Cost Evaluation of Hybrid-Hierarchical Optical Cross-connects based Optical Path Networks Hai-Chau Le, Hiroshi Hasegawa, Ken-ichi Sato Department of Electrical Engineering and Computer Science Nagoya University Japan le_hicho@echo.nuee.nagoya-u.ac.jp, {hasegawa, sato}@nuee.nagoya-u.ac.jp Abstract—In this paper, we present a hybrid-hierarchical optical cross-connect (hybrid-HOXC) architecture that employs a waveband cross-connect for adding/dropping or routing waveband paths and an electrical cross-connect to grooming wavelength paths. Then, we have proposed a heuristic design algorithm for the hybrid-HOXCs based optical path networks. Its effectiveness is evaluated through numerical experiments with different traffic loads and network parameters. Keywords- Hierarchical optical path networks, waveband, network design algorithm, routing and wavelength assignment I. INTRODUCTION The WDM technologies and the advances in related optical technologies have utilized the broad bandwidth of optical fibers and enhanced the capacity of backbone and metro networks. Current transport networks are widely employing point-to- point WDM transmission systems and electrical routing systems with OE/EO (Optical to Electrical/Electrical to Optical) conversions to meet the explosive traffic demand driven by world-wide deployments of broadband accesses such as xDSLs and FTTx. Since the traffic demand still rapidly increases, electrical routing and OEO conversions will create a system bottleneck and will result in an intolerable cost for realizing transport networks with sufficient capacity. Single layer optical path networks that utilize the wavelength path routing made possible with optical cross-connects (OXCs) and ROADMs have been introduced as the first step [1, 2] and a large number of ROADMs based ring networks has been implemented. In near future, an explosive growth of the Internet traffic expected due to the penetration of new broadband services including IP-TV and High-Definition TV, and the advances in WDM technology will result in an substantial increase in the switch size and cost of the OXCs and ROADMs in the single layer optical path networks. To support the ever-increasing traffic while avoiding the explosion of the cost and complexity of optical switches, hierarchical optical path networks that employ hierarchical optical cross-connects (HOXCs) using waveband switching have been developed [3-8]. The hierarchical optical path cross-connects are capable of switching optical signals at different granularities, a wavelength path, a waveband (a group of wavelengths) and even a fiber levels. It has been verified that the hierarchical optical path networks can substantially reduce network cost [9- 16]. In the hierarchical optical cross-connect architectures, switching fabrics required for switching higher-order optical paths (i.e. wavebands) are small, however, that required for lower-order optical paths (wavelength paths) are very large and have been not yet realizable with present optical switching technologies. Fortunately, such large wavelength switches are possible in the electrical domain. Usage of electrical switches can naturally provide wavelength conversion and 3R regeneration functions due to the necessity of using OE/EO converters at the inputs and outputs of the electrical switching matrices. Studies in [4, 5, 11] had proposed hybrid-hierarchical optical cross-connect architectures (hybrid-HOXCs) in which the optical wavelength cross-connect (WXC) is entirely replaced by an electrical one or an electrical TDM switch at wavelength speeds. As the results, the required electrical switch scale must be as large as that of the WXC. However, since such large electrical switches are costly devices and consume huge power supplies, their implementations should be limited. On the other hand, optical path network design problem to minimize the given network cost function, even for the single layer optical path networks, is known as NP-complete problem due to the difficulties of RWA (Routing and Wavelength Assignment). In the hierarchical optical path networks, we must solve not only the RWA problem but also waveband path routing and waveband assignment problem. Moreover, ingress/egress nodes of waveband paths must be optimally selected. In the hierarchical optical path networks based on the hybrid-HOXCs, although the wavelength conversion capability reduces the difficulty of RWA for wavelength paths, we still have to solve the waveband placement and routing and waveband assignment problems. In this paper, firstly, to take advantages of the electrical switches while carefully restricts the usages of expensive electrical switch ports and power consumption of the node, we present here a new hybrid-waveband cross-connect architecture that consists of an optical waveband cross-connect (BXC) for adding/dropping or routing large granular optical paths and an electrical cross-connect (EXC) only to intermediately groom wavelength paths. The architecture exploits large throughput of a BXC and grooming wavelength paths accompanied with 3R and wavelength conversion capabilities of the EXC at 978-1-4244-7057-0/10/$26.00 ©2010 IEEE 40