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Abstract--We present algorithms for the design of optimal virtual topologies embedded on wide-area wavelength-routed optical networks. The physical network architecture employs wavelength-conversion-enabled wavelength-routing switches (WRS) at the routing nodes, which allow the establishment of circuit-switched all-optical wavelength-division multiplexed (WDM) channels, called lightpaths. We assume packet-based traffic in the network, such that a packet travelling from its source to its destination may have to multihop through one or more such lightpaths.We present an exact integer linear programming (ILP) formulation for the complete virtual topology design, including choice of the constituent lightpaths, routes for these lightpaths, and intensity of packet flows through these lightpaths. By minimizing the average packet hop distance in our objective function and by relaxing the wavelength-continuity constraints (i.e., assuming wavelength converters at all nodes), we demonstrate that the entire optical network design problem can be considerably simplified and made computationally tractable.Although an ILP may take an exponential amount of time to obtain an exact optimal solution, we demonstrate that terminating the optimization within the first few iterations of the branch-and-bound method provides high-quality solutions. We ran experiments using the CPLEX optimization package on the NSFNET topology, a subset of the PACBELL network topology, as well as a third random topology to substantiate this conjecture.Minimizing the average packet hop distance is equivalent to maximizing the total network throughput under balanced flows through the lightpaths. The problem formulation can be used to design a balanced network, such that the utilizations of both transceivers and wavelengths in the network are maximized, thus reducing the cost of the network equipment. We analyze the tradeoffs in budgeting of resources (transceivers and switch sizes) in the optical network, and demonstrate how an improperly designed network may have low utilization of any one of these resources. We also use the problem formulation to provide a reconfiguration methodology in order to adapt the virtual topology to changing traffic conditions.

We consider an IP-over-WDM network in which network nodes employ optical crossconnects and IP routers. Nodes are connected by fibers to form a mesh topology. Any two IP routers in this network can be connected together by an all-optical wavelength-division multiplexing (WDM) channel, called a lightpath, and the collection of lightpaths that are set up form a virtual topology. In this paper, we concentrate on single fiber failures, since they are the predominant form of failures in optical networks. Since each lightpath is expected to operate at a rate of few gigabits per second, a fiber failure can cause a significant loss of bandwidth and revenue. Thus, the network designer must provide a fault-management technique that combats fiber failures. We consider two faultmanagement techniques in an IP-over-WDM network: 1) provide protection at the WDM layer (i.e., set up a backup lightpath for every primary lightpath) or 2) provide restoration at the IP layer (i.e., overprovision the network so that after a fiber failure, the network should still be able to carry all the traffic it was carrying before the fiber failure). We formulate these fault-management problems mathematically, develop heuristics to find efficient solutions in typical networks, and analyze their characteristics (e.g., maximum guaranteed network capacity in the event of a fiber failure and the recovery time) relative to each other.

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Multicasting is the ability of a communication network to accept a single message from an application and to deliver copies of the message to multiple recipients at different locations. Recentlv. there has been an exolosion of research literature on < .multicast communication. This ulticasling is the ability of a communication network to acccpt a single message from an application and to dclivcr copies of the mcssagc to multiple recipients at different locations. One of the cliallcngcs is to minimize tho amount of network resources employcd by multicasting. To illustrate this point, k t us assume that a vidco server wants to transmit a movie to 1000 recipients ( Fig. l a ) . If the server wcre to cmploy 1000 scparate point-to-point connections (e.g., TCP connections), 1000 copies of the movie may have ti) be sent over a singlc link, thus making poor usc of the availablc bandwidth. An efficient implementation of multicasting permits much bettcr nsc of the availeblc bandwidth by transmitting at most onc copy of the movic on cach link in the nctwork, as shown in Fig. lb. Rcccntly, there has bccn a lot of research in the area of multicast communication. Although many excellent surveys and books cxist which cxamine varions aspccts of multicasting [I-61, i n thc course of our studies wc have found a need for a tutorial-cum-survcy of the various multicast routing algorithms and their relationship with mnlticast routing protocols. In this work we present a tutorial-cum-survcy of the following two important topics in multicasting:-Multicast routing algorithms * Multicast routing protocols Communication networks can be classificd into two categories: local area networks (LANs) and wide area nctworks (WANs). A LAN spans a small geographical area, typically a single building or a cluster of buildings, while a WAN spans a large geographical area (e.g., a nation). Often, nodes connccted to a LAN communicatc over a broadcast network, while nodes connected to a WAN communicate via a switched network. In a broadcast LAN, a transmission from any one node is received by all the nodes on the network; thus, multicasting is easily implementThis work has heor supporied inpar( />y ike Nalional Suie,rce Foundution (NSF) under G,owts Nos. NCR 9508238 nnd ANI-9XO52U5. . _ I u&"g a movie" 1000 di&ent users; b) multicasting the movie. (R= slandard router, MR= rnullicast router.) 90 ox~o-xn4~~nn1~io.oo o 2uuo IEEE IEEE Network * JanuaryiFebrualy 2000

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