Materials reliability issues with coaxial cable systems for the information superhighway

Abstract

A frequently advocated design for theInformation Superhighwaythat will provide voice, video, and two-way digital communication for businesses and residences calls for transmission on optical fibers from the source (head end) to neighborhood nodes and coaxial cable for distribution from the neighborhood nodes to individual subscribers. Materials reliability challenges abound from the head end to the customer's premise. The maximum allowed service outage per customer is expected to be less than one hour per year. The fiber portion of the network, including lasers, amplifiers, lenses, and the fiber itself, carries two-way transmission to large neighborhoods. With one fiber serving hundreds of customers, cost/reliability tradeoffs emphasize reliability. At neighborhood nodes, where the network branches, coaxial components carry the signal to the street level, where each subscriber is served by at least one drop cable and one network interface unit. The volume of components is much higher. The challenge is to achieve high reliability at low cost. Indeed, one of the impediments to replacing coaxial distribution in the neighborhood with fiber is the difficulty of achieving low-cost, reliable packaging for lasers and other optical components, while maintaining reliability. Since covering all the many materials aspects of system reliability would fill an entire book, the important design considerations, materials issues, and test methods that bear on the reliability of hybrid fiber coaxial systems will be illustrated by reviewing several aspects of coaxial reliability. The intent is to illustrate the methods used to identify and prevent failure mechanisms and the information that must be assembled for making appropriate decisions on reliability/cost tradeoffs. While coaxial systems have been used for over 20 years to bring cable TV to homes across North America, the requirements for reliability and signal quality with these one-way analog rf systems are less demanding than those for two-way digital data transmission. Many of the reliability issues that could potentially degrade performance characteristics of coaxial cable systems are materials and environment related. This paper discusses these issues to the extent that they are currently understood, presents results from some ongoing research to improve our understanding of the potential materials/environment related causes of signal deterioration, describes some prevention strategies, and discusses some of the many materials challenges that remain.