Over the last several years, there has been a tremendous growth in data traffic spurred primarily by the World Wide Web. This growth has posed a great need for large amounts of bandwidth. At the same time, revenues from the data traffic have not been growing as fast, causing carriers to look for the cheapest possible deployments. It is now clear that optical networks more than satisfactorily satisfy these needs by providing large amounts of bandwidth over a single fiber.
Since the 1980's, optical networks have been the primary point-to-point transmission links in a voice or data network. Here, traffic is converted into light at the source, transmitted through fiber over large distances, and converted back to the electrical domain at the destination. If the distance between the source and the destination is too high, the signal may have to be regenerated through an optical-electrical-optical (OEO) conversion along the way. Often, these systems are deployed as links in ring topologies carrying Synchronous Optical Network (SONET) signals, with each ring consisting of two or four fibers for protection. However, as traffic grows, a single ring quickly becomes insufficient and multiple SONET rings are stacked on top of each other to support growing traffic. This solution is not economically scalable because each ring of fibers requires its own transmission system equipment.
Dense Wave Division Multiplexing (DWDM) technology addressed this problem by enabling the transmission of multiple wavelengths (up to 128 currently) on a single fiber. Also, new amplification technologies like Erbium Doped Fiber Amplifiers (EDFAs) and Raman pumps supported lumped amplification of a large spectrum of wavelengths and increased the reach of the signal. Together, these technologies allowed multiple traffic flows over a single strand of fiber using a common set of equipment, whether they are SONET signals or Gigabit Ethernet (GigE) channels or Multi-Protocol Label Switched (MPLS) tunnels. Further, they allowed the signal to go far without expensive OEO conversions. This dramatically reduced the costs of the networks while increasing their reach.
All of these advances in transmission technologies bring in a new set of issues. The modern transmission systems obtained high capacity and long reach by exploiting the physics of transmission media to the extreme. This has made them expensive and quite sensitive to fiber impairments. As a result, while designing an optical network for deployment, one has to select the amplification equipment carefully to guarantee error-free propagation of signal. Also, it is critical to minimize the costs associated with a transmission system deployment because these systems typically contribute the most to the overall cost of a network. Given a variety of amplifiers with varying costs and amplification strengths, the modern network designer has plenty of design choices.
Although optical network design has been an active area of research, most of the existing studies have focused on designing ring and mesh networks which contain optical transmission systems as links. They treat the links as black-boxes with simple cost functions and do not suggest the amplifiers that need to be placed on the links. Even where link cost was considered in the design, the cost was mostly a linear function of distance and was not derived from an accurate link design.
Accordingly, improved optical network design techniques are needed.