I would answer this question differently if I were considering the use of the network to be a service providers network or an enterprise network in that I could achieve a different cost model based on single versus multiple customers on the same networks. Also, the end needs of each customer are different, and if they were completely defined could lead to a different cost model for the services rendered.
Generally, a service providers network requires support for TDM and data protocols, requires stringent Service Level Agreements, and generally either owns their own or has a lower cost per mile for fiber than an enterprise. For this reason, a SONET/SDH based network with data capabilities provides the most cost effective way to transport the variety of circuits and streams from the end users location to the point-of-presence (POP) and a WDM system based network is used for the long haul and inter-POP traffic is used. Data can be Ethernet over SONET/SDH or WDM, or it can be done with RPR (802.17).
The enterprise is usually seen as more fiber constrained, and therefore uses a metro WDM (Coarse or Dense) to transport SAN and Ethernet connections between buildings. The ability for the enterprise to convert most legacy traffic into Ethernet, and the ability to combine Ethernet ports into ever-larger trunk speeds, can also lead to very cost effective Ethernet over dark fiber networks up to the new 10 Gigabit per second standard. RPR systems can also be used to extend SONET/SDH recovery mechanisms to Layer 2 and 3 networks.
With voice and video moving to native IP, and the ability to tunnel most other legacy traffic over IP, Metro Ethernet systems over dark fiber, or extended over WDM or SONET/SDH systems are beginning to be the most cost effective way for businesses to connect within the campus or metro today.
The question we have to ask whenever we build a network is what will the next 10 years bring us. My vision is one of lots of devices with wireless mobility, the mixing of SIP and HTTP services to the mobile devices, and lots of high speed Global Area Networking overhead to figure out who and where you are. What is your vision of the traffic and protocols you will be required to support in ten years?
