

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