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This paper describes two key network architecture design concepts that relate to evolving existing transport networks into economically viable next-generation optical networks. Today's metropolitan transport networks largely consist of synchronous optical network/synchronous digital hierarchy rings or switch-to-switch fiber connections for some form of optical Ethernet. The result is an optical-electrical-optical infrastructure that has limited use in providing wavelength services. Wavelength-division multiplexing (WDM) is the enabling technology for wavelength services, but it has limited penetration in the metropolitan area due to its cost justification being dependent primarily on fiber relief. The first part of this paper shows how existing services, primarily using time-division-multiplexing (TDM) transport, can be used to economically justify a WDM infrastructure while achieving significantly lower costs than legacy design techniques would produce. Dynamic bandwidth-on-demand (BoD) service is another elusive goal envisioned for next-generation metropolitan networks. This paper addresses how an economically viable BoD infrastructure can be built based on revenues from existing enterprise services rather than relying on revenues from new and unproven services. Quantitative analyses, presented in the paper, show the key parameters that determine when BoD services will be used, how bandwidth granularity affects BoD decisions, and how the customer's use of BoD drives service provider network design considerations.