Impact of transmission performance on path routing in all-opticaltransport networks

The impact of transmission related issues on the routing strategies for transparent all-optical wavelength division multiplexed (WDM) transport networks is analyzed in this paper. Three different categories of routing algorithms are analyzed: algorithms based on the wavelength path (WP) strategy, based on the virtual wavelength path (VWP) strategy and requiring only a limited number of wavelength converters in the network partial virtual wavelength path (PVWP). It results that the PVWP allows a saving in network devices with respect to the WP similar those permitted by the VWP also attaining transmission performances near those attained by the WP that are quite better that those attained by the VWP     

WDM passive network design for small wavelength-count in local area/home applications

Passive optical networks are attractive due to their enhanced reliability, reduced electrical power consumption, and small latency. This paper determines the feasibility of WDM (Wavelength Division Multiplexing) passive networks with low wavelength count for small environments such as buildings/homes from the viewpoint of the physical network configuration. After describing the general characteristics of WDM passive networks in the bus, star, and ring configurations, we elucidate the optical path losses to find design solutions when the three physical configurations are used to create small networks. It is concluded that the star configuration is the most feasible approach. The bus and ring are also feasible only for small areas and low wavelength counts.     

Wavelength assignment for multicast in all-optical WDM networks with splitting constraints

Multicast is an important application in all-optical WDM networks. The wavelength assignment problem for WDM multicast is to assign a set of wavelengths to the links of a given multicast tree. In an all-optical WDM network without wavelength conversions, wavelength assignment is the key to guarantee the quality of service and to reduce communication costs. In this paper, we study wavelength assignment for WDM multicast with two criteria, to cover the maximum number of destinations, and to minimize the wavelength costs. The computational complexity of the problem is studied. Three heuristic algorithms are proposed and the worst-case approximation ratios for some heuristic algorithms are given. We also derive a lower bound of the minimum total wavelength cost and an upper bound of the maximum number of reached destinations. The efficiency of the proposed heuristic algorithms and the effectiveness of the derived bounds are verified by the simulation results.     

QoS-Aware Wavelength Assignment With BER and Latency Constraints for All-Optical Networks

Physical impairments originating from optical fiber components and intermediate switching nodes can be the dominant reason calls are blocked in wide-area all-optical wavelength division multiplexing (WDM) networks. When a centralized network controller is used, estimating the impact of the physical impairments on the quality of a lightpath before provisioning it can cause a significant delay. In this paper, quality of service aware wavelength assignment algorithms are proposed that consider both bit-error rate (BER) and latency constraints. A novel wavelength assignment technique called wavelength ordering is shown via simulation to reduce the call blocking probability resulting from both physical impairments and excessive processing delay caused by channel BER estimation.     

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     

Transmission capacity of optical path overhead transfer schemeusing pilot tone for optical path network

Photonic networks based on the optical path concept and wavelength division multiplexing (WDM) technology require unique operation, administration, and maintenance (OAM) functions. In order to realize the required OAM functions, the optical path network must support an effective management information transfer method. The method that superimposes a pilot tone on the optical signal appears very interesting for optical path overhead transfer. The pilot tone transmission capacity is determined by the carrier to noise ratio which depends on the power spectral density of the optical signal. The pilot tone transmission capacity of an optical path network employing WDM technology is elucidated; 4.5 kb/s transmission can be realized when the pilot tone modulation index is set at 3%     

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.     

OA&M framework for multiwavelength photonic transport networks

Photonic networks based on wavelength division multiplexing (WDM) and optical path technologies are expected to realize flexible, transparent, and cost-effective transport networks with a large transmission capacity. This paper explores the design framework of photonic transport networks taking into consideration the operation administration and maintenance (OA&M) functions required for the successful introduction of WDM systems based on the optical path concept. From the view point of network maintenance, clear distinction is made between the optical path layer and the optical section layer to facilitate accurate and smooth failure localization. The digital multiplexing span between physical multiplexing interfaces at the end-to-end digital nodes should have the same maintenance span as the corresponding optical path. We argue that cooperative maintenance by OA&M functions at both the digital and optical layers can be a practical way of network supervision. A supervisory (SV) signal transfer method and a configuration that is suitable for the terrestrial trunk network are also indicated. As an example, a practical SV system design methodology and an actual procedure developed for a single channel optical transmission system based on optical in-line amplifiers are introduced. Furthermore, application of the developed SV system and network restoration schemes is discussed for future WDM-based photonic networks. The OA&M aspects introduced will be valuable for creating future photonic network systems     

WDM光网络中一种优先共享通路保护算法

为了提高波分复用光网络的可靠性,常采用分段共享通路保护算法,该方法通常要求保护通路要均匀分段并且应满足共享风险链路组约束,网络业务的阻塞率也就比较高,因此提出了用优先共享通路保护算法来降低业务阻塞率。通过计算机仿真进行了理论分析和实验验证,取得了两种保护算法下的业务阻塞率和资源预留比的数据。结果表明,优先共享保护通路算法能够有效地融合分段共享通路保护算法的优点,同时在业务的阻塞率和资源预留比方面优于分段共享保护通路算法。     

Heuristic and Hybrid Methods for Solving Optimal Multiple Multicast Problem on WDM Ring Network

The Optimal Multiple Multicast Problem (OMMP) on wavelength division multiplexing (WDM) ring networks without wavelength conversion is considered in this paper. When the physical network and the set of multicast requests are given, OMMP is the problem that selects a suitable path (or paths) and wavelength (or wavelengths) among the many possible choices for each multicast request such that not any pair of paths using the same wavelength pass through the same link. In this paper, a formulation of OMMP is given; this problem is NP-hard since the famous RWA problem which has been proved NP-hard is a special case of OMMP. In this paper, the OMMP is divided into two subproblems: path routing and wavelength assignment subproblems. For each subproblem, two heuristic algorithms are proposed to solve it. Moreover, a hybrid method which combines heuristic and simulated annealing algorithm is proposed to find the near optimal solution. Experimental results indicate that these algorithms are efficient.     

Optimal regenerator assignment and resource allocation strategies for translucent optical networks

Physical layer impairments in wavelength-routed networks limit the maximum distance, a signal can travel in the optical domain, without significant distortion. Therefore, signal regeneration is required at some intermediate nodes for long-haul lightpaths. In translucent WDM networks, sparsely located regenerators at certain nodes can be used to offset the impact of physical layer impairments. The routing and wavelength assignment (RWA) techniques in such translucent networks need to take into consideration the availability of regenerators and the maximum optical reach of the transparent lightpaths (without any regeneration). Although there has been significant research interest in RWA algorithms for translucent networks, much of the research has focused on dynamic RWA techniques. Only a handful of recent papers have considered the static (offline) case, and they typically propose heuristic algorithms to solve this complex design problem for practical networks. In this paper, we propose a generalized integer linear program (ILP) based formulation for static regenerator assignment and RWA in translucent WDM optical networks, with sparse regenerator placement. To the best of our knowledge, such a formulation that optimally allocates resources for a set of lightpaths for translucent networks, given the physical network, the locations of the regenerators, and the maximum optical reach has not been considered before. The proposed formulation is important for two reasons. First, it can serve as a benchmark for evaluating different heuristic approaches that may be developedin the future. Second, we show that using a novel node representation technique, it is possible to drastically reduce the number of integer variables. This means that unlike existing ILP formulations, our approach can actually be used to generate optimal solutions for practical networks, with hundreds of lightpath demands.     

WDM光网络中基于传输损伤的公平RWA算法

在WDM光网络中,物理层上存在使信号传输质量恶化的因素,当信号质量劣化到一定程度会导致连接失败,因此有必要在选路和波长分配RWA（Routing and Wavelength Assignment）算法中考虑物理层的传输损伤。连接公平性问题是指网络中长路径连接请求的阻塞率表现比短路径差,文中分析传输损伤约束的引入对连接公平性的影响,提出基于传输损伤的动态RWA算法：固定波长配额FWQ（Fixed Wavelength Quota）算法和非固定波长配额UFWQ（UnFixed Wavelength Quota）算法。     

全光网中的波长变换器放置问题研究

波长变换器的出现可有效地提高WDM全光网的网络性能。文童对WDM全光网中的波长变换器放置问题进行了深入的研究,详细阐述了各种波长变换器放置算法的设计思想,并对各种算法的性能以及时间复杂度等参数进行了分析,并在典型拓扑上进行了仿真。     

Design Method of Logical Topologies with Quality of Reliability in WDM Networks

As the bandwidth capacity of WDM networks continues to grow rapidly,traffic loss caused by a failure of network components is becoming unacceptable. To prevent such traffic loss and thus enhance network reliability, a protection method that prepares backup lightpaths for each working path is now being developed. In this paper, we first introduce the concept of QoR (quality of reliability), which is a realization of QoS with respect to the reliability needed in a WDM network. We define QoR in terms of the recovery time from when a failure occurs to when traffic on the affected primary lightpath is switched to the backup lightpath. After that, we propose a heuristic algorithm that can be used to design a logical topology that satisfies the QoR requirement for every node pair. The objective is to minimize the number of wavelengths needed for a fiber in the logical topology to carry the traffic with the required QoR. We compare this algorithm with two existing algorithms and show that it enables more effective use of wavelength resources; with the proposed algorithm, up to 25% fewer wavelengths are needed than with the other algorithms.     

Architectural analysis of multiple fiber ring networks employingoptical paths

Analyzes the performance of various types of multiple fiber ring networks employing optical paths (OP's). The multiple fiber ring network architecture is suitable for achieving failure resilient networks that have extremely large bandwidth but are still upgradable against future increases in traffic. This architecture will overcome the limitation of conventional WDM rings in terms of network expansion capabilities, the number of nodes within the ring, and the number of OP's accommodated in the network. The generic node architecture suitable for multiple fiber ring networks is presented and functionality requirements are identified. The OP accommodation design algorithms that minimize the required node system scale are proposed. Based on the generic node architecture and proposed OP accommodation design algorithms, we evaluated the performance of several types of multiple fiber rings in terms of the required node system scale for rings under various conditions. The effect of the ring architecture (uni-/bidirectional rings), optical path schemes (wavelength path/virtual wavelength path), and different node connectivity patterns are demonstrated for the first time. The obtained results elucidate the criteria for selecting the most suitable multiple fiber ring architecture     

SUCCESS-HPON: A next-generation optical access architecture for smooth migration from TDM-PON to WDM-PON

Optical access networks are considered to be a definite solution to the problem of upgrading current congested access networks to ones capable of delivering future broadband integrated services. However, the high deployment and maintenance cost of traditional point-to-point architectures is a major economic barrier. Current TDM-PON architectures are economically feasible, but bandwidth-limited. In this article we first discuss the possible role of WDM in access networks and investigate the associated issues. We then present the Stanford University Access Hybrid WDM/TDM Passive Optical Network (SUCCESS-HPON), a next-generation hybrid WDM/TDM optical access architecture that focuses on providing a smooth migration path from current TDM-PONs to future WDM-PONs. The first testbed for this architecture is described, along with the experimental results obtained, including feasibility of bidirectional transmission on the same wavelength on the same fiber for access networks and ONU modulation of upstream data on continuous waves provided by the OLT, eliminating the need for tunable components at the ONUs. The development of a second testbed and the issues it will address, including the implementability of the SUCCESS-HPON MAC protocol and scheduling algorithms, are also described.     

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.     

Complexity of Wavelength Assignment in Optical Network Optimization

We study the complexity of a set of design problems for optical networks. Under wavelength division multiplexing (WDM) technology, demands sharing a common fiber are transported on distinct wavelengths. Multiple fibers may be deployed on a physical link. Our basic goal is to design networks of minimum cost, minimum congestion and maximum throughput. This translates to three variants in the design objectives: 1) MlN-SUMFlBER: minimizing the total cost of fibers deployed to carry all demands; 2) MlN-MAXFlBER: minimizing the maximum number of fibers per link to carry all demands; and 3) MAX-THROUGHPUT: maximizing the carried demands using a given set of fibers. We also have two variants in the design constraints: 1) CHOOSEROUTE: Here we need to specify both a routing path and a wavelength for each demand; 2) FIXEDROUTE: Here we are given demand routes and we need to specify wavelengths only. The FIXEDROUTE variant allows us to study wavelength assignment in isolation. Combining these variants, we have six design problems. Previously we have shown that general instances of the problems MIN-SUMFIBER-CHOOSEROUTE and MIN-MAXFIBER-FIXEDROUTE have no constant-approximation algorithms. In this paper, we prove that a similar statement holds for all four other problems. Our main result shows that MIN-SUMFIBER-FIXEDROUTE cannot be approximated within any constant factor unless NP-hard problems have efficient algorithms. This, together with the previous hardness result of MIN-MAXFIBER-FIXEDROUTE, shows that the problem of wavelength assignment is inherently hard by itself. We also study the complexity of problems that arise when multiple demands can be time-multiplexed onto a single wavelength (as in time-domain wavelength interleaved networking (TWIN) networks) and when wavelength converters can be placed along the path of a demand.     

WDM protocol-transparent distance extension using R2 remodulation

In computer networks using wavelength-division multiplexing (WDM), it is often necessary to extend the length of a given WDM path beyond that of a single segment whose length Is limited by the link budget. While wavelength-flattened optical amplifiers are the most obvious solution, this paper calls attention to a simple (albeit less efficient) alternative that works for any number of wavelength channels, but only a modest number of segments and modest bit rates. This scheme substitutes for each optical amplifier stage a multiwavelength 2R remodulator consisting of a WDM demultiplexor, followed by wavelength-by-wavelength zero-crossing detectors, then remodulation wavelength by wavelength, and finally wavelength-division multiplexing back onto a single fiber. Theory and experimental results confirm the usefulness of this primitive technique, which can also be used for add-and-drop links, for wavelength routing or wavelength conversion in scalable wavelength routing networks, for internetwork gateways, and for other applications     

Design Considerations for Transmission Systems in Optical Metropolitan Networks

Wavelength division multiplexing (WDM) has become a key technology in optical long-haul communications and wide area networks. Recently, interest in introducing WDM into metropolitan networks has increased. We look at some issues pertinent to the design of WDM transmission systems deploying optical add/drop multiplexers in these networks. In particular, we investigate the interaction of node-induced crosstalk with fiber nonlinearities in metropolitan ring networks. Two transmission systems are considered; namely a system of nonzero dispersion shifted fiber operating in the anomalous dispersion regime and another one of single mode fiber with post dispersion compensation. Data rate is assumed to be 10 Gb/s per wavelength. The dependence of the crosstalk/nonlinearity interaction and its system penalty on the frequency difference between signal and crosstalk carriers, input signal power, extinction ratio, number of nodes, and physical span of the network is examined in detail. It is shown that node-induced crosstalk can interact with fiber nonlinearities and introduce limitations on transmission performance. This interaction should therefore be taken into consideration in designing regional metropolitan networks.