波分复用波长路由节点的阻塞特性分析

利用概率统计理论的方法，从节点层次上定量分析了节点规模、复用波长数目以及波长转换对波分复用(WDM)波长路由网络中波长路由节点的影响。提出了基于概率统计的节点阻塞模型。数值结果突出表明波长转换能力越强的全光节点，其性能越优。为了提高网络资源的使用效率并增强全光网络的灵活性，必须实现全光网络中的虚波长路由波长转换器。通过数值计算找到了阻塞性能和代价的折中，研究中发现配置较低波长转换能力波长转换器的波长路由节点将会具备更强的性价比优势，当前在构建光通信系统时使用弱波长转换能力的光节点更可行。     

Wavelength converters in dynamically-reconfigurable WDM networks

In simple wavelength-division multiplexed (WDM) networks, a connection must be established along a route using a common wavelength on all of the links along the route. This constraint may be removed by the introduction of wavelength converters, which are devices which take the data modulated on an input wavelength and transfer it to a different output wavelength. Wavelength converters thus improve network blocking performance. However, the introduction of wavelength converters into WDM cross-connects increases the hardware cost and complexity. Thus, it is important to establish precisely what advantages wavelength converters offer WDM networks. There has been considerable interest in the literature in the performance improvements offered by the introduction of wavelength converters into dynamically-reconfigurable WDM networks. This article provides a review of the conclusions drawn from these investigations. The performance improvements offered by wavelength converters depend on a number of factors, including network topology and size, the number of wavelengths, and the routing and wavelength assignment algorithms used. We discuss these factors here. However, it has been shown that wavelength converters offer only modest performance improvements in many networks. We also consider networks with limited wavelength conversion, in which the set of allowable conversions at a network node is constrained by having limited numbers of wavelength converters, or by using non-ideal wavelength converters. Limited wavelength conversion has been shown to provide performance which is often close to that achieved with ideal wavelength conversion in networks with tunable transmitters and receivers.     

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.     

Multicast connection capacity of WDM switching networks with limited wavelength conversion

Currently, many bandwidth-intensive applications require multicast services for efficiency purposes. In particular, as wavelength division multiplexing (WDM) technique emerges as a promising solution to meet the rapidly growing demands on bandwidth in present communication networks, supporting multicast at the WDM layer becomes an important yet challenging issue. In this paper, we introduce a systematic approach to analyzing the multicast connection capacity of WDM switching networks with limited wavelength conversion. We focus on the practical all-optical limited wavelength conversion with a small conversion degree d (e.g., d=2 or 3), where an incoming wavelength can be switched to one of the d outgoing wavelengths. We then compare the multicast performance of the network with limited wavelength conversion to that of no wavelength conversion and full wavelength conversion. Our results demonstrate that limited wavelength conversion with small conversion degrees provides a considerable fraction of the performance improvement obtained by full wavelength conversion over no wavelength conversion. We also present an economical multistage switching architecture for limited wavelength conversion. Our results indicate that the multistage switching architecture along with limited wavelength conversion of small degrees is a cost-effective design for WDM multicast switching networks.     

A novel survivable routing algorithm for shared segment protection in mesh WDM networks with partial wavelength conversion

In this paper, a survivable routing algorithm is proposed for shared segment protection (SSP), called optimal self-healing loop allocation (OSHLA), which dynamically allocates spare capacity for a given working lightpath in mesh wavelength-division-multiplexing (WDM) networks with partial wavelength conversion capability. Two novel graph transformation approaches, namely graph of cycles and wavelength graph of paths, are introduced to solve this problem, in which the task of survivable routing is formulated as a series of shortest path searching processes. In addition to an analysis on the computation complexity, a suite of experiments is conducted to verify OSHLA on four networks with different topologies and traffic loads. We find that the blocking probability and computation complexity are dominated by the upper bound on the length of the working and protection segments. Comparison is made between OSHLA and four other reported schemes in terms of blocking probability. The results show that OSHLA can achieve the lowest blocking probability under the network environment of interest. We conclude that OSHLA provides a generalized framework of survivable routing for an efficient implementation of SSP in mesh WDM partial wavelength convertible networks. With OSHLA, a compromise is initiated by manipulating the upper bound on the length of working and protection segments such that the best performance-computation complexity gain can be achieved.     

Wavelength conversion technologies for WDM network applications

WDM networks make a very effective utilization of the fiber bandwidth and offer flexible interconnections based on wavelength routing. In high capacity, dynamic WDM networks, blocking due to wavelength contention can he reduced by wavelength conversion. Wavelength conversion addresses a number of key issues in WDM networks including transparency, interoperability, and network capacity. Strictly transparent networks offer seamless interconnections with full reconfigurability and interoperability. Wavelength conversion may be the first obstacle in realizing a transparent WDM network. Among numerous wavelength conversion techniques reported to date, only a few techniques offer strict transparency. Optoelectronic conversion (O/E-E/O) techniques achieve limited transparency, yet their mature technologies allow deployment in the near future. The majority of all-optical wavelength conversion techniques also offer limited transparency but they have a potential advantage over the optoelectronic counterpart in realizing lower packaging costs and crosstalk when multiple wavelength array configurations are considered. Wavelength conversion by difference-frequency generation offers a full range of transparency while adding no excess noise to the signal. Recent experiments showed promising results including a spectral inversion and a 90 nm conversion bandwidth. This paper reviews various wavelength conversion techniques, discusses the advantages and shortcomings of each technique, and addresses their implications for transparent networks     

Analytical model of sparse-partial wavelength conversion in wavelength-routed WDM networks

Wavelength conversion is one of the key techniques to improve the blocking performance in wavelength-routed WDM networks. Given that wavelength converters nowadays remain very expensive, how to make effective use of wavelength converters becomes an important issue. In this letter, we analyze the sparse-partial wavelength conversion network architecture and demonstrate that it can significantly save the number of wavelength converters, yet achieving excellent blocking performance. Theoretical and simulation results indicate that, the performance of a wavelength-routed WDM network with only 1-5% off wavelength conversion capability is very close to that with full-complete wavelength conversion capability.     

Performance Analysis of Multi-Fiber WDM Network with Limited-Range Wavelength Conversion

Wavelength routed optical networks have emerged as a technology that can effectively utilize the enormous bandwidth of the optical fiber. Wavelength conversion technology and wavelength converters play an important role in enhancing fiber utilization and in reducing the overall call blocking probability of the network. In this paper, we develop a new analytical model to calculate the average blocking probability in multi-fiber link networks using limited range wavelength conversion. Based on the results obtained, we conclude that the proposed analytical model is simple and yet can effectively analyze the impact of wavelength conversion ranges and number of fibers on network performance. Also a new heuristic approach for placement of wavelength converters to reduce blocking probabilities is explored. Finally, we analyze network performance with the proposed scheme. It can be observed from numerical simulations that limited range converters placed at a few nodes can provide almost the same blocking probability as full range wavelength converters placed at all the nodes. We also show that being equipped with a multi-fiber per-link has the same effect as being equipped with the capability of limited range wavelength conversion. So a multi-fiber per-link network using limited range wavelength conversion has similar blocking performance as a full wavelength convertible network. Since a multi-fiber network using limited range wavelength conversion could use fewer converters than a single-fiber network using limited range wavelength conversion and because wavelength converters are today more expensive than fiber equipment, a multi-fiber network in condition with limited range wavelength conversion is less costly than a single fiber network using only limited range wavelength conversion. Thus, multi-fiber per-link network using limited range wavelength conversion is currently a more practical method for all optical WDM networks. Simulation studies carried out on a 14-node NSFNET, a 10-node CERNET (China Education and Research Network), and a 9-node regular mesh network validate the analysis.     

Performance analysis of multi-fiber WDM network with limited-range wavelength conversion

Wavelength routed optical networks have emerged as a technology that can effectively utilize the enormous bandwidth of the optical fiber. Wavelength conversion technology and wavelength converters play an important role in enhancing fiber utilization and in reducing the overall call blocking probability of the network. In this paper, we develop a new analytical model to calculate the average blocking probability in multi-fiber link networks using limited-range wavelength conversion. Based on the results obtained, we conclude that the proposed analytical model is simple and yet can effectively analyze the impact of wavelength conversion ranges and number of fibers on network performance. Also a new heuristic approach for placement of wavelength converters to reduce blocking probabilities is explored. Finally, we analyze network performance with the proposed scheme. It can be observed from numerical simulations that limited-range converters placed at a few nodes can provide almost the same blocking probability as full range wavelength converters placed at all the nodes. We also show that being equipped with a multi-fiber per-link has the same effect as being equipped with the capability of limited-range wavelength conversion. So a multi-fiber per-link network using limited-range wavelength conversion has similar blocking performance as a full wavelength convertible network. Since a multi-fiber network using limited-range wavelength conversion could use fewer converters than a single-fiber network using limited range wavelength conversion and because wavelength converters are today more expensive than fiber equipment, a multi-fiber network in condition with limited-range wavelength conversion is less costly than a single-fiber network using only limited-range wavelength conversion. Thus, multi-fiber per-link network using limited-range wavelength conversion is currently a more practical method for all optical WDM networks. Simulation studies carried out on a 14-node NSFNET, a 10-node CERNET (China Education and Research Network), and a 9-node regular mesh network validate the analysis.     

Nonblocking WDM switching networks with full and limited wavelength conversion

In previous years, with the rapid exhaustion of the capacity in wide area networks led by Internet and multimedia applications, demand for high bandwidth has been growing at a very fast pace. Wavelength-division multiplexing (WDM) is a promising technique for utilizing the huge available bandwidth in optical fibers. We consider efficient designs of nonblocking WDM permutation switching networks. Such designs require nontrivial extensions from the existing designs of electronic switching networks. We first propose several permutation models in WDM switching networks ranging from no wavelength conversion, to limited wavelength conversion, to full wavelength conversion, and analyze the network performance in terms of the permutation capacity and network cost, such as the number of optical cross-connect elements and the number of wavelength converters required for each model. We then give two methods for constructing nonblocking multistage WDM switching networks to reduce the network cost.     

Effects of wavelength routing and selection algorithms onwavelength conversion gain in WDM optical networks

Wavelength-division multiplexing (WDM) technology is emerging as the transmission and switching mechanism for future optical mesh networks. In these networks it is desired that a wavelength can be routed without electrical conversions. Two technologies are possible for this purpose: wavelength selective cross-connects (WSXC) and wavelength interchanging cross-connects (WIXC), which involve wavelength conversion. It is believed that wavelength converters may improve the blocking performance, but there is a mix of results in the literature on the amount of this performance enhancement. We use two metrics to quantify the wavelength conversion gain: the reduction in blocking probability and the increase in maximum utilization, compared to a network without converters. We study the effects of wavelength routing and selection algorithms on these measures for mesh networks. We use the overflow model to analyze the blocking probability for wavelength-selective (WS) mesh networks using the first-fit wavelength assignment algorithm. We propose a dynamic routing and wavelength selection algorithm, the least-loaded routing (LLR) algorithm, which jointly selects the least-loaded route-wavelength pair. In networks both with and without wavelength converters the LLR algorithm achieves much better blocking performance compared to the fixed shortest path routing algorithm. The LLR produces larger wavelength conversion gains; however, these large gains are not realized in sufficiently wide utilization regions and are diminished with the increased number of fibers     

Design of the optical path layer in multiwavelength cross-connectednetworks

We discuss the wavelength requirement for optical networks based on wavelength-division multiplexing (WDM). A mathematical model, to represent the routing and wavelength assignment in optical networks with or without wavelength conversion, is described, and metrics are defined to express the performance. A new heuristic for routing and wavelength assignment is proposed and compared with the Dijkstra algorithm and with a solution based on integer linear programming. The different techniques are applied to a variety of network examples with different traffic loads     

Blocking performance analysis on adaptive routing over WDM networks with finite wavelength conversion capability

Analytical blocking probability analysis is important for network design. In this paper, we present an analytical model for the blocking probability analysis on adaptive routing over the WDM networks with finite wavelength conversion capability. Modeling the finite nature of wavelength conversion has been a difficult task. We make use of the idea of segmented route to handle the finite wavelength conversion property. In this approach, a route is divided into a number of segments separated by wavelength converting nodes. We then combine the single-link model and the overflow model to derive the network-wide blocking probability. There are two distinct features in our technique. First, a concept of segmented route is used. Second, link state is considered when calculating the traffic flow. The latter ensures that the analytical results match closely to practical network status. Extensive simulations show that the analytical technique is effective in modeling the blocking probability performance for sparse networks.     

On-line integrated routing in dynamic multifiber IP/WDM networks

This paper focuses on dynamic integrated routing in multifiber Internet protocol/wavelength-division multiplexing (IP/WDM) networks, which can be implemented through either one-step routing (OSR) or two-step routing (TSR) approach. Based on an extended layered-graph, two resource assignment strategies, termed channel-level balance (CLB) and link-level balance (LLB), are proposed to balance the traffic in the network at different levels. To further improve the performance, a parameter K is introduced to make a dynamic tradeoff between the logical-layer links and the optical-layer links. Simulation studies are carried out for various topologies. The results show that LLB is better than CLB in most cases, and LLB combined with OSR has the optimal performance. Also, we find that the routing approach and the resource assignment strategy individually play different roles with different values of r/sub l/ that is introduced to indicate the resource richness of the network. As a multifiber network is functionally equivalent to a single-fiber network with limited wavelength conversion, we investigate the effects of wavelength conversion by studying the multifiber IP/WDM networks. The analysis shows that, when the granularity of each connection request is much smaller than the wavelength granularity, wavelength conversion may increase the request blocking probability in the network.     

Multiwavelength optical networks with limited wavelength conversion

This paper proposes optical wavelength division multiplexed (WDM) networks with limited wavelength conversion that can efficiently support lightpaths (connections) between nodes. Each lightpath follows a route in a network and must be assigned a channel on each link along the route. The load λ_max of a set of lightpaths is the maximum over all links of the number of lightpaths that use the link. At least λ_max wavelengths will be needed to assign channels to the lightpaths. If the network has full wavelength conversion capabilities, then λ_max wavelengths are sufficient to perform the channel assignment. Ring networks with fixed wavelength conversion capability within the nodes are proposed that can support all lightpath sets with load λ_max at most W-1, where W is the number of wavelengths in each link. Ring networks with a small additional amount of wavelength conversion capability within the nodes are also proposed that allow the support of any set of lightpaths with load λ_max at most W. A star network is also proposed with fixed wavelength conversion capability at its hub node that can support all lightpath sets with load λ_max at most W. These results are extended to tree networks and networks with arbitrary topologies. This provides evidence that significant improvements in traffic-carrying capacity can be obtained in WDM networks by providing very limited wavelength conversion capability within the network     

Routing and wavelength assignment strategies in optical networks

We consider the routing and wavelength assignment (RWA) problem on wavelength division multiplexing (WDM) networks without wavelength conversion. When the physical network and required connections are given, RWA is the problem to select a suitable path and wavelength among the many possible choices for each connection such that no two paths using the same wavelength pass through the same link. In WDM optical networks, there is need to maximize the number of connections established and to minimize the blocking probability using limited resources. This paper presents efficient RWA strategies, which minimizes the blocking probability. Simulation results show that the performance of the proposed strategies is much better than the existing strategy.     

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.     

Heuristic algorithm for allocation of wavelength convertible nodesand routing coordination in all-optical networks

It is true that in all-optical networks, network performance can be improved by wavelength conversion. However, the switching node with wavelength conversion capability is still costly, and the number of such nodes should he limited in the network. In this paper, a performance optimization problem is treated in all-optical networks. We propose a heuristic algorithm to minimize an overall blocking probability by properly allocating a limited number of nodes with wavelength conversion capability. The routing strategy is also considered suitable to the case where the number of wavelength convertible nodes are limited. We validate the minimization level of our heuristic algorithm through numerical examples, and show that our algorithm can properly allocate nodes with conversion and decide routes for performance optimization     

Performance study on a WDM packet switch with limited-rangewavelength converters

We consider a slotted WDM packet switch with limited-range wavelength converters. The performance of this switch is studied using simulations with various types of data traffic. Results show that the slotted WDM packet switch with a small range of wavelength conversion capability can achieve a performance close to that of a switch with the full range of wavelength conversion capability     

一种波长转换受限WDM网络的动态路由和波长分配算法

本文基于分层图模型,提出了在节点波长转换范围受限和波长转换器数目受限情况下,解决WDM网络的动态路由和波长分配问题的一种算法.通过计算机仿真,研究了本算法的性能以及这两种波长转换受限情况对网络阻塞率的影响.