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Feature: Upgraded Cable: What New Cables Offer and Old Cables Hide
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Aging cable plants can hamper modern broadband network deployment, making replacement a necessary solution. How do you decide whether to patch your existing network or plunge into system upgrades?

Each year, highway maintenance is necessary to counter the effects of the environment. Fortunately for us, we are not responsible for making sure there are high-quality paths for transit—or are we? Coaxial cable, the RF boulevard of the broadband network, is the path of transit for providing high quality signals to customers. We must ensure those new services, such as digital video and high-speed data, can travel without damaging the RF vehicle.

While coaxial cable is forgiving, it is still susceptible to the effects of the environment and aging, which even the best system maintenance cannot prevent. However, a good maintenance program with a proactive replacement effort may minimize these effects on the system.

Getting older

Even the best maintained plant cannot overcome aging. Cables will start to show signs of deterioration and RF leakage. We have all seen it before—radial cracking of the outer conductor, squirrel chew, projectile holes, etc. The usual repair approach is to cut out the bad section and splice in a new section of cable. This is appropriate if a cable is fairly new, but always consider replacing the span as an alternative.

The more straight splices in a system, the greater the system performance degradation. Also, a repair can lead to early failure in a span, requiring another costly truck roll.

Yesterday’s cable

Remember the days of the P1, P2 and CopperGuard? Unfortunately for many operators, these cables are diminishing network capabilities. Engineers make an effort to install the most advanced line gear and headend equipment, but they need to pay the same attention to the existing cable in the plant. While possible, trying to provide customers with an enhanced service broadband network through aged cables is difficult.

Those cables served their purposes well in the days of 300 MHz plants, but were only qualified to that bandwidth. Looking at the history of cable bandwidth qualification from 1971 through 1985, cable was only qualified for 5 MHz to 300 MHz. Then in 1985, qualification was extended to 450 MHz before changing to 600 MHz in 1988. The introduction of new amplifiers and the desire to make use of even more bandwidth changed the upper limit to 1 GHz in 1992, which remains the target today.

It is important to consider that the measurement of attenuation graphed in Figures 1 and 2 is the measurement of new cable, not cable installed in a plant 15 or more years ago. For those numbers, you would also have to take into consideration structural return loss (SRL) damage, the change in the velocity of propagation (Vp) of the dielectric, water damage and corrosion. You’ll need to perform sample testing of existing cable to the bandwidths anticipated for the new network to determine its capabilities.

When testing your older cable, keep the following in mind:

• Attenuation of cable is most accurately known below the frequency at which the cable was qualified. It is difficult to accurately predict how the cable would perform beyond the test range, although theoretical predictions do exist.
• The use of SRL measurements to evaluate cable is a standard practice. Unknown electrical performance issues beyond the frequencies at which they were originally swept and tested may exist in cables. The testing referred to provides SRL data inherent to the manufacturing process and does not predict field-created SRL.
• Thermal cycling adversely affects the molecular properties of the dielectric, which significantly changes the Vp. Aged cables more than 15 years old that have been tested have demonstrated this with a greater than 1 percent change in Vp.
• Water damage and corrosion have a significant impact on a cable’s electrical performance. The resistance of a cable may be dramatically increased, which can create plant performance and reliability issues.
• Mechanical failures, such as radial cracking of the outer conductor, can lead to water migration, as well as signal ingress and egress.

Today’s cable

Recent advances in coaxial cable engineering have made the economics of deploying them into the network worth the expense. They provide a more robust signal performance than their predecessors, having been designed with 1 GHz bandwidth, digital video, data, telephony and 90 volt powering applications in mind.

There also have been significant developments in cable’s craft-friendliness. Complete systems have been introduced that consider connectorization critical and provide craftsmen with the ability to perform better and more reliable splicing. The advent of these "low-loss" cables provides a more flexible and longer-lasting cable in a smaller, lighter package.

The introduction of 90-volt powering has demonstrated a need for cables designed specifically for the task of powering today’s broadband networks. Also, architectures employing centralized powering have been on the increase. These issues created a need for a dedicated power feeder cable. These cables are available on the market today, and are designed and engineered with a much lower resistance, allowing power to be carried further than ever before.

Drop cables have been particularly troublesome to system operators, especially when it comes to corrosion. Recent advancements in engineering have led to the development of non-corrosive center conductors and shields exceeding the Society of Cable Telecommunications Engineers (SCTE) requirements for corrosion resistance. Also available on the market are products meeting the new National Electrical Code (NEC) Article 830 requirements, which enhance safety in the deployment of telephony services.

In our competitive environment, reliability and quality of service is paramount. These new cable choices will allow you to provide these quality levels more consistently, at higher frequencies and with better reliability than was possible in the past.

Paul Gemme is vice president of plant engineering for Time Warner Cable. He may be reached at . Christopher Gemme is a technical services applications engineer for CommScope. He may be reached at .

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