Signal distribution / Control

Phil Hippensteel on AV Over IP Cabling

9/20/2017 12:45 PM Eastern
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Few of us would argue IP is relatively new to the AV industry. There are many lessons we can learn from the experiences that telecom encountered when IP was first used in telephony. One of the notable lessons involved the enormous learning curve that faced telecom directors when they used copper cabling. Before VoIP, cables usually carried a 3 kz band of frequencies, which is enough to carry voice. However, when IP was introduces, LANs were installed primarily to support 100 Mbps Ethernet. Suddenly, old telephone cables were rarely able to support voice in its new transport format. We may face the same type of challenges as we move from analog video and audio to gigabit Ethernet

So, let’s review some important facts about category cables. The important connection isn’t the patch cable that we use to connect the device to the wall plate, unless we are foolish enough to build the cable ourselves. A commercially available cable is usually not a source of the problem. But, the distribution cable, the one from the wall plate to the wiring closet, is very distinct. Here, the how the cable is manufactured, installed, and tested makes a very significant difference. Let’s discover why.

Copper cables carry electrical patterns that operate at varying frequencies. If the signal being transmitted is digital, it will be composed of a fundamental frequency and several higher frequencies. For example, 10 Mbps Ethernet actually has a fundamental frequency of 5 MHz and also contains odd harmonic frequencies which include 15, 25, and 35 MHz. It’s the combination of the waveforms that allow the representation of the square shaped wave that is transmitted. Now, think about good quality audio that might be used in a collaboration session. If it operates with 22 kHz sound, the cable needs to support the range of frequencies up to that value. This requirement is well below the frequency demands of 10 Mbps Ethernet. Now consider the fact that 100 Mbps Ethernet requires a fundamental frequency of 62.5 MHz with odd harmonics well above that level, you see quickly that carrying sound in IP form requires very good copper cables.

Suppose the distribution cable is Category 6, what characteristics are important? According to the current TIA or ISO standards (they are the same), there are over a dozen parameters that should be measured.  Wikipedia has a good description at

https://en.wikipedia.org/wiki/Copper_cable_certification#Power_Sum_ACR_.28PSACR.29

They are well beyond the scope of this newsletter, but I’ll mention a few that are relatively easy to understand.

The cable needs to be tested for signal loss and it appears in several forms. Signal loss is a decrease in the signal level from the transmitting point to the receiving point. There are also several forms of crosstalk. It can occur between each of the four pairs and can be measured at either the near-end or far-end of the cable. Due to twisting, the length of each pair can be different from the length of the other pairs. That means the electrical signal may take longer to reach the other end of the cable in some pairs. Bear in mind, this is only a brief introduction to the measurements that should be made.

How do you test or verify the cable for use? You use a qualified contractor who uses a reputable testing device. Require a printed report for each cable run, even if the reports seem daunting. A device that will do this will likely cost the installer several thousand dollars. I’ve experience installations where the installer knew just to press the “autotest” button. The results showed quite a few cable runs that did not pass. We had to insist that the installer to retest or re-cable many of the cable runs.

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