Throughout this book the approaches taken to system design and performance evaluation are based on the constraints of the enabling technology. Available fiber capacity is assumed to be limited by the constraints and imperfections of optical transceivers, amplifiers, and cross-connects. These constraints affect maximum available spectrum, wavelength spacing, and maximum bit rates per channel. Optical connections are assumed to have limited reach, both geographically and in terms of the number of optical cross-connects they may traverse. Sizes of switches as well as their speed, complexity, and functionality are also assumed to be limited by cost and performance constraints, ultimately going back to the limits of the underlying technologies. Trade-offs between optical and electronic methods of implementing connectivity and routing are suggested, in which the optimal design point depends again on relative cost and performance of the enabling technologies.

Although emphasizing that these technological constraints are paramount, we purposely keep as much of a separation as possible between the architectures discussed in the book and the limitations of any specific technology. The reason is obvious: Today's technology is likely to be obsolete tomorrow. After more than a decade of gestation in the laboratory, photonic and optoelectronic technology has matured to the point where a wide range of technological choices are available for implementing each function in a network, so that cost-effectiveness and viability in the field are the primary issues now rather than proof of concept, which was the issue in the network testbeds just a few years ago.