S. Ramamurthy

Survivable WDM mesh networks

In a wavelength-division-multiplexing (WDM) optical network, the failure of network elements (e.g., fiber links and cross connects) may cause the failure of several optical channels, thereby leading to large data losses. This study examines different approaches to protect a mesh-based WDM optical network from such failures. These approaches are based on two survivability paradigms: 1) path protection/restoration and 2) link protection/restoration. The study examines the wavelength capacity requirements, and routing and wavelength assignment of primary and backup paths for path and link protection and proposes distributed protocols for path and link restoration. The study also examines the protection-switching time and the restoration time for each of these schemes, and the susceptibility of these schemes to multiple link failures. The numerical results obtained for a representative network topology with random traffic demands demonstrate that there is a tradeoff between the capacity utilization and the susceptibility to multiple link failures. We find that, on one hand, path protection provides significant capacity savings over link protection, and shared protection provides significant savings over dedicated protection; while on the other hand, path protection is more susceptible to multiple link failures than link protection, and shared protection is more susceptible to multiple link failures than dedicated protection. We formulate a model of protection-switching times for the different protection schemes based on a fully distributed control network. We propose distributed control protocols for path and link restoration. Numerical results obtained by simulating these protocols indicate that, for a representative network topology, path restoration has a better restoration efficiency than link restoration, and link restoration has a faster restoration time compared with path restoration.

Survivable WDM mesh networks. Part I-Protection

This investigation considers optical networks which employ wavelength cross-connects that enable the establishment of wavelength-division-multiplexed (WDM) channels, between node-pairs. In such and other networks, the failure of a network element (e.g., fiber link, cross-connect, etc.) may cause the failure of several optical channels, thereby leading to large data losses. This study examines different approaches to protect mesh based WDM optical networks from single-link failures. These approaches are based on two basic survivability paradigms: (a) path protection/restoration, and (b) link protection/restoration. In path- and link-protection schemes, backup paths and wavelengths are reserved in advance at the time of call setup. Path- and link-restoration schemes are dynamic schemes in which backup paths are discovered (from the spare capacity in the network) upon the occurrence of a failure. In part 1 of this study presented in this paper, we formulated integer linear programs to determine the capacity requirements for the above protection schemes for a static traffic demand.

Fault management in IP-over-WDM networks: WDM protection versus IP restoration

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 fault-management 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.

Survivable WDM mesh networks. II. Restoration

This investigation considers optical networks which employ wavelength cross-connects that enable the establishment of wavelength-division-multiplexed (WDM) channels, between node-pairs. In such and other networks, the failure of a network element may cause the failure of several optical channels, thereby leading to large data losses. Ramamurthy and Mukherjee formulated integer linear programs to determine the capacity requirements for different protection schemes for a static traffic demand. In this paper, we formulate a model of protection switching times for the different protection schemes, and propose distributed control protocols for path and link restoration, assuming a fully distributed control network. Based on our assumptions, we find that when the cross connect configuration time is low (/spl les/10 /spl mu/s), the protection schemes in increasing order of average protection-switching times are as follows: (a) shared-link, (b) dedicated-path, and (c) shared-path. When the cross-connect configuration time is high (/spl ges/500 /spl mu/s), the protection schemes in increasing order of average protection-switching times are as follows: (a) dedicated-path, (c) shared-link, and (d) shared-path. Numerical results obtained by simulating the distributed restoration protocols indicate that, for a representative network topology, path restoration has a better restoration efficiency than link restoration, and link restoration has a better restoration time compared to path restoration.

Fixed-alternate routing and wavelength conversion in wavelength-routed optical networks

This paper considers optical networks which employ wavelength-routing switches that enable the establishment of wavelength-division-multiplexed (WDM) connections between node-pairs. Alternate routing improves the blocking performance of such networks by providing multiple possible paths between node-pairs. Wavelength conversion improves the blocking performance of such networks by allowing a connection to use different wavelengths along its route. This paper proposes an approximate computational model that incorporates alternate routing and sparse wavelength conversion. The model is shown to give reasonably good estimates of different network parameters. Empirical studies based on discrete-event simulation, illustrate the importance of alternate routing in improving the blocking performance of a wavelength-routed optical network.

Connection management for wavelength-routed WDM networks

In wavelength-routed WDM networks, a control mechanism is required to set up and take down all-optical connections. Upon the arrival of a connection request, this mechanism must be able to select a route, assign a wavelength to the connection, and configure the appropriate optical switches in the network. The mechanism must also be able to provide updates to reflect which wavelengths are currently being used on each link so that the nodes may make informed routing decisions. We investigate and compare different distributed control mechanisms for establishing all-optical connections in a wavelength-routed WDM network.

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>>

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>>

Optimized partitioning of a wavelength distributed data interface ring network

A fiber optic ring network, such as fiber distributed data interface (FDDI), can be operated over multiple wavelengths on its existing fiber plant consisting of point-to-point fiber links. Using wavelength division multiplexing (WDM) technology, FDDI nodes can be partitioned to operate over multiple subnetworks, with each subnetwork operating independently on a different wavelength, and inter-subnetwork traffic forwarding performed by a bridge. For this multiwavelength version of FDDI, which we refer to as wavelength distributed data interface (WDDI), we examine the necessary upgrades to the architecture of a FDDI node, including its possibility to serve as a bridge. The main motivation behind this study is that, as network traffic scales beyond (the single-wavelength) FDDIs information-carrying capacity, its multiwavelength version, WDDI, can gracefully accommodate such traffic growth. A number of design choices exist in constructing a good WDDI network. Specifically, we investigate algorithms using which, based on prevailing traffic conditions, partitioning of nodes into subnetworks can be performed in an optimized fashion. Our algorithms partition the nodes into subrings, such that the total traffic flow in the network and/or the network-wide average packet delay is minimized. >