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.     

Wavelength Conversion Placement in WDM Mesh Optical Networks*

Wavelength conversion helps improve the performance of wavelength division multiplexed (WDM) optical networks that employ wavelength routing. In this paper, we address the problem of optimally placing a limited number of wavelength converters in mesh topologies. Two objective functions, namely, minimizing the average blocking probability and minimizing the maximum blocking probability over all routes, are considered. In the first part of the paper, we extend an earlier analytical model to compute the blocking probability on an arbitrary route in a mesh topology, given the traffic and locations of converters. We then propose heuristic algorithms to place wavelength converters, and evaluate the performance of the proposed heuristics using the analytical model. Results suggest that simple heuristics are sufficient to give near-optimal performance.     

XGM波长变换器网络的性能分析

目前对有限范围波长变换器的研究或者是针对能变换到较长和较短波长的FWM(SOA(Semiconductor Optical Amplifier) in Four-Wave Mixing,基于半导体光放大器的四波混合)波长变换器的,或者是针对具有卷绕(Wraprounds)功能的变换器.本文研究了只具有单边变换且不具有卷绕的XGM(SOA(Semiconductor Optical Amplifier) in Cross-Gain Modulation,基于半导体光放大器的交叉增益调制)有限范围波长变换器对网络性能的影响,从网络的阻塞率方面比较了FWM与XGM波长变换器对网络性能的影响.研究结果表明,XGM波长变换器的性能虽然只相当于具有10-20%左右变换范围的FWM波长变换器,但较无波长变换器网络的性能已经有了较大的改善,同时与全范围波长变换器网络的性能相差并不太远.     

Sparse Wavelength Conversion in Wavelength-Routed WDM Optical Networks*

A wavelength-routed optical network can suffer inefficiencies due to the wavelength-continuity constraint (under which a signal has to remain on the same wavelength from the source to the destination). In order to eliminate or reduce the effects of this constraint, a device called a wavelength converter may be utilized. Due to the high cost of these wavelength converters, many studies have attempted to determine the exact benefits of wavelength conversion. However, most of these studies have focused on optical networks that implement full wavelength conversion capabilities. An alternative to full wavelength conversion is to employ only a sparse number of wavelength converters throughout the network, thereby reducing network costs. This study will focus on different versions of sparse wavelength conversion--namely, sparse nodal conversion, sparse switch-output conversion, and sparse (or limited) range conversion--to determine if most of the benefits of full conversion can be obtained using only sparse conversion. Simulation and analytical results on these three different classes of sparse wavelength conversion will be presented. In addition, this study will present heuristic techniques for the placement of sparse conversion facilities within an optical network.     

抗毁WDM网络中支持多优先级的波长分配算法

WDM网络将在未来的骨干网中扮演重要的角色,具备抗毁能力和支持多优先级都很必要.本文提出了两种基于优先级的波长分配算法——波长编号法和分配限额法,并研究了动态业务下,抗毁WDM网络使用这两种波长分配方法后网络的阻塞率性能.研究的内容包括业务量负载、光纤链路的波长通道数和每条链路包含的光纤数对网络性能的影响.本文还给出了仿真结果.     

全光网络中的波长选择器件

随着光通信技术的发展,特别是光波分复用（WDM）技术的广泛应用,光交叉连接技术（OXC）和光分插复用技术（OADM）受到了越来越多的关注。波长选择器件因具有对不同波长自由选取的功能而在以上两种技术中起到了十分重要的作用。本文介绍了目前研究的几种典型的波长选择器件,对它们的特点进行了分析和比较。     

稀疏多光纤光网配置算法对波长连续性限制的影响

针对波分复用（WDM）光网络在实现端到端业务链接时的波长连续性限制问题,提出了对光网络进行稀疏多光纤配置的方法。通过分析发现：稀疏多光纤配置能够使整个网络的性能受波长连续性限制的影响得到明显改善;具有稀疏多光纤配置的光网络长链路建链阻塞概率在全网阻塞率中的比例要高于使用相同数目波长转换器的光网络,这说明,波长转换器对于光网络的长链建链仍具有一定的优势。     

A Comparison of Allocation Policies in Wavelength Routing Networks*

We consider wavelength routing networks with and without wavelength converters, and several wavelength allocation policies. Through numerical and simulation results we obtain upper and lower bounds on the blocking probabilities for two wavelength allocation policies that are most likely to be used in practice, namely, most-used and first-fit allocation. These bounds are the blocking probabilities obtained by the random wavelength allocation policy with either no converters or with converters at all nodes of the network. Furthermore, we demonstrate that using the most-used or first-fit policies gives an improvement on call blocking probabilities that is equivalent to employing converters at a number of nodes in a network with the random allocation policy. These results have been obtained for a wide range of loads for both single-path and general mesh topology networks. The main conclusion of our work is that the gains obtained by employing specialized and expensive hardware (namely, wavelength converters) can be realized cost-effectively by making more intelligent choices in software (namely, the wavelength allocation policy).     

Routing and Wavelength Assignment in Optical Networks Using Logical Link Representation and Efficient Bitwise Computation

In this paper, we propose and evaluate a new approach for implementing efficient routing and wavelength assignment (RWA) in wavelength division multiplexing (WDM) optical networks. In our method, the state of a fiber is given by the set of free wavelengths in this fiber and is efficiently represented as a compact bitmap. The state of a multiple-fiber link is also represented by a compact bitmap computed as the logical union of the individual bitmaps of the fibers in this link. Likewise, the state of a lightpath is represented by a similar bitmap computed as the logical intersection of the individual bitmaps of the links in this path. The count of the number of 1-valued bits in the bitmap of the route from source to destination is used as the primary reward function in route selection. A modified Dijkstra algorithm is developed for dynamic routing based on the bitmap representation. The algorithm uses bitwise logical operations and is quite efficient. A first-fit channel assignment algorithm is developed using a simple computation on the bitmap of the selected route. The resulting bitwise routing algorithm combines the benefits of least loaded routing algorithms and shortest path routing algorithms. Our extensive simulation tests have shown that the bitwise RWA approach has small storage overhead, is computationally fast, and reduces the network-wide blocking probability. The blocking performance of our RWA method compares very favorably with three routing methods: fixed alternate routing, shortest path using flooding, and Dijkstra’s algorithm using mathematical operations. Our simulation experiments have also evaluated the performance gain obtained when the network access stations are equipped with finite buffers to temporarily hold blocked connection requests.     

Wavelength Conversion in WDM Optical Networks: Strategies and Algorithms for Limiting the Number of Converters in the Optical Cross-Connects

This paper considers the problem of wavelength conversion in optical networks using wavelength division multiplexing technique. In the previous literature, two main wavelength routing and assignment strategies have been introduced: wavelength path (WP) and virtual wavelength path (VWP), depending on whether the signal stays on the same wavelength or is converted to another during its travel throughout the network. While the former method does not require any wavelength conversion, the latter needs wavelength conversion in each optical node and, in particular, a wavelength converter per each signal handled by the node itself. From the previous literature emerged that the VWP leads to optical cross-connect (OXC) with lower dimensions compared to the ones required by the WP scheme, and that the difference between the WP and VWP schemes increases as the number of wavelengths carried by each fiber increases. In this paper a new strategy is introduced, named partial virtual wavelength path (PVWP), with the related wavelength routing and assignment algorithm, which makes limited use of wavelength conversion compared to the VWP scheme, and allows the same advantages of VWP to be attained with lower OXC dimensions. The paper reports a comparative analysis among the different strategies, considering both the cases of a network without failures and a network with the possibility of failure restoration. The main result is that the proposed PVWP strategy allows the same advantages of the VWP scheme with a strongly reduced number of wavelength converters (around 5% of the number required by VWP scheme). This figure does not vary appreciably if failure restoration is considered. The new strategy can be adopted by using an opportune OXC architecture, as illustrated in the paper, which allow a limited number of converters to be shared among all the channels as a common pool.     

Cost Effective Shared Path Protection for WDM Optical Mesh Networks with Partial Wavelength Conversion

In this paper, we study routing and wavelength assignment of connection requests in survivable WDM optical mesh networks employing shared path protection with partial wavelength conversion while 100% restorability is guaranteed against any single failures. We formulate the problem as a linear integer program under a static traffic model. The objective is to minimize the total cost of wavelength-links and wavelength converters used by working paths and protection paths of all connections. A weight factor is used which is defined as the cost ratio of a wavelength converter and a wavelength-link. Depending on the relative cost of bandwidth and wavelength conversion, the optimization objective allows a proper tradeoff between the two. The proposed algorithm, the shortest-widest-path-first (SWPF) algorithm, uses a modified Dijkstra's algorithm to find a working path and a protection path for each connection request in the wavelength graph transformed from the original network topology. When there are multiple candidate paths that have the same minimum total cost, the path along which the maximum number of converters used at each node is minimized is chosen by the SWPF algorithm. We have evaluated the effectiveness of the proposed algorithm via extensive simulation. The results indicate that the performance of the proposed algorithm is very close to that of the optimal solutions obtained by solving the ILP formulation and outperforms existing heuristic algorithms in terms of total number of converters used and the maximum number of converters required at each node in the network. The proposed algorithm also achieves slightly better performance in terms of total cost of wavelength-links and converters used by all connections. We also investigated shared path protection employing converter sharing. The results show that the technique can reduce not only the total number of converters used in the network but also the maximum number of converters required at each node, especially when a large number of converters are needed in the network. In this study, although the ILP formulation is based on static traffic, the proposed algorithm is also applicable to routing dynamic connection requests.     

Dynamic Deflection Routing with Virtual Wavelength Assignment in Optical Burst-Switched Networks

In optical burst-switched networks, one of the most significant issues is contention resolution. There have been several deflection routing techniques as contention resolution. While contention is resolved by traditional deflection routing, it cannot guarantee that the control packet will reserve all the wavelengths successfully to the destination on the alternate path, especially when traffic load in a network is high. Therefore, in this paper, we propose a Deflection Routing with Virtual Wavelength Assignment (DR-VWA) algorithm in order to provide a higher resource guarantee for loss-sensitive traffic bursts. The proposed DR-VWA scheme (1) dynamically decides the alternate path with the least traffic load and (2) allows high-priority bursts in terms of loss to be assigned available wavelengths over the path virtually. The proposed scheme is evaluated through simulation, and it is shown that significant improvement with regard to burst loss and wavelength conversion cost can be achieved.This work was supported in part by the Korea Science and Engineering Foundation (KOSEF) through OIRC project and by the US National Communications System (NCS).     

混合光波长转换在波分复用光网络中的应用

在全光网络中，如何合理利用波长转换来降低光网络的阻塞率是一个非常关键的问题。研究了最新的波长转换体系结构和波长转换手段，提出一种全新的混合波长转换方法，在减少网络中波长转换器个数的同时，维持拥塞概率类似于全波长转换。提出了5种不同的波长转换器使用策略，并利用数值模拟的方法，比较了这5种不同的波长转换器使用策略，分析结果，得出了最小化光网络的阻塞概率的波长转换使用策略。结合混合波长转换和波长转换器使用策略，进一步提出了光网络中优化波长转换器配置的遗传算法，通过对14个节点的美国自然科学基金网(NSF Net)的数值模拟，结果表明它是十分有效的，在减少光网络中波长转换器数量，且不增加光网络波长数量的情况下，基本保持原有网络性能。     

Strictly Non-Blocking All-Optical-Cross-Connect Demonstrator for WDM Wavelength Path Networks

In this paper, we assess the design and performances of a strictly non-blocking all-optical cross-connect demonstrator node for WDM wavelength path networks. The all-optical cross-connect (AOXC) prototype was experimentally tested in a small 2 x 2 WDM network with a STM-16 bit stream per channel. The single and cascaded performance of the AOXC was also simulated and its results were validated with the experimental ones.     

波长变换器均匀放置的Mesh-Torus的性能分析

本文主要研究了在WDM的Mesh Torus中 ,部分网络节点使用波长转换器能给网络带来的好处 .文中比较了在非泊松业务下 ,波长转换器按二项式分布和均匀分布放置时在阻塞率、转换增益和公平性方面能带来的好处 ,证明了波长转换器均匀放置在链路负荷均匀的情况下的Mesh Torus中是一种优化配置     

WDM网络中基于阻塞公平性的业务疏导和波长分配

在具有多速率级别连接请求的WDM网络中，通过为较高速率的请求预留更多的波长集合，结合网络节点对较低速率业务更强的输导能力，解决网络的阻塞公平性和全网的流量问题。在中国教育科研网的仿真实验，本文的策略比First-fit算法取得了更好的阻塞公平性能，并接纳了更多的网络流量。当负载为170Erlang时，多接纳的流量约为12％。     

Study on a Distributed Wavelength Routing Algorithm in WDM Optical Transport Networks

A new design scheme for a distributed algorithm for routing and wavelength assignment (RWA) is developed in this paper, and the communication rules between the nodes to exchange signaling packets are discussed. The Adaptive-Alternate-Routing-Least-Load (AARLL) algorithm is implemented in the distributed scheme for the first time. Under dynamic traffic circumstances, the influence of the race condition, which does not exist in centralized scheme, on the network performance is analyzed and the analyzed results show that the race condition has a major impact on network performance only under light traffic load, while under medium and heavy traffic load the impact is very small. To analyze the performance loss caused by adopting the distributed algorithm for RWA, the capacity loss factor (CLF) is introduced and the calculated results show that CLF does not exceed 6% under medium traffic load.     

Dimensioning of the Wavelength Converters in a WDM Optical Packet Switch

This paper analyzes the performances of a WDM optical packet switch making use of fiber delay lines to resolve output packet contentions; the optical packet switch is equipped with tunable wavelength converters which can shift optical packets to any wavelength of the output link which they are directed to. As not all packets need conversion, we propose a dimensioning technique allowing to reduce the number of converters and to improve the signal quality by reducing unnecessary conversions. The obtained results show that a remarkable reduction of the number of converters is obtained with respect to that needed by other switches described in literature. Such a saving is achieved by maintaining the packet loss probability below a prefixed threshold.     

Lightpath Versus Semi-Lightpath: Some Studies on Optimal Routing in WDM Optical Networks

This paper deals with the problem of optimal dynamic routing in WDM optical networks with wavelength-changing facilities available at some of the nodes. The route may be either a lightpath (i.e., wavelength continuous channel) or a semi-lightpath (i.e., wavelength-converted channel). We attempt to estimate in this work the gain in blocking probability, when we move from lightpath routing to semi-lightpath routing, keeping the number of wavelengths fixed, in a given circuit switched network. We ensure optimal (minimum cost) routing in both the cases by using the algorithm of Banerjee et al. [7,8] (called Algorithm-I in this paper) for lightpaths and that of Chlamtac et al. [6] (called Algorithm-II) for semi-lightpaths. Our results indicate that, for both the algorithms, the blocking probability (P _B), as expected, increases with network load. At light load, P _B for Algorithm-I is always larger than that for Algorithm-II. But the rate of increase in P _B is slightly higher in case of Algorithm-II, so that there is a crossover point where P _B for Algorithm-II exceeds P _B for Algorithm-I. This probably happens due to the irregularities in the semi-lightpaths at heavy loads when almost all routes are exhausted in the network. However, since this crossover phenomenon occurs at a very congested status of the network, it has little significance over the real life operation of a network. It only suffices to indicate that, under heavy load, both the algorithms are equally insufficient, and conversion does not improve the situation as might have been expected intuitively.     

Performance Models for Multi-rate Circuits in Switched Multi-Channel Optical Networks

We consider optical networks with nodes interconnected by links comprising multiple channels. The blocking performance of such networks depends on the channel-switching capabilities of the nodes. In this paper, we focus on developing analytical models for evaluating the blocking performance in circuit-switched optical networks. Several architectures for channel-switching are presented. Multi-rate circuits may be established if different circuits are allocated different number of channels. Depending on how the network can assign the channels to circuits that request more than one channel, it is classified as a Data Splitting Network (DSN) or a non-DSN. We consider multi-rate circuits which require either a single channel or a given number of channels k (>1). Analytical models for computing blocking probabilities are developed for various channel-switching architectures at the nodes. The validity of the models is shown by comparing the analytical results with simulations. Numerical results in a single-fiber TDM wavelength-routing network suggest that limited channel-switching may be sufficient even for circuits that require more than one channel or slot. A comparison of DSN and non-DSN shows that data splitting can accommodate more multi-slot circuits at the expense of blocking more single-slot circuits.This work was supported in part by NSF Grants ANI-9973098 and ANI-9973111