Multi-Granular Optical Switching: A Classified Overview for the Past and Future

The wavelength routed network (WRN) accommodates traffic demands by establishing lightpaths along the corresponding routing paths. The wavelength of each lightpath is switched individually by traditional wavelength optical cross-connects (T-OXCs) to transit the traffic. Due to the traffic explosion and the resulted growth in wavelength number, WRNs face a challenge of the increase in node-size (i.e., the port number of a T-OXC) as well as the associated cost and control complexity. As an alternative solution, waveband switching (WBS) is introduced to group multiple wavelengths together as a band or fiber. Whenever possible, the group of wavelengths requires just a single port at a multi-granular optical cross-connect (MG-OXC). One fundamental problem in WBS networks is the routing and wavelength assignment (RWA). With the major goal of minimizing the port numbers in WBS networks, the optimal RWA problem was shown to be NP-Hard. In the literature, various Integer Linear Programming models are proposed to optimally solve a small-size RWA problem, and many heuristic algorithms are proposed to provide a practical solution for the large-scale RWA problem in WBS networks. In this work, we comprehensively review literature studies on waveband switching networks. The topics covered include architecture, RWA problem solving strategies, and future challenges of wavelength conversion, protection, and lightpath rerouting in WBS networks. We aim at presenting a classified view of WBS networks, based on various aspects including the traffic pattern, node and network architecture, grouping policy, and the band configurations. We investigate factors that affect the goal of port reduction and blocking minimization in WBS networks. In addition, we explore several unique features of waveband switching in protection, wavelength conversion and rerouting, along which we point out multiple open challenges in WBS networks that deserve further studies.