Optical fiber networks are growing in importance as a medium for both short-haul and long-haul communications. One category of optical fiber networks consists of dense wavelength-division multiplexed (DWDM) optical networks. In a DWDM network, each optical fiber carries data in each of many distinct wavelength channels. With present technology, it is feasible for each fiber to carry eighty or more such channels. At the various nodes of the network, wavelength-selective optical elements are used to combine traffic from disparate sources onto a single fiber, and from a single fiber, to distribute traffic for disparate destinations into distinct fibers.
After a connection between a given source node and a given destination node has been established, the connection will typically traverse a route through zero, one, or plural intermediate nodes and through the optical fiber links connecting the source, intermediate, and destination nodes. The number of possible alternative routes for calls between a given source-destination pair depends upon the number of intermediate nodes, and upon the connectivity of the network; i.e., on the number of links converging on or diverging from each node. If the network is highly connected and the number of intermediate nodes is permitted to be large, e.g. on the order of ten or more, the number of alternative routes for a given call can be quite large.
In particular, flexibility in the routing, as well as the re-routing, of calls is afforded by the use of an optical element known as an Optical Cross-Connect (OXC). An OXC, which is typically deployed at an intermediate node, is used to pass transmissions between incoming and outgoing optical fibers. Through the use of an OXC, any one of a plurality of incoming fibers can be connected to any one of a plurality of outgoing optical fibers. If the OXC is used in tandem with wavelength converters, a given message can not only be switched from fiber to fiber, but also from one wavelength channel to another.
In one known OXC arrangement, for example, incoming transmissions from one or more fibers are wavelength demultiplexed into single-wavelength-channel signals, each on a separate input fiber. Then, wavelength converters are used to place each single-wavelength-channel signal onto a common working wavelength for the OXC. The OXC then couples each of the single-wavelength-channel signals, at the working wavelength, into a respective output fiber. Then, wavelength converters are used to place each signal on a respective output wavelength channel, which is not necessarily the channel on which that signal arrived. Finally, the output signals are wavelength multiplexed onto communication fibers for further transmission through the network.
Although many alternative routes may be available for calls between a given source node and a given destination node (to be referred to jointly as a “source-destination (S-D) pair”), it is often advantageous to limit the routing to only one or only a few of the possible routes. Route sets are often chosen, for example, on the basis of hop count or link count; i.e., the total number of links. To keep total accrued delay within reasonable limits, it is often desirable to limit route sets to those routes having the lowest link counts.
The term “network design” as used herein means the layout of the physical elements of a network, such as the fibers and OXCs, together with the prescribed route sets that avail themselves of those physical elements in order to meet a pre-specified set of connections or calls. A design may be static or dynamic. If dynamic, the design may be subject to periodic revision. The physical layout might be revised, for example, at periods measured in months or years. The route sets, on the other hand, could be revised on periods of, e.g., a week, day, hour, or even smaller unit of time.
A network design is economically efficient if it accommodates substantially all of the traffic demand, but does not, on average, have a substantial amount of unused capacity. To promote economic efficiency, it is advantageous when planning an initial or revised network design to consider the physical layout and the route sets together, in view of measured or anticipated demands. There is still a need for computationally tractable design procedures that take such an approach to achieving economic efficiency.