Migration from fixed grid to flexible grid in optical networks

Optical WDM backbone networks based on fixed spectrum grid have limitations such as low spectrum utilization and rigidity in provisioning for heterogeneous rates. Flexible-grid technologies can alleviate these limitations for on-demand provisioning. These technologies represent promising candidates for future optical networks supporting beyond-100-Gb/s signals. However, a one-time green-field deployment of flexible-grid technologies may not be practical, as the already-made investment in existing fixed-grid WDM networks needs to be preserved, and interruptions to ongoing services need to be minimized. Therefore, we envision that fixed-and flexible-grid technologies will coexist, which will bring the challenge of interoperating fixed-and flexible-grid equipment. It is also important to design the optimum migration strategy to maximize cost effectiveness and minimize service interruption. In this article, we discuss the key aspects of network architectures supporting coexistence of fixed and flexible grid technologies, and outline the challenges of network operations. We also propose and evaluate different migration strategies from fixed grid to flexible grid under different network scenarios.