Wavelength-routed All-optical Networks Research Articles

Routing and wavelength assignment for folded hypercube in linear array WDM optical networks

The folded hypercube is one of the hypercube variants and is of great significance in the study of interconnection networks. In a folded hypercube, information can be broadcast using efficient distributed algorithms. In the context of parallel computing, folded hypercube has been studied as a possible network topology as an alternative to the hypercube. The routing and wavelength assignment (RWA) problem is significant, since it improves the performance of wavelength-routed all-optical networks constructed using wavelength division multiplexing approach. Given the physical network topology, the aim of the RWA problem is to establish routes for the connection requests and assign the fewest possible wavelengths in accordance with the wavelength continuity and distinct wavelength constraints. This paper discusses the RWA problem in a linear array for the folded hypercube communication pattern by using the congestion technique.

Solving routing and wavelength assignment problem with maximum edge-disjoint paths

The routing and wavelength assignment (RWA) problem in wave-length-division multiplexing optical networks is critically important for increasing the efficiency of wavelength-routed all-optical networks. Given the physical network structure and a set of connection requests, the problem aims to establishing routes for the requests and assigning a wavelength to each of them, subject to the wavelength continuous and wavelength clash constraints. The objective is to minimize he number of required wavelengths to satisfy all requests. The RWA problem was proven to be NP-hard and has been investigated for decades. In this paper, an algorithm based on the maximum edge-disjoint paths is proposed to tackle the RWA problem. Its performance is verified by experiments on some realistic network topologies. Compared with the state-of-the-art bin-packing based methods and the particle swarm optimization algorithm, the proposed method can find the best solution among all testing instances in reasonable computing time.

Topological optimization for spare-sharing-based wavelength-routed all-optical networks

In this paper, we propose a two-level fault tolerance strategy for wavelength-routed all-optical networks. The first-level strategy is applied to handle the large-scale disaster induced failures while the second-level strategy protects the network against regular single-link failures. The first-level fault tolerance is achieved by solving a topological optimization problem to re-regulate the traffic away from the disaster-affected area with minimum resource cost. Shared lightpath protection is applied in the second-level fault tolerance design to reduce resource allocation. First, by comparing with a simple greedy approach that we develop, we show that the traditional Routing and Wavelength Assignment (RWA) method, in which the routing and wavelength assignment are considered in a separate fashion, cannot lead to satisfying performance. Next, in order to obtain better performance, based on drawback analysis of the greedy approach, we propose a two-phase heuristic algorithm, in which the first phase is designed to generate an initial feasible solution and the second phase iteratively perfects the initial solution until no improvement can be made. For the design of the first phase, two variations are proposed featuring different types of initial solution generation. The numerical results show that, combined with perfection phase, both design variations can lead to considerable performance improvement over the greedy solutions. Finally, we propose a Performance Indicator (PI) that provides insight into the reason for performance difference among algorithms.

Review of Dynamic Impairment-Aware Routing and Wavelength Assignment Techniques in All-Optical Wavelength-Routed Networks

Since light-paths are the basic connections in wavelength routed networks, their effective establishment is very important. Routing and Wavelength Assignment (RWA) techniques can be divided into two categories. The first category (pure RWA) concentrates on setting up light-paths under the assumption of an ideal physical layer. However, this assumption could be suitable for opaque networks, where a signal is regenerated at each optical switch along its path. On the other hand, as an optical signal propagates along a light-path to its destination in a transparent (all-optical) network, the signal's quality degrades because there is no signal regeneration, thus increasing Bit-Error-Rate (BER) of the signal. However, users would not accept a light-path with a high BER. Even it is not acceptable if the establishment of a light-path causes the BER of other existing light-paths to become unacceptably high. Therefore, considering physical layer impairments, the quality of a light-path must be checked during the light-path setup in the second category. In this article, the operations of dynamic RWA techniques proposed in transparent networks for the second category are reviewed in detail. These techniques are called Quality of Transmission Aware (QoT-aware) RWA and are grouped in two groups: integrated QoT and RWA, and QoT after Pure RWA. Each group can be further divided into direct modelling and indirect modelling sub-groups. A comprehensive discussion is also provided to compare dynamic QoT-aware RWA techniques based on different network and physical layer parameters.

Blocking probability calculation in wavelength-routed all-optical networks

In this paper we have proposed a mathematical model to reduce the blocking probability of the WDM optical network. The mathematical model proposed has a closed-form expression and does not require simulated statistics, it has low implementation complexity and the computation is quite efficient. This model suggests us to choose the best path and appropriate number of free wavelengths in the network. We can go for the compromise between the path length and number of free wavelength. The model is also used to evaluate the blocking performance of NSFNet topology and hence used to improve its performance.

Survivable routing and wavelength assignment algorithm for multiple link failures in wavelength-routed all-optical WDM networks

In this paper, we have investigated the problem of enhancing multiple-fault restorability in the path protected wavelength-routed all-optical WDM networks. The system architecture considered is circuit-switched with dynamic arrival of session requests. We have proposed a mechanism, which is used to combat multiple link failures. A survivable routing and wavelength assignment algorithm has been proposed which is very efficient for the networks having multiple failures. This algorithm deals with the optical networks with multiple faults and is effective for the varying load applied to nodes. This algorithm works well for the load applied to the nodes varying from low to high.

Dynamic Impairment-Aware RWA in Multifiber Wavelength-Routed All-Optical Networks Supporting Class-Based Traffic

A quality-of-transmission- (QoT-) aware routing and wavelength assignment (RWA) technique suitable for multifiber wavelength-routed all-optical networks supporting class-based lightpaths (LPs) is proposed, where QoT is evaluated not only for any candidate LP being set up but also for already-set-up LPs. In this paper, QoT refers to the quality of transmission at the physical layer and quality of service (QoS) refers to the network-level quality issues. Four performance improvement techniques are proposed: (1) preempting an existing low-quality (LQ) LP by a high-quality (HQ) connection considering bandwidth utilization to provide better QoS for HQ traffic; (2) class-based buffering of connection requests in edge switches to provide the chance of setup for the LPs that are going to be blocked, and therefore, improving the performance of the LP setup; (3) differentiated routing in which LQ LPs are routed through large dispersion paths and HQ LPs through small dispersion paths; and (4) wavelength ordering to reduce crosstalk in switches and interchannel crosstalk on fibers. Our performance evaluations show the effectiveness of the proposed improvements in reducing blocking probability.

Blocking in wavelength-routed all-optical WDM network with wavelength conversion

The blocking probability in wavelength-routed all optical networks is very important measure of performance of the network, which can be affected by many factors such as network topology, traffic load, number of links, algorithms employed and whether wavelength conversion is available or not. In this paper, we have proposed a mathematical model to reduce the blocking probability of the WDM optical network for wavelength-convertible networks. The model can be used to evaluate the blocking performance of any network topology also it can be useful to improve its blocking performance of the given network topology. The blocking probability variation of the network for a particular load (per link) has been studied based on the load variation and total number of wavelengths used in the network. This model gives good results for high load (per link).

Fault-tolerant routing and wavelength assignment algorithm for multiple link failures in wavelength-routed all-optical WDM networks

In this paper, we investigate the problem of enhancing multiple-fault restorability in the path protected wavelength-routed all-optical WDM networks. The system architecture considered is circuit-switched with dynamic arrival of session requests. We propose a mechanism, which is used to combat multiple link failures. A routing and wavelength assignment algorithm has been proposed with the name of fault-tolerant routing and wavelength assignment algorithm. The comparison of this algorithm has also been made with the best-fit and first-fit algorithms. This algorithm deals with the optical networks with multiple faults and is effective for the varying load applied to nodes. This algorithm works well for the load applied to the nodes varying from low to high.

Blocking in wavelength-routed all-optical WDM network with or without wavelength conversion

The blocking probability in wavelength-routed all-optical networks is a very important measure of performance of the network. This blocking probability can be affected by many factors such as network topology, traffic load, number of links, algorithms employed and whether wavelength conversion is available or not. In this paper we have proposed a mathematical model to reduce the blocking probability of the WDM optical network for both wavelength convertible networks as well as for wavelength non-convertible networks. The model is can be used to evaluate the blocking performance of any network topology also it can be useful to improve its performance of the given network topology.

Sparse-partial wavelength conversion: Converter placement and wavelength assignment

Wavelength conversion has been shown as one of the key techniques to improve blocking performance in a wavelength-routed all-optical WDM network. Given that wavelength converters nowadays remain very expensive, how to make effective use of a limited number of wavelength converters becomes an important issue. In this paper, we present and analyze the Sparse-Partial Wavelength Conversion (SPWC) network architecture, which has the inherent flexibility that can facilitate network carriers to upgrade the legacy optical backbone to support wavelength conversion. We explore the efficiency of partial wavelength conversion and demonstrate that SPWC architecture can significantly reduce the number of wavelength converters, yet achieve excellent blocking performance with a proper wavelength converter placement scheme. We also propose a wavelength assignment scheme called Minimum Converter Allocation (MCA), which can further improve the utilization of the wavelength converters. Simulation results indicate that, with the proposed MCA wavelength assignment algorithm, the performance of a wavelength-routed WDM network with only 1%–5% of wavelength conversion capability is very close to that with Full-Complete Wavelength Conversion capability.

Wavelength conversion and in-band crosstalk in WDM optical networks

Wavelength-routed all-optical networks have emerged as a popular architectural solution for wide area networks and are being conceived for future broadband communications. Call connection probability in such networks depends on the number of WDM wavelengths employed and on the capability for wavelength conversion at network nodes. Equipping all the network nodes with wavelength conversion capability to reduce the blocking probability is not a cost effective solution due to the high cost of wavelength converters (WC). Hence, networks equipped with converters at only some of the nodes are more practical. However, these nodes should be identified optimally so that network blocking probability is minimized. In this paper, wavelength converters based on four wave mixing (FWM) in semiconductor optical amplifiers (SOAs) is considered. In WDM all-optical networks, in-band crosstalk is regarded as one of the major transmission impairments. In-band crosstalk usually arises when multiple signals at identical or adjacent wavelengths pass through an optical crossconnect node. In-band crosstalk leads to increased receiver BER. This paper considers in-band crosstalk arising in a network which is equipped with wavelength converters. For every dynamically arriving connection request, a route is determined using Dijikstra’s algorithm and a free wavelength is determined using the random wavelength assignment. If no common free wavelength is available, connection is tried using wavelength converters, if available, on the given route. If wavelength assignment does not succeed, the call is blocked. If wavelength assignment succeeds, the BER at the destination node is computed before establishing the lightpath. If the estimated BER exceeds 10−12, the call is blocked. Otherwise, the call is admitted. The results reveal that wavelength converters are not useful in networks that already suffer from crosstalk conditions.

Routing and wavelength assignment in wavelength-routed all-optical WDM networks

In this paper, low-complexity mathematical model has been developed which is used for the calculation of the blocking probability of network and this model does not require any simulation statistics. The implementation of the model proposed has less complexity and the computation used in this model is quite efficient. The routing algorithm has also been proposed in this paper depending upon the proposed model, which is very efficient in calculation and minimization of the blocking probability. This algorithm suggests an optimum path as a solution to routing problem. The wavelength assignment technique has also been suggested in this paper to minimize the blocking probability. The proposed model and algorithms can be implemented on different network topologies. Furthermore, the model, routing algorithm and wavelength assignment technique is also used to evaluate the blocking performance of NSFNet and EUPAN Network topology and hence used to improve its performance on the basis of the blocking probability.

Blocking in wavelength-routed all-optical WDM networks

In this paper, we have developed a low complexity mathematical model which is used for the calculation of blocking probability of network and this model does not require any simulation statistics. The implementation of the model proposed has less complexity and the computation used in this model is quite efficient. This paper suggests an optimum path as a solution to routing problem and the appropriate number of wavelengths have also been suggested which should be free in a network to have the least blocking probability. This model can be implemented on different network topologies. Further, the model is also used to evaluate the blocking performance of NSFNet topology and hence used to improve its performance on the basis of blocking probability.

Adaptive alternate routing in WDM networks and its performance tradeoffs in the presence of wavelength converters

Routing in wavelength-routed all-optical WDM networks has received much attention in the past decade, for which fixed and dynamic routing methods have been proposed. Taking into account the observation that wavelength-routed all-optical WDM networks are similar to circuit-switched voice networks, except with regard to wavelength conversion, we propose an adaptive alternate routing (AAR) scheme for wavelength-routed all-optical WDM networks. A major benefit of AAR is that it can operate and adapt without requiring an exchange of network status, i.e., it is an information-less adaptive routing scheme. The scope of this work is to understand this scheme in its own right since no other dynamic routing schemes are known to have the information-less property. In this paper, we conduct a systematic study of AAR with regard to factors such as the number of converters, load conditions, traffic patterns, network topologies, and the number of alternate paths considered. We observe that the routing scheme with multiple alternate routes provides more gain at a lower load instead of requiring any nodes to be equipped with wavelength converters. On the other hand, the availability of wavelength converters at some nodes, along with adaptive routing, is beneficial at a moderate to high load without requiring all nodes to be equipped with wavelength converters. We also observed that a small number of alternate routes considered in a network without wavelength converters gives a much better performance than a network with full wavelength converters and fewer alternate routes. Throughout this study, we observed that the proposed adaptive alternate routing scheme adapts well to the network traffic condition.

Optimal RWA for Static Traffic in Transmission-Impaired Wavelength-Routed Networks

Physical-layer degradations can severely limit the capacity of all-optical wavelength-routed networks, and powerful routing and wavelength assignment (RWA) algorithms are needed to minimize their effect. An optimal RWA technique is presented that minimizes the number of wavelengths needed to serve a set of static call provisioning requests. The optimal solution is compared to classical suboptimal algorithms and results show that route selection is more critical than wavelength assignment if multiple paths are available. Physical impairments become less significant in mesh networks than in tandem networks.

Multi-hop all-to-all optical routings in Cartesian product networks

This paper considers the wavelength assignment problem for Cartesian product networks with multi-hops. An upper bound of the (uniform) wavelength index for Cartesian product networks with single-hop is established. This result leads to a consequence for the nth power of an arbitrary network with k-hops. In particular, if k = 1 , this bound partially generalizes the results of Pankaj [R.K. Pankaj, Architectures for linear lightwave networks, PhD thesis, Dept. of Electrical Engineering and Computer Science, MIT, Cambridge, MA, 1992], Bermond et al. [J.-C. Bermond, L. Gargano, S. Perennes, A.A. Rescigno, U. Vaccaro, Efficient collective communication in optical networks, Theoret. Comput. Sci. 233 (2000) 165–189] and Beauquier [B. Beauquier, All-to-all communication for some wavelength-routed all-optical networks, Networks 33 (1999) 179–187] for hypercubes and Hamming graphs. As an application, a tight upper bound for Hamming graph with k hops is established and a corresponding open problem is also proposed.

Multipoint-to-point lightpaths in all-optical networks: Dimensioning and cost analysis

One of the major concerns in optical networks is the bandwidth underutilization problem. In fact, as WDM technology keeps maturing, there is a bandwidth gap between the transmission speed of a wavelength channel (over a Gb/s) and the capacity requirement of customers’ connections. In this regard, building cost-efficient optical networks requires an efficient traffic grooming solution at the high speed optical access nodes. In this paper, we propose and evaluate a new concept of traffic aggregation in wavelength-division multiplexing (WDM) optical networks. Our objective is to reduce the network cost while preserving the benefits of all-optical wavelength-routed networks. In order to assess the efficiency of our proposal, all underlying network costs are compared. These costs include that of the transceivers required at node level as well as the number of wavelengths. Our results show that the proposed aggregation technique can significantly improve the network throughput while reducing its cost.

QoS-Friendly Wavelength Assignment in Dynamic Wavelength-Routed Optical Networks

In-band crosstalk has been widely considered as a major transmission impairment that significantly impacts the bit error rate (BER) performance of lightpaths in circuit-switched all-optical wavelength-routed networks. Such crosstalk usually occurs when multiple wavelengths pass through an optical crossconnect node, and the magnitude of the crosstalk is largely dependent on the wavelengths assigned to the lightpaths. Traditional wavelength assignment (WA) schemes pay little regard to the physical layer quality of service (QoS), and hence cannot provide optimized network performance in practical networks with imperfect physical transmission media. In this paper, we first present our categorization of in-band crosstalk based on the location of crosstalk generation, then we propose two QoS-friendly WA approaches, one of which selects the wavelength based on the estimated BERs, and the other based on the weighted crosstalk number counting. The two approaches have different computation complexities. Numerical results show that both approaches can significantly improve the BER blocking rate by suppressing the created in-band crosstalk, but the BER-based approach generally gives the better performance at the price of more extensive computation.

Dynamic routing and wavelength assignment in the presence of wavelength conversion for all-optical networks

Blocking probability has been one of the key performance indexes in the design of wavelength-routed all-optical WDM networks. Existing research has demonstrated that an effective Routing and Wavelength Assignment (RWA) algorithm and wavelength conversion are two primary vehicles for improving the blocking performance. However, these two issues have largely been investigated separately; in particular the existing RWA algorithms have seldom considered the presence of wavelength conversion. In this paper, we firstly demonstrate that the existing dynamic RWA algorithms do not work well in the presence of wavelength conversion as they usually only take into account the current traffic, and do not explicitly consider the route lengths. We then propose a weighted least-congestion routing and first-fit wavelength assignment (WLCR-FF) algorithm that considers both the current traffic load and the route lengths jointly. We further introduce an analytical model that can evaluate the blocking performance for WLCR algorithm. We carry out extensive numerical studies over typical topologies including ring, mesh-torus, and the 14-node NSFNET; and compare the performance of WLCR-FF with a wide variety of existing routing algorithms including static routing, fixed-alternate routing and least-loaded routing. The results conclusively demonstrate that the proposed WLCR-FF algorithm can achieve much better blocking performance in the presence of sparse or/and full wavelength conversion.