The fiber optic transmission technology has matured from its embryonic field trial status to the position of market where now it dominates in the long-haul transmission arena. The fiber optic has recently penetrated into the feeder portion of the local loop. This technological evolution is continuing into the distribution portion of the loop and then further into the drops to individual homes. The primary reason for this change is that fiber optics has become the most economical way to meet customer needs.
As discussed in "What Cost Local Loop?" by C. Nelson in Telephony, Oct. 29, 1990, pp 38-42, there are many factors to consider when comparing the economics of copper and fiber in the loop. Although the installed first cost of fully loaded systems is probably the single most visible factor, other variables such as depreciation, maintenance costs and the ability to provide service to initial residents-while deferring the cost of providing service to future residents-can have major effects on the economics of the competing systems. These life-cycle costs are the most difficult to analyze because they vary so dramatically based on the specifics of the neighbourhood under consideration.
The telephone companies at the same time must face another problem of providing cost-effective POTS (plain old telephone service) today using a system that will be able to carry tomorrow's enhanced services. Any FITL (fiber-in-the-loop) system deployed today must have this capability. Thus Bellcore (Bell Communications Research Ltd) lists the three key requirements of such a system. First, it must have only two fibers per pedestal. Second, the upgrade plan must provide for the simultaneous transmission of telephony, AM video and broadband integrated services digital network (BISDN) offerings in any mix at the pedestal. Finally, upgrading one subscriber interface unit should not affect customer service at other subscriber interface units.
Single-mode optical fibers are the future of the telephone loop and have been exclusively used in various portions of the loop. They are relatively robust, economical and have bandwidth capacity that far exceeds today's copper pairs. But placing fiber in the loop also presents significant technical challenges for telecommunications equipment suppliers and telephone company planners in the areas of cost, power, operations and architecture. The choices made in each of these areas will affect decisions in the others, which adds to the challenge. (See "Break On Through To The Other Side" by C. T. Hawley, Telephony Jan. 14, 1991, pp 38-48)
Telephone company planners continue to struggle with the placement of fiber in the distribution plant even though, as stated earlier, fiber optic networks can provide increased bandwidth, superior signal quality, greater immunity to electromagnetic interference and reduced maintenance when compared with their copper-based counterparts. It is discussed in "To PON or Not To PON? That is The Question" by J. S. McConnell, Telephony, Jan. 14, 1991, pp 50-56 that traditional approaches for placing fiber in the distribution plant vs. the copper cable alternative have not proven economically feasible.
In summary, as stated in the article by Hawley, telephone companies need an FITL architecture that supports current services and operations and provides an evolutionary path from a copper-dominated loop to an all-fiber loop. It must lead to the deployment of fiber in a way that conserves near-term capital resources and provides a fiber disposition in the loop that will allow a stream of new services to be provided with minimal rearrangements. It must gracefully integrate the loop into a survivable national synchronous optical network. The right architecture will allow the evolution of operations support to new generations of mechanized support systems in an increasingly intelligent and flexible network. It is also very important that the loop network should be as immune as possible to accident or the like which may cause faults in the network or the network must have a good survivability against such accidents. The present invention proposes a novel way of integrating fiber network into the loop and achieves these objectives.