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Friday, 26 October 2007 04:17

Fiber Challenges Copper But Brings Its Own Challenges

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The past decade has seen a gradual move by telephone companies to replace copper wire with fiber optics as a communications medium. This approach is based on a desire to reduce life cycle costs and maintenance, as well as to provide a platform for other services that take advantage of fiber. Fiber's major performance advantage is that it provides a much broader bandwidth than copper cable. A single fiber cable can handle thousands of telephone conversations or many cable TV channels. Therefore, telephone companies are anxious to bring fiber cable to the "curb," that is, distribute it throughout residential and industrial neighborhoods for future use. Some telecommunications companies have already deployed fiber cable for this purpose.

A paper from the 9th International Symposium on Subscriber Loops and Services, "Fiber to the Customer: Real World Constraints and Dependable Solutions," covers the subject from a power quality viewpoint. Authored by R. Rando, G. E. Strohl, J. Tardy and R. M. Wozniak of AT&T Bell Laboratories, the paper covers the subject as described by its title.

The authors note that fiber technology has brought with it a new element, the ONU, or Optical Network Unit. The ONU is part of what is referred to as a Distant Terminal (DT) located on a curbside pedestal. The DT includes the ONU, power apparatus, batteries and the Drop and fiber connections.

With copper-based transmission, power for the telephone service originates in the telephone exchange. In contrast, with a fiber optic system the curbside ONU must always be powered. There are two ways to power the ONU: locally, from available individual sources or from a shared central source (called network powering, or NWP). The major difference between the two approaches is in the location and dependability of the energy source. Regulatory agencies, local electrical codes and a practical operating system, along with the average and peak power demands of the DT limit the DT's powering alternatives.

A telephone standard and the National Electric Code (NEC) limit the amount of source power for the distribution wiring in communication circuits. Both codes define and classify voltage levels and protection rules for these circuits.

One local powering approach for the curbside DT is via the electric utility at 120VAC and 60Hz. An alternate approach is use of AC customer power to generate DC voltages at the living unit. Connections from the DC power source to the DT are made with the usual telephone methods. This customer-fed "back-powering" raises political questions about customer reimbursement for electric charges. It also raises issues related to availability of customer-delivered energy.

Local powering approaches require a rechargeable battery source in the event of power failure. This requires battery support elements: charger, disconnect switch, temperature sensor, heater etc.

Network powering supplies power from a central source over copper wire to the DT. This approach addresses the service continuity problem of battery reserve depletion in local powering and it also reduces the logistic/maintenance complexities associated with widely deployed battery installations. However, network powering requires considerable plant investment and operates with a lower power system efficiency. Network powering also introduces major maintenance issues for the deployment and protection of a copper network for power transmission. Plus, the addition of a copper wire with the fiber optic cable makes the communications medium more susceptible to the problems associated with lightning strikes. Adding copper wire defeats one of the reasons for going to fiber optics, namely its ability to be unaffected by lightning strikes.

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