Photonic Network Communications, 2:4, 335±348, 2000 # 2000 Kluwer Academic Publishers. Manufactured in The Netherlands. Wavelength Conversion in WDM Optical Networks: Strategies and Algorithms for Limiting the Number of Converters in the Optical Cross-Connects Marco Listanti INFO-COM Department, University of Rome, Via Eudossiana18, 00184 Rome, Italy Roberto Sabella Ericsson Telecomunicazioni, R & D Division, v. Anagnina 203, 00040 Rome, Italy Accepted May 18, 2000 Abstract. This paper considers the problem of wavelength conversion in optical networks using wavelength division multiplexing technique. In the previous literature, two main wavelength routing and assignment strategies have been introduced: wavelength path (WP) and virtual wavelength path (VWP), depending on whether the signal stays on the same wavelength or is converted to another during its travel throughout the network. While the former method does not require any wavelength conversion, the latter needs wavelength conversion in each optical node and, in particular, a wavelength converter per each signal handled by the node itself. From the previous literature emerged that the VWP leads to optical cross-connect (OXC) with lower dimensions compared to the ones required by the WP scheme, and that the difference between the WP and VWP schemes increases as the number of wavelengths carried by each ®ber increases. In this paper a new strategy is introduced, named partial virtual wavelength path (PVWP), with the related wavelength routing and assignment algorithm, which makes limited use of wavelength conversion compared to the VWP scheme, and allows the same advantages of VWP to be attained with lower OXC dimensions. The paper reports a comparative analysis among the different strategies, considering both the cases of a network without failures and a network with the possibility of failure restoration. The main result is that the proposed PVWP strategy allows the same advantages of the VWP scheme with a strongly reduced number of wavelength converters (around 5% of the number required by VWP scheme). This ®gure does not vary appreciably if failure restoration is considered. The new strategy can be adopted by using an opportune OXC architecture, as illustrated in the paper, which allow a limited number of converters to be shared among all the channels as a common pool. Keywords: optical networks, WDM, OXC, routing policies, wavelength conversion, wavelength routing algorithms, wavelength assignment algorithms I Introduction All-optical wavelength division multiplexed (WDM) networks using wavelength routing are widely considered to be valid candidates for the next generation of wide-area transport networks. A WDM optical network can conveniently exploit the large bandwidth of optical ®bers to realize many channels, each at a proper wavelength, can achieve channel routing without the need of opto-electronic conver- sion, and can relieve the electronic bottlenecks. WDM optical networks have been demonstrated in the framework of research projects both in Europe [1± 3], in USA [4,5], and in Japan [6,7]. In this networks dynamically controllable optical cross-connects (OXCs), connected by ®bers, provide purely optical transport between pairs of network nodes. Together with wavelength routing, wavelength conversion (or wavelength translation) is viewed as an important capability for better enabling scalability and improving the performance of all-optical networks. There is still a signi®cant debate on the fact whether it represents a real need and, in any case, whether the cost of wavelength converters is justi®ed by the advantages they provide. Mainly, two optical path (OP) strategies have been proposed: the wavelength path (WP) scheme and the virtual wavelength path (VWP) scheme [8,9]. In the former, one wavelength is assigned to each path, form the source node to the destination node. In the latter,