Distributed partial-express routing of broad-band transport network demands

There are a number of contexts in transport network design where cost savings arise from providing a dedicated transmission system between two nonadjacent nodes. This occurs when the cost to terminate demands at intermediate cross-connecting and grooming nodes exceeds the alternative of providing a dedicated, perhaps only partly filled, system. Today, dedicated end-to-end "express routes" are usually found where large demands flow between major centers or to/from hubbing sites where traffic can be aggregated to warrant an express system. Smaller demand flows are usually terminated at each flexibility point en route to permit remultiplexing with other demands, thereby keeping the fiber system fill high. We consider a new means to find economic express system opportunities among combinations of the smaller co-routed demands. The hypothesis is that certain collections of demand may warrant express treatment over some common portion of their routes. These demands need not share common end nodes or be aggregated at a hub. Rather, they are discovered among the natural pattern of demand flows as a group of demands that travel together over some sequence of spans on which an express system might be cost-effective. The search problem is to detect the maximally coherent subgroups of demands en route that yield the best economic payback. Test results were obtained in four representative transport network topology and demand matrix models. Assuming a 75% level of system fill in the nonexpress baseline case, opportunities worth up to $45 M in equipment savings, relative to conventional design, were obtained using industry-supplied cost data. The concept of distributed partial-express system design may also be applicable to WDM-based networks to assign OC-n level units of demand to various express or nonexpress wavelengths.