Gurusamy Mohan

Efficient algorithms for routing dependable connections in WDM optical networks

We consider the problem of establishing dependable connections in WDM networks with dynamic traffic demands. We call a connection with fault-tolerant requirements as a dependable connection (D-connection). We consider the single-link failure model in our study and recommend the use of a proactive approach, wherein a D-connection is identified with the establishment of the primary lightpath and a backup lightpath at the time of honoring the connection request. We develop algorithms to select routes and wavelengths to establish D-connections with improved blocking performance. The algorithms use the backup multiplexing technique to efficiently utilize the wavelength channels. To further improve channel utilization, we propose a new multiplexing technique called primary-backup multiplexing. Here, a connection may not have its backup lightpath readily available throughout its existence. We develop algorithms based on this technique to route D-connections with a specified restoration guarantee. We present an efficient and computationally simple heuristic to estimate the average number of connections per link that do not have backup lightpaths readily available upon a link failure. We conduct extensive simulation experiments on different networks to study the performance of the proposed algorithms.

Lightpath restoration in WDM optical networks

Optical networks employing wavelength-division multiplexing and wavelength routing are potential candidates for future wide area networks. Because these networks are prone to component failures and carry a large volume of traffic, maintaining a high level of service availability is an important issue. This article discusses providing tolerance capability to the optical layer in WDM-based transport networks. It presents a survey on restoration schemes available in the literature, explains the operation of these schemes, and discusses their performance.

Virtual source based multicast routing in WDM optical networks

Wavelength-division multiplexed (WDM) networks using wavelength-routing are considered to be potential candidates for the next generation wide-area backbone networks. Multicasting is the ability to transmit information from a single source node to multiple destination nodes and is becoming an important requirement in high-speed networks. As WDM technology matures and multicast applications become increasingly popular, supporting multicast routing at the WDM layer becomes an important and yet a challenging topic. This paper concerns with the problem of optical multicast routing in WDM networks. A few nodes in the network may have wavelength conversion and/or splitting capabilities. In this paper, a new multicast tree construction algorithm is proposed. This algorithm is based on a concept called virtual source. A virtual source is a node having both the splitting and wavelength conversion capabilities. By exploiting the presence of virtual source nodes, the proposed algorithm achieves improved performance. To further improve the performance, the algorithm assigns priorities to nodes based on their capabilities. The effectiveness of the proposed algorithm is verified through extensive simulation experiments.

Protection approaches for dynamic traffic in IP/MPLS-over-WDM networks

Due to the explosive growth of data-related traffic driven by the Internet, network reliability becomes an important issue. We investigate various protection approaches to handle failures for dynamic traffic demands in IP/MPLS-over-WDM networks. An LSP can be protected at either the IP/MPLS layer or the optical layer. In IP/MPLS layer protection, an LSP is protected by providing a link-disjoint backup LSP between its end nodes. In optical layer protection, an LSP is protected by the backup lightpath of each lightpath traversed by the LSP. We present two integrated routing algorithms: hop-based integrated routing algorithm and bandwidth-based integrated routing algorithm (BIRA) to set up the restorable bandwidth-guaranteed paths efficiently. Then we present a multilayer protection scheme for multiclass traffic in such networks. This scheme takes into account the different QoS and recovery requirements of the traffic to provide protection capability either at the MPLS layer or at the optical layer in a cost-effective manner. We use the connection blocking probability and number of optical-electrical-optical conversions as performance metrics to compare various protection approaches.

Dynamic load balancing in IP-over-WDM optical burst switching networks

In this paper, we consider the problem of dynamic load balancing in wavelength division multiplexing (WDM)-based optical burst switching (OBS) networks. We propose a load balancing scheme based on adaptive alternate routing aimed at reducing burst loss. The key idea of adaptive alternate routing is to reduce network congestion by adaptively distributing the load between two pre-determined link-disjoint alternative paths based on the measurement of the impact of traffic load on each of them. We develop two alternative-path selection schemes to select link-disjoint alternative paths to be used by adaptive alternate routing. The path selection schemes differ in the way the cost of a path is defined and in the assumption made about the knowledge of the traffic demands. Through extensive simulation experiments for different traffic scenarios, we show that the proposed dynamic load balancing algorithm outperforms the shortest path routing and static alternate routing algorithms.

Virtual source based multicast routing in WDM networks with sparse light splitting

Wavelength-division multiplexed (WDM) networks using wavelength-routing are considered to be potential candidates for the next generation wide-area backbone networks. This paper concerns with the problem of multicast routing in WDM networks. A node in the network may have both wavelength conversion and splitting capabilities which is called as virtual source. It is assumed that virtual sources are limited in number and are distributed evenly in the network. This paper proposes a new approach, which makes use of these special capable virtual sources to construct a multicast tree.

Dynamic routing with inaccurate link state information in integrated IP-over-WDM networks

In this paper, we investigate the problem of dynamically routing bandwidth-guaranteed label switched paths (LSPs) in integrated IP-over-wavelength division multiplexed (WDM) networks with inaccurate link state information. To select a good path, a routing algorithm needs up-to-date link state information. This leads to excessive update overhead and scalability problems. In real networks, from the practical point of view, in order to avoid extensive overhead of advertising and processing link state information, updates need to be made periodically or based on a threshold trigger. This leads to inaccuracies in the link state information. Our contribution is that we consider the routing problem taking into consideration the uncertainty of link state parameters due to wavelength inaccuracy in addition to bandwidth inaccuracy. Based on the threshold-triggered update scheme, we present a probabilistic method to model the uncertainty of link state parameters. We then define a cost function reflecting the uncertainty. Depending on different cost metrics chosen to be optimized, we propose two routing algorithms considering the uncertainty of link state parameters. The objective is to minimize the impact of inaccurate information so that the blocking probability as well as setup failures are reduced. We use various performance metrics such as total blocking probability, blocking probability due to setup failures, blocking probability due to routing failures, bandwidth update frequency, and wavelength update frequency to evaluate the effectiveness of the proposed algorithms. Through extensive simulation experiments, we show that our algorithms can significantly reduce the impact of inaccurate link state information and perform very well.

An efficient dynamic protection scheme in integrated IP/WDM networks

This paper presents an efficient protection scheme to dynamically allocate restorable bandwidth-guaranteed paths in integrated IP-over-WDM networks. A restorable bandwidth-guaranteed path implies an active path and another link-disjoint backup path both with the required bandwidth. Dynamic traffic where connection requests arrive one by one with no knowledge about future requests is assumed. We consider integrated IP/WDM networks with generalized multi-protocol label switching (GMPLS) capabilities. To optimize the network resources, we provide the protection at the multi-protocol label switching (MPLS) layer and backup resources are judiciously shared among different requests at the label switched path (LSP) level. To allocate the restorable bandwidth-guaranteed paths, we propose two integrated routing algorithms: hop-based integrated routing algorithm (HIRA) and bandwidth-based integrated routing algorithm (BIRA). We use connection blocking probability and number of optical-electrical-optical (o-e-o) conversions as performance metrics to evaluate these two algorithms. Through extensive simulation experiments, we demonstrate that our algorithms perform better in terms of the above two metrics than other existing approaches.

An efficient algorithm for virtual topology reconfiguration in WDM optical ring networks

Wavelength-division multiplexed (WDM) networks using wavelength routing are emerging to be the right choice for the future transport networks. In a WDM-based transport network, the optical layer provides circuit-switched lightpath services to the client layer such as IP, SONET, and ATM. The set of lightpaths in the optical layer defines the virtual topology. Since the optical switches (cross-connects) are reconfigurable, the virtual topology can be reconfigured in accordance with the changing traffic demand pattern at the client layer in order to optimize the network performance. On the other hand, changing the virtual topology can be disruptive to the network since the traffic at each node must be buffered or re-routed while the topology is being reconfigured. We develop a reconfiguration algorithm to reduce the cost of virtual topology reconfiguration in WDM optical ring networks. The algorithm is based on the concept of splitting and merging existing lightpaths, together with cost-benefit analysis to reduce the network reconfiguration cost. Our objective is to reduce the number of lightpaths that need to be reconfigured, while ensuring that the network congestion is low. The performance of the algorithm is verified through simulation experiments.

Multi-layer protection in IP-over-WDM networks with and with no backup lightpath sharing

We consider the problem of multi-layer protection in IP-over-WDM networks. In our multi-layer protection schemes, traffic is protected either at the lightpath level or at the label switched path (LSP) level based on the restoration time requirements. In lightpath-level protection, traffic on a path (LSP) is protected by routing it over primary lightpaths which are protected at the optical layer by their respective backup lightpaths. In LSP-level protection, the traffic on a primary LSP is protected at the IP-multi-protocol label switching (MPLS) layer by a backup LSP. In this case, both primary and backup LSPs traverse unprotected lightpaths. We allow two backup LSPs to share bandwidth resource on these unprotected lightpaths if their primary LSPs are link-disjoint. In lightpath-level protection, whether backup resources can be shared or not depends on the protection mechanisms used. In 1:1 dedicated protection where the backup lightpath is pre-configured at the time of setting up of the corresponding primary lightpath, sharing is not possible. However, sharing is allowed in shared protection where the backup lightpaths are configured only after failure. We consider these two schemes and refer them as multi-layer protection with no backup lightpath sharing (MLP-NLS) and multi-layer protection with backup lightpath sharing (MLP-LS). An inter-level sharing (ILS) method is proposed to improve resource utilization in MLP-NLS, by allowing dedicated backup lightpaths to be used by backup LSPs. Two integrated-routing algorithms are developed to select paths in lightpath-level protection and LSP-level protection with the objective to utilize the network resources efficiently. We verify the effectiveness of the proposed multi-layer protection schemes through simulation results on the NSFNET and Pan-European networks.