Performance Implications of Meshing Degree for Optical Burst Switched Networks Using One-Way Resource Reservation Protocols

This paper discusses performance implications of meshing degree (or nodal degree) for optical burst switching (OBS) mesh networks using one-way resource reservation protocols. The analysis is focused on the following topologies: rings, chordal rings with nodal degrees ranging from three to six, mesh-torus, NSFNET, ARPANET and the European Optical Network (EON). It is shown that the largest nodal degree gain, due to the increase of the nodal degree from two to around three, is observed for degree-three chordal ring topology, where as the smallest gain is observed for the ARPANET. For these cases, the magnitude of the nodal degree gain is slightly less than three orders for the degree-three chordal ring and less than one order of magnitude for the ARPANET. On the other hand, when the nodal degree increases from two to a value ranging from about four up to six, the nodal degree gain ranges between four and six orders of magnitude for chordal rings. However, EON, which has a nodal degree slightly less than four has the smallest nodal degree gain. The observed gain for this case is less than one order of magnitude. Since burst loss is a key issue in OBS networks, these results clearly show the importance of meshing degree for this kind of networks.     

Performance Assessment of Optical Burst Switching Ring and Chordal Ring Networks

This paper presents a performance analysis of optical burst switching (OBS) networks with ring and chordal ring topologies. The performance analysis considers signaling protocols with one-way reservation schemes, being focused on the following protocols: just-in-time (JIT), Jumpstart, JIT⁺, just-enough-time (JET) and Horizon. It is shown that for a network with 20 nodes, the nodal degree gain due to the increase of nodal degree from two (ring) to three (chordal ring) is about three orders of magnitude in the first hop of both topologies, and is between two and three orders of magnitude in the last hop of each topology. It is also shown that the largest chord length gain, in a 20-node OBS network with a chordal ring topology, is slightly less than two orders of magnitude.     

Reliability of mesh and torus topologies in the presence of faults

Mesh and torus topologies have been used to implement various multicomputer architectures. Faults in these networks significantly degrade their performance. In this paper, a new Markov model is used to derive a closed form analytical solution for the terminal reliability of the mesh and 2‐dimensional torus. The effect of the added links in the torus on reliability is studied. Networks of various dimensions are analyzed, and all the analytical results are verified by computer simulations. Although the mesh‐connected topology is considered in this paper, the analysis can be readily adopted for any network topology. This revised version was published online in June 2006 with corrections to the Cover Date.     

An analysis of oblivious and adaptive routing in optical networkswith wavelength translation

We present an analysis for both oblivious and adaptive routing in regular, all-optical networks with wavelength translation. Our approach is simple, computationally inexpensive, accurate for both low and high network loads, and the first to analyze adaptive routing with wavelength translation in wavelength division multiplexed (WDM) networks while also providing a simpler formulation of oblivious routing with wavelength translation. Unlike some previous analyses which use the link independence blocking assumption and the call dropping (loss) model (where blocked calls are cleared), we account for the dependence between the acquisition of wavelengths on successive links of a session's path and use a lossless model (where blocked calls are retried at a later time). We show that the throughput per wavelength increases superlinearly (as expected) as we increase the number of wavelengths per link, due both to additional capacity and more efficient use of this capacity; however, the extent of this superlinear increase in throughput saturates rather quickly to a linear increase. We also examine the effect that adaptive routing can have on performance. The analytical methodology that we develop can be applied to any vertex and edge symmetric topology, and with modifications, to any vertex symmetric (but not necessarily edge symmetric) topology. We find that, for the topologies we examine, providing at most one alternate link at every hop gives a per wavelength throughput that is close to that achieved by oblivious routing with twice the number of wavelengths per link. This suggests some interesting possibilities for network provisioning in an all-optical network. We verify the accuracy of our analysis for both oblivious and adaptive routing via simulations for the torus and hypercube networks     

Lightwave networks based on de Bruijn graphs

Proposes de Bruijn graphs as logical topologies for multihop lightwave networks. After deriving bounds on the throughput and delay performance of any logical topology, the authors compute the throughput and delay performance of de Bruijn graphs for two different routing schemes and compare it with their bounds and the performance of shufflenets. For a given maximum nodal in- and out-degree and average number of hops between stations, a logical topology based on a de Bruijn graph can support a larger number of stations than a shufflenet and this number is close to the maximum that can be supported by any topology. The authors also propose de Bruijn graphs as good physical topologies for wavelength routing lightwave networks consisting of all-optical routing nodes interconnected by point-to-point fiber links. The worst-case loss experienced by a transmission is proportional to the maximum number of hops (diameter). For a given maximum nodal in- and out-degree and diameter, a physical topology based on a de Bruijn graph can support a large number of stations using a relatively small number of wavelengths     

All-optical networks with sparse wavelength conversion

Unlike broadcast-and-select networks, wavelength-routing networks offer the advantages of wavelength reuse and scalability and are thus suitable for wide-area networks (WANs) We study the effects of topological connectivity and wavelength conversion in circuit-switched all-optical wavelength-routing networks. A blocking analysis of such networks is given. We first propose an analytical framework for accurate analysis of networks with arbitrary topology. We then introduce a model for networks with a variable number of converters and analyze the effect of wavelength converter density on the blocking probability. This framework is applied to three regular network topologies that have varying levels of connectivity: the ring, the mesh-torus, and the hypercube. The results show that either a relatively small number of converters is sufficient for a certain level of performance or that conversion does not offer a significant advantage. The benefits of conversion are largely dependent on the network load, the number of available wavelengths, and the connectivity of the network. Finally, the tradeoff between physical connectivity, wavelength conversion, and the number of available wavelengths is studied through networks with random topologies     

Failure protection methods for optical meshed-ring communications networks

We study the survivability of a meshed-ring communication network that employs cross-connect switches. For WDM networks, the cross-connect switches are implemented as wavelength routers. Nodes can also provide cross-connection at the ATM VP (virtual path) level. By meshing the ring, the nodal degree of connectivity is increased as compared to a ring topology, and thus more alternative (protection) paths are available. For routing purposes, wavelength subnetworks are embedded in the topology. Nodes communicate with each other across one of the subnetworks to which both belong. We consider two types of subnetwork topologies to simplify the routing in a normal (nonfailure) situation. For each type of subnetwork, different protection methods are proposed to protect against a single link and/or nodal failure. The throughput performance of such meshed-ring networks under failure conditions is clearly superior to that achieved by (SONET) ring networks. We show that certain protection methods even result in lower values of the protection capacity as well as the protection capacity ratio (i.e., the overall capacity used under a failure divided by the total capacity) as compared to ring networks. We also present methods for constructing wavelength subnetworks to achieve single-failure protection using the minimal number of wavelengths.     

典型网状拓扑全光网设计及配置性能分析(本期优秀论文)

基于一种结构简单、成本低廉的波长转换节点共享型光交叉连接设备建立起一个极具代表性的典型四节点四维度网状拓扑全光试验网,利用排队理论方法构建其数学模型,分析研究了光交叉连接节点配置对试验网性能的影响。数值结果表明,在网络受限波长转换条件下,合理配置的该光交叉连接节点设备具有一定的工程应用前景。     

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。     

A Bandwidth Guaranteed Integrated Routing Algorithm in IP over WDM Optical Networks

In this paper, we have developed an integrated online algorithm for dynamic routing of bandwidth guaranteed label switched paths (LSPs) in IP-over-WDM optical networks. Traditionally, routing at an upper layer (e.g., IP layer) is independent of wavelength routing at the optical layer. Wavelength routing at the optical layer sets up a quasi-static logical topology which is then used at the IP layer for IP routing. The coarse-grain wavelength channels and the pre-determined virtual topologies with respect to some a priori assumed traffic distribution are barriers to efficient resource use and inflexible to changing traffic. We take into account the combined knowledge of resource and topology information at both IP and optical layers. With this added knowledge, an integrated routing approach may extract better network efficiencies, be more robust to changing traffic patterns at the IP layer than schemes that either use dynamic routing information at the IP layer or use a static wavelength topology only. LSP set-up requests are represented in terms of a pair of ingress and egress routers as well as its bandwidth requirement, and arrive one-by-one. There is no a priori knowledge regarding the arrivals and characteristics of future LSP set-up requests. Our proposed algorithm considers not only the importance of critical links, but also their relative importance to routing potential future LSP set-up requests by characterizing their normalized bandwidth contribution to routing future LSP requests with bandwidth requirements. Moreover, link residual bandwidth information that captures the link's capability of routing future LSPs is also incorporated into route calculation. Extensive simulation was conducted to study the performance of our proposed algorithm and to compare it with some existing ones, such as the integrated minimum hop routing algorithm and the maximum open capacity routing algorithm. Simulation results show that our proposed algorithm performs better than both routing algorithms in terms of the number of LSP set-up requests rejected and the total available bandwidth between router pairs.     

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     

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.     

A new architecture and a new metric for lightwave networks

The notion of a logically routed network was developed to overcome the bottlenecks encountered during the design of a large purely optical network. In the last few years, researchers have proposed the use of torus. Perfect shuffle, hypercube, de Bruijn graph, Kautz graph, and Cayley graph as an overlay structure on top of a purely optical network. All these networks have regular structures. Although regular structures have many virtues, it is often difficult in a realistic setting to meet these stringent structural requirements. In this paper, we propose generalized multimesh (GM), a semiregular structure, as an alternate to the proposed architectures. In terms of simplicity of interconnection and routing, this architecture is comparable to the torus network. However, the new architecture exhibits significantly superior topological properties to the torus. For example, whereas a two-dimensional (2-D) torus with N nodes has a diameter of Θ(N^0.5), a generalized multimesh network with the same number of nodes and links has a diameter of Θ(N^0.25). In this paper, we also introduce a new metric, flow number, that can be used to evaluate topologies for optical networks. For optical networks, a topology with a smaller flow number is preferable, as it is an indicator of the number of wavelengths necessary for full connectivity. We show that the flow numbers of a 2-D torus, a multimesh, and a de Bruijn network, are Θ(N^1.5), Θ(N^1.25), and Θ(N log N), respectively, where N is the number of nodes in the network. The advantage of the generalized multimesh over the de Bruijn network lies in the bet that, unlike the de Bruijn network, this network can be constructed for any number of nodes and is incrementally expandable     

Virtual Fiber Configuration for Dynamic Lightpath Establishment in Large-scaled Optical Networks

Recently, progress has been made in the Generalized Multi-Protocol Label Switching (GMPLS) and Automatic Switched Optical Networks (ASON) standardizations. These technologies realize construction of large-scaled optical networks, interconnections among single-domain Wavelength Division Multiplexing (WDM) networks, and direct communication over multi-domain WDM networks. Meanwhile, it is known that the topology of the Internet exhibits the power-law attribute. Since the topology of the Internet, which is constructed by interconnecting ASs, exhibits the power-law, there is a possibility that large-scale WDM networks, which are constructed by interconnecting WDM networks, will also exhibit the power-law attribute. One of the structural properties of a topology that adheres to the power-law is that most nodes have just a few links, although some have a tremendous number of them. Another property is that the average distance between nodes is smaller than in a mesh-like network. A natural question is how such a structural property performs in WDM networks. In this paper, we first investigate the property of the power-law attribute of physical topologies for WDM networks. We compare the performance of WDM networks with mesh-like and power-law topologies, and show that links connected to high-degree nodes are bottlenecks in power-law topologies. To relax this, we introduce a concept of virtual fiber, which consists of two or more fibers, and propose its configuration method to utilize wavelength resources more effectively. We compare performances of power-law networks with and without our method by computer simulations. The results show that our method reduces the blocking probabilities by more than one order of magnitude.     

Efficient routing and wavelength assignment for reconfigurable WDMnetworks

Through the use of configurable wavelength-division-multiplexing (WDM) technology including tunable optical transceivers and frequency selective switches, next-generation WDM networks will allow multiple virtual topologies to be dynamically established on a given physical topology. For N node P port networks, we determine the number of wavelengths required to support all possible virtual topologies (PN lightpaths) on a bidirectional ring physical topology. We show that if shortest path routing is used, approximately N wavelengths are needed to map N lightpaths. We then present novel adaptive lightpath routing and wavelength assignment strategies that reduce the wavelength requirements to [(N/2)] working wavelengths per port for protected networks and [(N/3)] wavelengths in each direction per port for unprotected networks. We show that this reduced wavelength requirement is optimal in the sense that it is the minimum required to support the worst case logical topology. Furthermore, we prove that a significant number of logical topologies require this minimum number of wavelengths. We also develop joint routing and wavelength assignment strategies that not only minimize the number of wavelengths required to implement the worst case logical topologies but also reduce average wavelength requirements. Finally, methods for extending these routing and wavelength assignment results to general two-connected and three-connected physical topologies are presented     

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

Consider an optical network which employs wavelength-routing crossconnects that enable the establishment of wavelength-division-multiplexed (WDM) connections between node pairs. In such a network, when there is no wavelength conversion, a connection is constrained to be on the same wavelength channel along its route. Alternate routing can improve the blocking performance of such a network by providing multiple possible paths between node pairs. Wavelength conversion can also improve the blocking performance of such a network by allowing a connection to use different wavelengths along its route. This work proposes an approximate analytical model that incorporates alternate routing and sparse wavelength conversion. We perform simulation studies of the relationships between alternate routing and wavelength conversion on three representative network topologies. We demonstrate that alternate routing generally provides significant benefits, and that it is important to design alternate routes between node pairs in an optimized fashion to exploit the connectivity of the network topology. The empirical results also indicate that fixed-alternate routing with a small number of alternate routes asymptotically approaches adaptive routing in blocking performance     

Scalability of Wavelength Division Multiplexed Optical Passive Star Networks with Range Limited Tunable Transceivers

The number of stations attached to a single optical passive star is limited by current state of the art in optical technology. Also, the wavelength range of tunable optical transceivers is limited by current technology. Many high performance computing applications require the use of large size regular topologies for communication between computing nodes. Scalability of passive star networks built with these two limitations becomes an important issue for building larger networks. This is the subject of our study in this paper. In a previous related work we explored the design issues for networks built on a single passive star employing transceivers of a limited tuning range. Here we extend that study by considering the problem of connecting several optical passive stars, each embedded with a given virtual topology, to create larger aggregate networks. The design issues are analyzed and a number of design rules are proposed for building such aggregate networks. We study the scalability of embedded optical passive stars by considering the most commonly employed virtual topologies—complete graph, mesh and hypercube.     

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。     

A Node-Level Blocking Probability Analysis for WRONs

This work presents the blocking performance of a single node with (full or limited) wavelength conversion in wavelength routed optical networks (WRON) based on the theory of probability. A blocking probability model is proposed. Particularly, we pay more attention to investigate wavelength routing node performance improvement by using the more feasible case of limited wavelength conversion. Based on our analytical model, we calculate the blocking probability for a single wavelength routing node and then make a simulation to validate it. It is shown that a node with low conversion degrees having a small number of fiber link ports and a large number of wavelengths per link is a more realistic choice.     

Performance Comparison of Overlay and Peer Models in IP/MPLS over Optical Networks

We study the connection establishment of label switched paths (LSPs), and compare the LSP blocking performance of the overlay and peer models in IP/MPLS over optical networks. We consider two dynamic routing algorithms for the establishment of LSP connections, of which one is for the overlay model and the other is for the peer model. Our investigations on two typical network topologies, namely NSFNET and ARPA2 networks, show that the number of add/drop ports (or transceivers) on optical cross-connects (OXCs) has a significant impact on the LSP blocking performance for both models. We show by computer simulation that in each case, there is a threshold value for the add/drop ratio, which can achieve almost the best blocking performance. This threshold value remains virtually unchanged as the traffic load varies, but it does depend on the network topology and the number of wavelengths per fiber. This will then indicate the number of add/drop ports to be used so that one can achieve a near optimal blocking performance without incurring unnecessarily excessive network costs. Our investigations reveal that the peer model achieves a much better blocking performance than the overlay model when the number of add/drop ports is relatively high, but that this is not always true when the number of add/drop ports is small.