IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 51, NO. 4, APRIL 2003 607 Wavelength Converter Placement Under Different RWA Algorithms in Wavelength-Routed All-Optical Networks Xiaowen Chu, Student Member, IEEE, Bo Li, Senior Member, IEEE, and Imrich Chlamtac, Fellow, IEEE Abstract—Sparse wavelength conversion and appropriate routing and wavelength assignment (RWA) algorithms are the two key factors in improving the blocking performance in wave- length-routed all-optical networks. It has been shown that the optimal placement of a limited number of wavelength converters in an arbitrary mesh network is an NP-complete problem. There have been various heuristic algorithms proposed in the literature, in which most of them assume that a static routing and random-wavelength assignment RWA algorithm is employed. However, the existing work shows that fixed-alternate routing and dynamic routing RWA algorithms can achieve much better blocking performance. Our study further demonstrates that the wavelength converter placement and RWA algorithms are closely related in the sense that a well-designed wavelength converter placement mechanism for a particular RWA algorithm might not work well with a different RWA algorithm. Therefore, the wave- length converter placement and the RWA have to be considered jointly. The objective of this paper is to investigate the wavelength converter placement problem under the fixed-alternate routing (FAR) algorithm and least-loaded routing (LLR) algorithm. Under the FAR algorithm, we propose a heuristic algorithm called minimum blocking probability first for wavelength converter place- ment. Under the LLR algorithm, we propose another heuristic algorithm called weighted maximum segment length. The objective of the converter placement algorithms is to minimize the overall blocking probability. Extensive simulation studies have been car- ried out over three typical mesh networks, including the 14-node NSFNET, 19-node EON, and 38-node CTNET. We observe that the proposed algorithms not only outperform existing wavelength converter placement algorithms by a large margin, but they also can achieve almost the same performance compared with full wavelength conversion under the same RWA algorithm. Index Terms—Routing and wavelength assignment (RWA), wavelength converter placement, wavelength-division multi- plexing (WDM), wavelength routing. Paper approved by W. C. Kwong, the Editor for Optical Communications of the IEEE Communications Society. Manuscript received November 6, 2001; revised June 7, 2002; August 24, 2002; October 15, 2002. This work was sup- ported in part by Research Grants Council (RGC) grants under Contract AoE/E- 01/99, Contract HKUST 6163/00E, and Contract HKUST 6195/02E. This paper was presented in part at the SPIE Optical Networking and Communications Conference (OptiComm) 2002, Boston, MA, July 29-August 2, 2002. X. Chu and B. Li are with the Department of Computer Science, Hong Kong University of Science and Technology, Kowloon, Hong Kong (e-mail: chxw@cs.ust.hk; bli@cs.ust.hk). I. Chlamtac is with the Erik Johnsson School of Engineering and Computer Science, The University of Texas at Dallas, Richardson, TX 75083 USA (e-mail: chlamtac@utdallas.edu). Digital Object Identifier 10.1109/TCOMM.2003.810834 I. INTRODUCTION W AVELENGTH-ROUTED all-optical networks are considered to be candidates for the next generation wide-area backbone networks [4], [14]. In such networks, wavelength conversion (or translation) plays an important role in improving the fiber link utilization and reducing the call blocking probability [10]. Since the wavelength converters 1 are still very expensive nowadays, much research work focuses on sparse wavelength conversion, which means that only part of the network nodes have the capability of wavelength conversion, while others have no conversion capability [15]. If all the network nodes are capable of wavelength conversion, this is referred to as full wavelength conversion. It has been shown in [15] that, by using sparse wavelength conversion, a relatively small number of converters can achieve satisfactory performance. However, the problem of wavelength converter placement was not considered. That is, given a net- work topology, a certain number of wavelength converters, and traffic statistics, how can the wavelength converters be placed into the network in order to minimize the overall blocking prob- ability? Usually, this is addressed as a separate issue that is solved by converter placement algorithms. The algorithms for optimal converter placement in simple topologies, such as bus and ring, have been provided in [16]. However, optimal con- verter placement for more realistic topologies such as arbitrary mesh is considered to be very hard. Hence, a number of heuristic algorithms have been proposed [1], [9], [11], [18]. All of them assume that the static routing and random wavelength assign- ment (RWA) algorithm is employed. Nevertheless, the literature results show that the blocking probabilities of wavelength-routed networks are heavily de- pendent on the RWA algorithms [5], [8], [12]. Our studies also demonstrate that a well-designed wavelength converter place- ment mechanism for the static RWA algorithm does not work well under a different RWA algorithm. Therefore, we argue that wavelength converter placement and RWA algorithms should be considered jointly. In this paper, we investigate the problem of wavelength converter placement under two RWA algorithms, both of which have exhibited that better blocking performance can be obtained. The first one is the fixed-alternate routing and first-fit wavelength assignment (FAR-FF RWA) algorithm. The second 1 In this paper, wavelength converter means a wavelength router with the ca- pability of wavelength conversion. 0090-6778/03$17.00 © 2003 IEEE