星形波长路由光网络中的波长分配

文章主要研究了网络中心与其它节点之间由两条光纤成双向传输链路的情况，给出了均匀和顾机两种业务条件下网络中所需波长数的理论下限范围，利用图着色理论构造启发式算法解决了星形网络中的波长分配问题，最后通过实际波长分配验证了算法的有效性。     

Design of logical topologies for wavelength-routed optical networks

The problem of designing a logical topology over a wavelength-routed all-optical network (AON) physical topology is studied. The physical topology consists of the nodes and fiber links in the network. On an AON physical topology, we can set up lightpaths between pairs of nodes, where a lightpath represents a direct optical connection without any intermediate electronics. The set of lightpaths along with the nodes constitutes the logical topology. For a given network physical topology and traffic pattern, our objective is to design the logical topology and the routing algorithm so as to minimize the network congestion while constraining the average delay seen by a source-destination pair and the amount of processing required at the nodes (degree of the logical topology). Ignoring the delay constraints can result in fairly convoluted logical topologies with very long delays. On the other hand, in all our examples, imposing it results in a minimal increase in congestion. While the number of wavelengths required to imbed the resulting logical topology on the physical all optical topology is also a constraint in general, we find that in many cases of interest this number can be quite small. We formulate the combined logical topology design and routing problem described above as a mixed integer linear programming problem which we then solve for a number of cases of a six-node network. This programming problem is split into two subproblems: logical topology design, and routing. We then compare the performance of several heuristic topology design algorithms against that of randomly generated topologies, as well as lower bounds     

具有容错能力的波长路由光网络的拓扑设计

研究了具有容错能力的波长路由光网络的拓扑设计问题.提出了一种基于业务流的初始物理拓扑产生算法,与随机拓扑生成算法相比,该算法产生的物理拓扑更接近于最优拓扑.另外,还提出了一种新的链路拥塞计算方法,它能够更好地反应链路的实际状态.数值结果显示文章提出的物理拓扑设计策略具有更快的收敛速度.     

Design of wavelength-routed optical networks for packet switchedtraffic

We consider the problem of designing a logical optical network topology for a given physical topology (or fiber layout) and a given traffic demand matrix between the end-users. Traffic between the end-users is carried in a packet-switched form and the objective of our logical topology design is to minimize the maximum congestion on the logical connections in the logical topology. The logical connections are realized by wavelength continuous paths or lightpaths between end-users and they are routed via wavelength-selective routers. Note that a topology with lower maximum link congestion will allow its traffic demand matrix to be scaled up by a larger factor. In the logical topology each node is equipped with a limited number of optical transceivers, hence logical connections cannot be set up between every pair of nodes. In this paper we present an improved lower bound for maximum congestion on any link In the logical topology. The bound is shown to be up to 50% higher than the existing ones. An analytical model for obtaining the maximum and average logical connection loads for a given logical network and traffic demand matrix is also formulated, and it has been confirmed via simulation. Finally, two heuristic algorithms for constructing a logical topology that reduces maximum logical connection congestion are presented     

Estimating the blocking probability in wavelength-routed optical networks

In this work, a new methodology to compute the blocking probability in wavelength-routed optical networks is presented. The proposal is based on an interactive procedure, named Interactive Matrix Methodology (IMM), that executes actualization of the network traffic distribution in order to reach a precise blocking performance. The IMM updates an initial network link load continuously and computes the blocking probability for each output link considering that the traffic among the links is dependent and related with all links and nodes in the network, not only with all links in a given path or route. The simulation results obtained in the same conditions and in several optical network scenarios match very well with the theoretical approximation achieved with this methodology. The advantage of this theoretical methodology is to be fast, accurate and applicable in low load regions, where a discrete event simulation is not precise. Furthermore, this method can be used to compute the estimative of blocking probabilities per node and in the network, including the cases where the number of wavelengths is different on each node.     

Optimization of splitting node placement in wavelength-routed optical networks

This paper addresses the placement of multicast nodes in wavelength-routed all-optical networks. This problem is motivated by the expected high cost of optical multicast cross-connects due to fabrication complexity and power considerations. An analytical model for the approximate blocking probability in multicast networks is developed. The blocking performance is used to guide an iterative algorithm, Multicast-ADD, for the placement of multicast nodes. We provide validation of the model and the techniques used. In addition, a simulation study is provided to evaluate the performance of the heuristic. Insights obtained from the simulation results reveal that: (1) only a subset of the nodes (found to be 50%) need to be multicast-capable for acceptable blocking performance; (2) the blocking performance of Multicast-ADD outperforms that of random placement; and (3) employing alternate routing trees is not helpful in the optimization procedure.     

Comprehensive design methodology for control and data planes in wavelength-routed optical networks

We propose a comprehensive design methodology for control and data planes of wavelength-routed optical networks (WRONs) employing mixed-line-rate (MLR) transmission for cost-effective resource provisioning. The proposed approach attempts to minimize the maximum lightpath capacity demand in Gbps (representing the measure of lightpath congestion) in network for a given traffic matrix by using a mix of a heuristic scheme and linear programming (LP). In the first step of the proposed three-step design, some lightpaths are set up on a set of judiciously selected fiber links (with point-to-point lightpaths between neighboring nodes), on a specific wavelength throughout the network, and an appropriate fraction of the same set of lightpaths is utilized for carrying control information, forming therefore the control plane (CP) of the WRON. The remaining bandwidth of these lightpaths is utilized to carry the data traffic along with all other designed lightpaths of the WRON using appropriate algorithm, forming the overall data plane (DP) of the WRON. In the second step, traffic routing is carried out through LP to minimize lightpath congestion in the network. In the third step, we utilize the results of LP to assign rates to lightpaths, such that the cost (considering only the transceiver cost) of the network is minimized. This design leads to congestion-aware MLR network with due consideration to cost-effectiveness without compromising the network restoration response against link failures. We carry out simulation studies employing possible CPs using both symmetric (CP topology being same as the physical topology) as well as asymmetric (using fewer fiber links than the symmetric case) topology. The results of our simulations indicate that the proposed design of CP with symmetric/asymmetric topology and in-band transmission with sub-lightpath capacity can bring down network congestion and cost with respect to symmetric out-of-band transmission (using fully reserved lightpaths for CP), without any perceptible sacrifice in respect of the network restoration time. Failure can occur either in CP or DP, or in both the planes. We investigate the effect of design of CP with symmetric/asymmetric topology on network restoration time for single- and double-link failures. We further present DP design methodology with hybrid restoration scheme, i.e., combination of dedicated (1:1) path protection and path restoration. We analyze the effect of symmetric CP topology and degree of protection on the congestion of the network. Some lightpaths, that support more traffic, are protected against failures, while the others are left for path restoration in the event of failures. As more lightpaths are protected, the congestion and power consumption of network increase. We provide an analysis of the factors that come into play while altering the degree of protection and observe how the choice for the degree of protection in DP can be arrived at using an appropriate design methodology.     

Comparison of failure dependent protection strategies in optical networks

The criticality of survivable network design and operation increases with increasing transmission speed. Path protection strategies achieve better network utilization compared to link protection strategies; however, the recovery time of connections in path protection strategies are higher than that in link protection strategies. This paper evaluates and compares the performance of three failure dependent strategies: (1) failure dependent path protection; (2) link protection; and (3) Diversion—a variant of the segmented path protection approach. In addition, a framework for evaluating the connection recovery time is also developed. The protection strategies are compared for their recovery time and blocking performance using extensive simluations.     

Cost efficient traffic grooming and regenerator placement in impairment-aware optical WDM networks

In this paper, we address the problem of traffic grooming and regenerator placement in a WDM optical network in which lightpaths are hop-constrained by physical impairments. The efficient placement of regenerators and electronic grooming equipment at ROADM nodes for a given network topology is required such that all traffic demands can be supported with minimum cost. We present a detailed ROADM node architecture together with an associated cost model. We model the problem by Integer Linear Programming (ILPs) and propose an auxiliary-graph-based heuristic for jointly placing regenerators and electronic grooming equipment in the network. To evaluate the performance of the proposed heuristic, we also derive a lower bound on the network cost. The numerical results show that combining the grooming problem with the placement of regenerators reduces the network cost significantly compared to the cases in which traffic grooming and regenerator placement are handled separately. The performance of the proposed polynomial-time heuristic is very close to the lower bound and approaches the bound as the network load increases.     

A novel distributed control protocol in dynamic wavelength-routed optical networks

Path selection has long been one of the most important issues in the network design and management, which is also referred to as routing and wavelength assignment in the optical network domain. In this article we investigate the RWA problem for dynamic wavelength-routed mesh networks in a fully distributed controlled environment. We present a novel routing and signaling protocol called Asynchronous Criticality Avoidance (ACA), which is devised to reduce the mutual interference between lightpaths launched by different source-destination pairs to improve network performance in terms of blocking probability. With the fixed alternate routing architecture, the ACA protocol dynamically marks a set of wavelength channels as critical if the occupancy of the channels causes a bottleneck between an S-D pair with a width equal to or narrower than a predefined threshold. To support a distributed control environment, a suite of signaling processes is devised to realize the criticality avoidance mechanism, with which a two-stage routing and wavelength assignment (or criticality avoidance routing) is performed. The ACA protocol has been shown by simulations to be capable of better performance than existing schemes.     

Restoration in wavelength-routed optical networks by means of ant colony optimization

Because of the distributed control of the network, the dynamic nature of the traffic and the unpredictability of a failure event, the flexibility and robustness of ant colony optimization (ACO) make it a suitable candidate for provisioning lightpaths in an optical network. In this work, we propose a fault-tolerant dynamic routing and wavelength assignment (RWA) algorithm based on the ACO framework, presenting its integration into the Generalized multi-protocol label switching (GMPLS) control plane. By simulating two different scenarios, we demonstrate the effectiveness of this algorithm when a single link or node failure occurs.

A kind of topology aggregation algorithm in hierarchical wavelength-routed optical networks

In large-scale networks composed of several hierarchical subnetworks, topology aggregation (TA) is implemented for both scalability and security consideration. However, due to the special characteristic of optical wavelength-routed networks, existing TA algorithms cannot be applied directly for topology compression. In this paper, we propose a kind of TA algorithm suitable for hierarchical wavelength-routed optical networks, thereby considering information of residual wavelengths on network links. Basically, our TA algorithm is based on a transition matrix technique for constructing a full mesh topology and asymmetric star approach for a more compact topology. Simulations with randomly-generated networks show that topology information compression can be achieved with a little loss of its accuracy: the ratio of border node pairs with representation deviation being well below 10%. Results also indicate that the advantage of our algorithm is also behaved on reducing the amount of link-state-advertisement (LSA) messages and making networks less vulnerable to propagation delay of these messages over communication links.     

A dynamic impairment-aware networking solution for transparent mesh optical networks - [Topics in optical communications]

Core networks of the future will have a translucent and eventually transparent optical structure. Ultra-high-speed end-to-end connectivity with high quality of service and high reliability will be realized through the exploitation of optimized protocols and lightpath routing algorithms. These algorithms will complement a flexible control and management plane integrated in the proposed solution. Physical layer impairments and optical performance are monitored and incorporated in impairment-aware lightpath routing algorithms. These algorithms will be integrated into a novel dynamic network planning tool that will consider dynamic traffic characteristics, a reconfigurable optical layer, and varying physical impairment and component characteristics. The network planning tool along with extended control planes will make it possible to realize the vision of optical transparency. This article presents a novel framework that addresses dynamic cross-layer network planning and optimization while considering the development of a future transport network infrastructure.     

抗毁WDM光网络中SRLG约束下p-Cycles配置算法

基于共享风险链路组(SRLG,shared risk link group)和P圈(P-Cycles,pre-configured cycles)的概念,研究了SRLG约束下p-Cycles的构造问题,引入SRLG完全分离p-Cycles的概念,基于SRLG的简单p-Cycles构造算法和获得更多p-Cycles的SRLG约束下的圈扩展算法提出的SRLG约束下的p-Cycles配置算法(SCAA),实现在光网络中优化配置SRLG完全分离的p-Cycles。通过计算机仿真表明,SCAA最小容量配置方案可以预留更少的网络资源,而SCAA优化容量配置方案可以实现p-Cycles快速配置容量,SCAA算法可以保障配置SRLG分离p-Cycles的高保护效能,使网络具备单SRLG故障恢复能力。     

Routing, wavelength and time-slot-assignment algorithms for wavelength-routed optical WDM/TDM networks

This paper studies the connection-assignment problem for a time-division-multiplexed (TDM) wavelength-routed (WR) optical wavelength-division-multiplexing (WDM) network. In a conventional WR network, an entire wavelength is assigned to a given connection (or session). This can lead to lower channel utilization when individual sessions do not need the entire channel bandwidth. This paper considers a TDM-based approach to reduce this inefficiency, where multiple connections are multiplexed onto each wavelength channel. The resultant network is a TDM-based WR network (TWRN), where the wavelength bandwidth is partitioned into fixed-length time slots organized as a fixed-length frame. Provisioning a connection in such a network involves determining a time-slot assignment, in addition to the route and wavelength. This problem is defined as the routing, wavelength, and time-slot-assignment (RWTA) problem. In this paper, we present a family of RWTA algorithms and study the resulting blocking performance. For routing, we use the existing shortest path routing algorithm with a new link cost function called least resistance weight (LRW) function, which incorporates wavelength-utilization information. For wavelength assignment, we employ the existing least loaded (LL) wavelength selection; and for time-slot allocation, we present the LL time-slot (LLT) algorithm with different variations. Simulation-based analyses are used to compare the proposed TDM architecture to traditional WR networks, both with and without wavelength conversion. The objective is to compare the benefits of TDM and wavelength conversion, relative to WR networks, towards improving performance. The results show that the use of TDM provides substantial gains, especially for multifiber networks.     

Differentiated-resilience provisioning for the wavelength-routed optical network

This paper evaluates the performance of a new scheme for the differentiated-resilience provisioning of lightpaths in a wavelength-routed optical network. A novel differentiated-resilience optical-services model (DROSM) is proposed to enhance the performance of the optical network. First, an analysis of optical-resilience options is presented. Then, a classification of optical services according to their resilience requirement is proposed with respective resilience options. In particular, a novel link-management mechanism is proposed to assist the routing and wavelength allocation. Our numerical results show that differentiated-resilience provisioning has significant merit over that of single resilience provisioning in the optical network.     

Impact of transmission impairments on the teletraffic performanceof wavelength-routed optical networks

In a wavelength-routed optical network, a transmitted signal remains in the optical domain over the entire route (lightpath) assigned to it between its source and destination nodes. The optical signal may have to traverse a number of crossconnect switches (XCSs), fiber segments, and optical amplifiers, e.g., erbium-doped fiber amplifiers (EDFAs). Thus, while propagating through the network, the signal may degrade in quality as it encounters crosstalk at the XCSs and also picks up amplified spontaneous emission (ASE) noise at the EDFAs. Since these impairments continue to degrade the signal quality as it progresses toward its destination, the received bit error rate (BER) at the destination node might become unacceptably high. Previous work on the lightpath routing and wavelength assignment (RWA) problem assumed an ideal physical layer and ignored these transmission impairments. The main contribution of our work is to incorporate the role of the physical layer in setting up lightpaths by employing appropriate models of multiwavelength optical devices (XCSs and EDFAs) such that the BER of a candidate lightpath can be computed, in advance, to determine if this lightpath should be used for the call. Features from existing RWA algorithms are integrated with our on-line BER calculation mechanism. Our simulation studies indicate that employing BER-based call-admission algorithms has a significant impact on the performance of realistic networks     

Reducing the lightpath establishing time of FWM-aware dynamic RWA for wavelength-routed optical networks

A dynamic routing and wavelength allocation technique with an interplay between physical and network layer parameters encompassing Four-wave mixing (FWM) awareness and teletraffic performance of wavelength-routed optical networks has previously been proposed for a distributed approach. In this article, we present a fast computational algorithm for our routing and wavelength assignment (RWA) encompassing FWM-induced crosstalk. The objective is to minimize the time of establishing a dynamic lightpath. For this purpose, a precomputed matrix of FWM crosstalk products is used in an adapted version of the FWM-aware dynamic RWA algorithm. The approach is validated through simulations showing improvement up to 30–50% on the provisioning time of lightpaths for different network topologies compared to an online full computational scheme.

Joint optimization of delay and congestion in wavelength-routed optical networks using genetic algorithms

A new multipurpose genetic algorithm, based on Pareto optimality, is proposed to design logical topologies for wavelength-routed optical networks with the aim of minimizing both the congestion and the end-to-end delay. Simulation results show its efficiency when compared with other previously proposed algorithms, achieving in most cases optimal or near-optimal solutions, and in less time than other methods. Moreover, since the algorithm relies on Pareto optimality, not only does it obtain a single logical topology but a set of them, so that the network designer can easily select the most appropriate one according to the current network requirements.