Dhritiman Banerjee

A practical approach for routing and wavelength assignment in large wavelength-routed optical networks

We consider large optical networks in which nodes employ wavelength-routing switches which enable the establishment of wavelength-division-multiplexed (WDM) channels, called lightpaths, between node pairs. We propose a practical approach to solve routing and wavelength assignment (RWA) of lightpaths in such networks. A large RWA problem is partitioned into several smaller subproblems, each of which may be solved independently and efficiently using well-known approximation techniques. A multicommodity flow formulation combined with randomized rounding is employed to calculate the routes for lightpaths. Wavelength assignments for lightpaths are performed based on graph-coloring techniques. Representative numerical examples indicate the accuracy of our algorithms.

Wavelength-routed optical networks: linear formulation, resource budgeting tradeoffs, and a reconfiguration study

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, 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 trade-offs 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.

Optical components for WDM lightwave networks

Recently, there has been growing interest in developing optical fiber networks to support the increasing bandwidth demands of multimedia applications, such as video conferencing and World Wide Web browsing. One technique for accessing the huge bandwidth available in an optical fiber is wavelength-division multiplexing (WDM). Under WDM, the optical fiber bandwidth is divided into a number of nonoverlapping wavelength bands, each of which may be accessed at peak electronic rates by an end user. By utilizing WDM in optical networks, we can achieve link capacities on the order of 50 THz. The success of WDM networks depends heavily on the available optical device technology. This paper is intended as a tutorial on some of the optical device issues in WDM networks. It discusses the basic principles of optical transmission in fiber and reviews the current state of the art in optical device technology. It introduces some of the basic components in WDM networks, discusses various implementations of these components, and provides insights into their capabilities and limitations. Then, this paper demonstrates how various optical components can be incorporated into WDM optical networks for both local and wide-area applications. Finally, the paper provides a brief review of experimental WDM networks that have been implemented.

Wavelength-routed optical-networks: linear formulation, resource budgeting tradeoffs, and a reconfiguration study

We consider a wavelength-routed optical network operated as a lightpath-based virtual topology. We present an exact linear programming formulation for the complete virtual topology design, including choice of constituent lightpaths, routes for these lightpaths, and intensity of packet flows through these lightpaths. By making a shift in the objective function to minimal hop distance 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 linearized and hence solved optimally. The linear formulation can be used to design a balanced network, such that the utilizations of both transceivers and wavelengths are high, i.e., neither of these expensive resources are under-utilized. We also use the linear formulation to provide a reconfiguration methodology in order to adapt the virtual topology to changing traffic conditions.

Passive optical network architecture based on waveguide grating routing

We explore an optical network architecture which employs dense wavelength division multiplexing (WDM) technology and passive waveguide grating routers (WGRs) to establish a virtual topology based on lightpath communication. We examine the motivation and the technical challenges involved in this approach, propose and examine the characteristics of a network design algorithm, and provide some illustrative performance results.

The multidimensional torus: analysis of average hop distance and application as a multihop lightwave network

The torus network, whose two-dimensional version is often referred to as the Manhattan Street Network (MSN), has received significant attention in the literature. Analysis of the hop distance properties for this network when its links are bidirectional is straightforward. However, the analysis of this network with unidirectional links is more complex, and the literature only provides the mean hop distance for the unidirectional version when the nodal degree is two. The authors provide a closed-form, analytical formula for the average hop distance in a three-dimensional torus network with unidirectional links, and hypothesize an approximate result for higher dimensions. The importance of this work stems from the fact that the torus is a useful candidate for the construction of an optical network with a multihop virtual topology based on wavelength division multiplexing (WDM). So far, it has not been possible to conduct fair comparisons between the torus and other multihop topologies with nodal degree greater than two, but the present work will now enable such comparisons.<<ETX>>

Constraint satisfaction in optical routing for passive wavelength-routed networks

A wavelength-routed, optical network employs all-optical channels (lightpaths) on multiple wavelengths to establish a rearrangeable interconnection pattern (virtual topology) for transport of data. A lightpath may span multiple fiber-links, and may be routed optically at an intermediate node without undergoing electronic conversion. We examine the problem of establishing a set of lightpaths in an optical network, which employs a passive wavelength routing device called a Latin Router (LR). Latin Routers are attractive for optical network design because of their fault-tolerance and low cost, but make traditional routing algorithms difficult to implement due to the complexity of the constraints they impose on legitimate routes and colors. We employ a local search algorithm to search the space of virtual topologies in order to satisfy a maximum number of given lightpath requests. We use the same algorithm to maximize the number of single-hop connections for a given network. We show that the algorithm can satisfy a high percentage of lightpaths under low to moderate network loads. Experiments reveal that we can establish O(N) lightpaths in an optical network with N nodes. We believe that our work is the first known attempt at designing optical wide-area networks using Latin Routers.

Optical interconnects for multiprocessor architectures using wavelength-division multiplexing

Multiprocessor architectures based on optical interconnects employing wavelength division multiplexing (WDM) are considered. WDM is a technique used to divide the tremendous bandwidth of a single strand of fiber into many non-interfering wavelengths. System components (processing, memory, and/or I/O elements) can use these wavelengths as communication channels. The authors present the background needed to understand WDM architecture, outline several categories of single-hop communication protocols, and show how a multihop WDM-based multiprocessor can reconfigure itself to any virtual topology.<<ETX>>

Towards passive wavelength-routed optical networks

We explore an optical network architecture which employs dense WDM technology and passive Waveguide Grating Routers (WGRs) to establish a virtual topology based on lightpaths. We examine the motivation and the technical challenges involved in this approach, propose a network design algorithm, and provide some illustrative performance results.

Upgrading a nationwide, fiber-based, electronic packet-switched network

We develop algorithms for the design of optical packet-switched networks. These algorithms are aimed at upgrading an existing fiber-based, network infrastructure, such as the NSFNET backbone, by using wavelength division multiplexing (WDM) technology. The network architecture employs wavelength-routing optical switches which enable the establishment of all-optical, WDM channels, called lightpaths. A set of lightpaths may be used as a virtual topology over which packet-switched traffic may be transported ina store-and-forward manner. The packet forwarding is done by electronic packet routers which are attached to the wavelength-routing optical switches. This paper examines issues related to the upgrading of an existing fiber-based, packet-switched, electronic network to accommodate WDM, and to the optimal choice of the virtual topology based on changing traffic demands. We demonstrate how the total information carrying capacity of the network can be enhanced by using WDM.