International Journal of Hybrid Information Technology Vol.2, No.2, April, 2009 71 Routing in Degree-constrained FSO Mesh Networks Ziping Hu, Pramode Verma, and James Sluss Jr. School of Electrical & Computer Engineering The University of Oklahoma, Schusterman Center 4502 E. 41 st Street, Tulsa, OK 74135-2512, USA {ziping.hu, pverma, sluss}@ou.edu Abstract This paper addresses the routing problem in packet switching free-space optical (FSO) mesh networks. FSO mesh networks are emerging as broadband communication networks because of their high bandwidth (up to Gbps), low cost, and easy installation. Physical layer topology design of degree-constrained FSO mesh networks has been studied in a recent communication [1]. In this paper, we propose four different routing algorithms, and evaluate their performances through simulations for a number of FSO mesh networks with different topologies and nodal degrees. The performance parameter against which we evaluate these algorithms is the mean end-to-end delay. Our proposed least cost path (LCP) routing algorithm, which is based on minimizing the end-to-end delay, is considered as the bench mark. The performance of each of other three proposed algorithms is evaluated against the bench mark. Our proposed minimum hop count with load-balancing (MHLB) routing algorithm is based on the number of hops between the source and the destination node to route the traffic. Simulations show that the MHLB routing algorithm performs best in most cases compared with the other two. It results in minimum average delay and least blocked traffic. 1. Introduction FSO networks are emerging as broadband communication networks because of their high bandwidth (up to Gbps), low cost, and easy installation. An FSO network consists of a set of geographically distributed FSO nodes and FSO links interconnecting the nodes. Each FSO node can carry a router and several transceivers. An FSO node can carry only limited number of transceivers due to size, weight and power issues. Each transceiver operates both in transmitting and receiving modes. FSO links that form the communication channels of FSO networks are point-to-point directional light beams. To improve the performance of FSO networks through network design, the two major issues are topology design and routing. Traditionally, for wired communication networks such as fiber-optic networks, a fixed physical layer topology is formed based on external traffic flow requirements and/or other requirements. Routing is then a task of finding optimal logical connections that can be mapped on the physical layer topology in order to achieve low delay, high throughput, or reduced congestion. Research in [2] presents a delay-constrained minimum hop (DCMH) distributed routing algorithm for real time communication applications. An optimal diverse routing algorithm is proposed in [3] to find the shortest pair of physically-disjoint paths in order to improve the reliability of fiber optical networks. Reference [4] presents an algorithm that computes the shortest path from a given source to a destination for any number of hops for QoS routing. Research in [5] extends the work in [4],