Wavelength Division Multiplexing Layer Research Articles

Optimization Technology of Optical Fiber Communication Network Based on Service Classification

Optical fiber has huge bandwidth, as a physical medium, it has good anti electromagnetic interference characteristics and excellent security performance, which is very suitable for the establishment of backbone transmission network. Wavelength division multiplexing(WDM) network based on optical fiber has been widely used. Using optical fiber for information transmission can give full play to the nearly unlimited transmission capacity of optical fiber. With the advent of the information age, data service has become the mainstream business, replacing the original telephone voice service. The great development of data transmission service further promotes the continuous development of WDM network technology. Among all kinds of services in optical network, IP service accounts for the largest proportion. In optical networks, switching is an important technology. Network performance is directly related to switching technology. With the development of optical communication technology, a variety of optical switching devices are introduced into WDM optical network, such as optical packet switching, optical burst switching and so on. This makes WDM layer have the ability of dynamic path establishment and removal similar to IP layer. In order to improve the performance of the optical network and reduce the blocking rate of the network, in WDM optical network, the services are designed in different levels. For some very important services, the highest priority is given, and the optical path channel with better survivability and higher fault tolerance is selected. For general services, it can be designed as a lower level service, and choose the appropriate optical path channel for transmission. The experimental results show that this hierarchical service design method can make more effective use of network resources, reduce the network blocking rate, and improve the overall performance of the network.

Spatial Channel Cross-Connect Architectures for Spatial Channel Networks

A spatial channel network (SCN) is a novel optical network architecture targeting the space division multiplexing (SDM) era where the optical layer is explicitly evolved into the hierarchical SDM layer and wavelength division multiplexing layer, and an optical node is decoupled into a spatial channel cross-connect (SXC) and a wavelength cross-connect to form a hierarchical optical cross-connect. In this article, we discuss a wide variety of SXC architectures based on optical matrix switches (MSs) and newly proposed core selective switches (CSSs) from the viewpoints of connection flexibility and architectural complexity. After briefly reviewing the architecture and functionalities of an SCN and its building-block optical devices, novel SXC architectures based on sub-MSs and CSSs are described and formulas are derived for each SXC architecture that give the required number of optical devices, switching mirrors, and internal fiber connections. Based on the formulations, these architectures are compared with traditional SXC architectures based on a high-port count full-MS pair and a Clos network from the viewpoints of the freedom of connection functionalities, operational benefits, and potential node costs associated with the architectural complexity in order to obtain insights regarding which network application is suited to each SXC architecture.

Multi-layer Virtual Topology Design in Optical Networks Under Physical Layer Impairments and Multi-hour Traffic Demand

In this paper, we consider the virtual topology design (VTD) problem in multi-protocol label switching (MPLS)/wavelength division multiplexing (WDM) networks with an hourly changing traffic pattern, in the presence of physical layer impairments. We propose a multi-layer VTD scheme that satisfies the constraints in both layers with guaranteed bit error rates assuming no wavelength conversion. The proposed scheme involves interaction of the control planes of MPLS and WDM layers and can be applied without the necessity of a unified control plane. When the virtual topology is designed by considering only MPLS layer constraints, some lightpaths in the designed topology cannot be established, and this may result in a large amount of blocked traffic. We show that, by applying interaction between MPLS and WDM layers during topology design, the blocking ratio is reduced by an amount between 53% and 89% in the investigated cases. We also propose different information sharing strategies between the layers and show that, if physical layer information is available to the MPLS layer, a better blocking performance can be achieved and physical layer resource usage can be reduced by an amount up to 21%.

Design and Analysis of a WDM EPON for Supporting Private Networking and Differentiated Services

Although EPONs have been proposed as a means of overcoming the bandwidth bottleneck problem in local access networks, they may not be able to meet the ever-growing bandwidth demand from users in the near future. Accordingly, this study presents what we believe to be a novel wavelength-division-multiplexing (WDM) Ethernet passive optical network (EPON) system based on an arrayed-waveguide-grating (AWG) module to meet the rapidly increasing bandwidth demand anticipated in the future. The use of the AWG enables the proposed WDM EPON architecture to achieve wavelength spatial reuse and to provide an intercommunication capability between optical network units (ONUs). As a result, the architecture not only allows upstream access to the central office, but also facilitates a truly shared LAN capability among the end users. Furthermore, the proposed WDM EPON scheme takes account of the requirement for backward compatibility with the IEEE 802.3ah multipoint control protocol and incorporates a dynamic bandwidth allocation scheme and a quality-of-service provisioning mechanism to arbitrate the access of the individual ONUs over the WDM layer. An analytical framework is developed for evaluating the mean packet delay and mean queue length of the proposed WDM EPON system. The analytical results derived using this framework are found to be in good agreement with those obtained from computer simulations.

Differentiated resilience services support in heterogeneous IP over wavelength division multiplexing networks

The authors develop an integrated scheme that supports differentiated resilience services (DiffResServ) for dynamic provisioning in heterogeneous IP-over- wavelength division multiplexing (WDM) networks. By integrated, the authors mean that the provisioning mechanism takes into account network topology, resource and resilience information in both the IP and WDM layers. By dynamic, the authors mean that the establishment process of connections and lightpaths is based on the fact that the arrivals and departures of IP traffic connections are not known in advance. By heterogenous, the authors mean that the state routing algorithms function on the fact that optical nodes in WDM network may support different switching capabilities. This has been achieved through our proposed scheme which operates based on a dual link metrics (bandwidth,delay)routing algorithm inter-operating with efficient connections and lightpaths rerouting algorithms. A genetic algorithm is introduced to operate based on the solution space generated by this scheme in order to minimise the total cost of network operation. The authors have conducted simulations, where the results show that this scheme enables IP-over-WDM networks to guarantee the quality-of-service and resilience required for heterogenous traffic classes with effective and efficient allocation of network resources.

Hop-Constrained Node Survivable Network Design: An Application to MPLS over WDM

In this paper, we address a hop-constrained node survivable network design problem that is defined in the context of multi-protocol label switching (MPLS) over wavelength division multiplexing (WDM) networks. At the lower WDM layer, we consider a maximum length constraint for optical connections between MPLS routers. At the upper MPLS layer, we consider survivability as well as maximum delay constraints. Survivability is guaranteed by routing each demand through D node-disjoint paths and maximum delay is guaranteed by constraining all paths to a maximum number of hops. An Integer Linear Programming model, based on the previous works by Gouveia et al. (Proc of IEEE INFOCOM, 2003, and Telecommunications network planning: innovations in pricing, network design and management, pp 167–180, 2006) is used to model the network design problem considering two different survivability mechanisms: path diversity (where each demand is equally split over the D paths) and path protection (where any D–1 out of the D paths have enough capacity to support the total demand). For both mechanisms, we use the NSFNet and EON real world networks to make a cost analysis of the design solutions for different values of D. In the path diversity mechanism, the results consistently show that greater values of D impose a cost penalty that is greater than the gain in the percentage of demand that is protected. In the path protection mechanism, where all traffic is totally protected, the results show that the network solutions obtained with D=3 node-disjoint paths have consistently lower costs than the network solutions obtained with D=2 node-disjoint paths. However, using values of D that are greater than 3 led to network solutions with larger costs.

Introduction to the Special Issue on Telecommunications

This issue contains a selection of the papers representative of the best work presented at the 8th INFORMS Telecommunications conference which was held in Dallas, Texas, from March 30 to April 1, 2006. These articles represent the broad spectrum of topics in network design and management that were presented at the conference and each underwent a rigorous refereeing process. We thank the authors for their contributions, and the group of academics and telecommunications industry practitioners who served as referees. Gouveia et al. present an integer linear programming (ILP) model to address a survivable network design problem for networks employing Multiprotocol Label Switching (MPLS) over Wavelength Division Multiplexing (WDM). At the lower WDM layer, they consider a maximum-hop limit on optical connections between MPLS routers. At the upper MPLS layer, they consider survivability requirements and maximum delay constraints. Using their ILP model they conduct an empirical study on problem instances derived from real world networks to analyze the cost structure of designs based on two different protection schemes. Two of the papers in this issue are motivated by the anticipated growth in demand for Video-on-Demand (VOD) services. Luss presents a model for bandwidth allocation for VOD and near-VOD service in tree networks where a server at the root node broadcasts copies of programs requested by customers. The solution to the model determines an optimal bandwidth allocation for the given set of programs and demand data, as well as insight into how the capacity of the network should be expanded in the future. Kalvenes and Keon develop a game-theoretic model to study the optimal economic conditions for a provider to enter the VOD market. Their Netw Spat Econ (2008) 8:1–2 DOI 10.1007/s11067-007-9039-2

Protection Schemes for IP-over-WDM Networks: Throughput and Recovery Time Comparison

This article presents a novel protection approach using Generalized Multi-Protocol Label Switching (GMPLS). This strategy provides protection at the Wavelength Division Multiplexing (WDM) layer, meaning that all Internet Protocol Label-Switched Path (IP LSPs) nested inside a lightpath are protected in an aggregated way. It uses resources efficiently since spare capacity of working primary lightpaths can be used for backup purposes whenever necessary. The IP and WDM layers are treated together as a single integrated network from a control plane point of view, so that network state information from both layers can be used. Besides discussing the strategy proposed and the key features of GMPLS that will allow its implementation, we mathematically formulate the maximum throughput problem. Thereafter, we propose and compare heuristic algorithms for IP-over-WDM networks using three protection approaches: WDM lightpath protection, IP LSP protection, and the proposed protection scheme. Their throughputs and recovery times are analyzed and compared. Our results show that, for a representative mesh network, the proposed aggregated protection scheme presents better protection efficiency and good scalability properties when compared with the other two schemes.

Dynamic reconfiguration and routing algorithms for IP-over-WDM networks with stochastic traffic

We develop algorithms for joint IP-layer routing and WDM logical topology reconfiguration in IP-over-WDM networks experiencing stochastic traffic. At the wavelength division multiplexing (WDM) layer, we associate a nonnegligible overhead with WDM reconfiguration, during which time tuned transceivers cannot service backlogged data. The Internet Protocol (IP) layer is modeled as a queueing system. We demonstrate that the proposed algorithms achieve asymptotic throughput optimality by using frame-based maximum weight scheduling decisions. We study both fixed and variable frame durations. In addition to dynamically triggering WDM reconfiguration, our algorithms specify precisely how to route packets over the IP layer during the phases in which the WDM layer remains fixed. We demonstrate that optical-layer constraints do not affect the results, and provide an analysis of the specific case of WDM networks with multiple ports per node. In order to gauge the delay properties of our algorithms, we conduct a simulation study and demonstrate an important tradeoff between WDM reconfiguration and IP-layer routing. We find that multihop routing is extremely beneficial at low-throughput levels, while single-hop routing achieves improved delay at high-throughput levels. For a simple access network, we demonstrate through simulation the benefit of employing multihop IP-layer routes.

A Lagrangian-Relaxation Based Network Profit Optimization For Mesh SONET-Over-WDM Networks

In Wavelength Division Multiplexing (WDM) networks, the huge capacity of wavelength channels is generally much larger than the bandwidth requirement of individual traffic streams from network users. Traffic grooming techniques aggregate low-bandwidth traffic streams onto high-bandwidth wavelength channels. In this paper, we study the optimization problem of grooming the static traffic in mesh Synchronous Optical Network (SONET) over WDM networks. The problem is formulated as a constrained integer linear programming problem and an innovative optimization objective is developed as network profit optimization. The routing cost in the SONET and WDM layers as well as the revenue generated by accepting SONET traffic demands are modelled. Through the optimization process, SONET traffic demands will be selectively accepted based on the profit (i.e., the excess of revenue over network cost) they generate. Consiering the complexity of the network optimization problem, a decomposition approach using Lagrangian relaxation is proposed. The overall relaxed dual problem is decomposed into routing and wavelength assignment and SONET traffic routing sub-problems. The subgradient approach is used to optimize the derived dual function by updating the Lagrange multipliers. To generate a feasible network routing scheme, a heuristic algorithm is proposed based on the dual solution. A systematic approach to obtain theoretical performance bounds is presented for an arbitrary topology mesh network. This is the first time that such theoretical performance bounds are obtained for SONET traffic grooming in mesh topology networks. The optimization results of sample networks indicate that the roposed algorithm achieves good sub-optimal solutions. Finally, the influence of various network parameters is studied.

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.

Framework for dynamic optical virtual private networks (VPNs): architecture and analysis

The authors explore the problem of dynamic virtual private networks (VPNs) over wavelength division multiplexing (WDM) networks to facilitate the guarantee of diverse quality of service (QoS) requirements of different traffic streams, especially those with very stringent delay jitter requirements and with bursty characteristics. Each VPN consists of different traffic streams with various quality sensitivities. The proposed QoS model provides three types of QoS services for various VPNs: static, optical burst switching (OBS) and alternative VPNs. All the three types provide transmission in the optical core networks without any O/E/O conversion. The static type has a set of dedicated or shared lightpaths which have been pre-allocated and the traffic from this VPN has a constant bit rate (CBR). The OBS type need to setup the lightpath using so-called optical burst switching while a traffic stream with variable bit rate (VBR) request, is occurring. The alternative type has to set up a lightpath based on two-way reservation protocols such as tell-and-wait (TAW) for the traffic with unspecified bit rate (UBR) or available bit rate (ABR). The proposed model is suitable for carrying the multimedia applications which have various sensitivities to delay and jitter. By employing dynamic VPNs directly over the WDM layer, one can eliminate the often complex QoS related functions in the upper protocol layers, and thus increase the overall communication efficiency. Furthermore, this could be the first step towards providing fine grained service over optical wide area networks. In addition, basic performance analysis and preliminary results are presented, based on simulation, and the network designer is provided with a quantitative assessment of proportion for different traffic types with different QoS service requirements.

Traffic engineering in next-generation optical Networks

In this article traffic-engineering issues regarding network survivability, traffic grooming, impairment-aware routing, virtual-topology engineering, and coordination among multiple layers of network architecture will be reviewed for next-generation optical networks based on wavelength-division multiplexing (WDM). Due to the recent progress and development of WDM technology, increasing traffic demands can be readily accommodated in the next-generation optical networks. In spite of the huge amount of capacity (e.g., OC-192) provided by a WDM channel, enhanced network services and network performance improvement can only be achieved with efficient traffic-engineering mechanisms. The fault-tolerant function is essential in order to provide seamless services to users by protecting their traffic against failures in the optical network because many connections can be carried on a fiber. Because the capacity of a WDM channel is very large, its bandwidth may not be efficiently utilized by a single connection. Hence, low-rate user connections need to be efficiently aggregated through the traffic-grooming scheme. An intelligent routing algorithm is especially necessary in the optical network where signal impairments due to device imperfections might degrade the signal quality. In addition, the virtual network connectivity (topology) should be flexibly maintained such that dynamic changes to the traffic demands can be easily absorbed, which can be implemented by the virtual-topology engineering method in a WDM network. As the dominant usage of Internet protocol (IP) of the Internet is expected to reside directly above the WDM layer in the future network, the coordinated traffic-engineering scheme should be deliberately designed for the multi-layer network by judiciously choosing where to put many overlapping functions in the different network layers.

Strategy for protection and restoration of optical paths in WDM backbone networks for next-generation Internet infrastructures

This paper proposes and analyzes a strategy for protection and restoration of optical paths in wavelength division multiplexing (WDM) networks for next-generation Internet infrastructure. Assuming a network model in which a multiprotocol label switching (MPLS) layer is overlaid on top of a WDM layer, and a segregation of the traffic on a wavelength basis, the strategy is based on a network dimensioning aiming at: 1) the support of different types of traffic, relating to different service categories; 2) the guarantee that all the wavelength paths carrying mission-critical services (with stringent quality of service requirements) are protected against failure; and 3) the possibility of restoring a large amount of wavelength paths carrying Internet best-effort services, i.e., low-priority (LP) preemptible traffic with no QoS requirements. The reference network scenario is an overlay model in which the optical network interacts with an MPLS network with a separated control plane; the optical network manages its resources to better serve the traffic coming from the MPLS network. Two path protection schemes, namely, disjoint path (DP) and single-link basis (SLB), and a link protection mechanism, namely, local repair (LP), are investigated. Furthermore, the paper considers both the case of using wavelength conversion in the optical nodes and the case in which wavelength conversion is not used at all. The analysis compares the performance of the different strategies in terms of the percentage of optical paths carrying LP traffic not served by the optical network after a failure and of the dimensions of the optical nodes for all the schemes. The analysis reveals that the proposed approach allows the restoration of a large amount of best-effort traffic with a reasonable increase of network redundancy.

Fault management in IP-over-WDM networks: WDM protection versus IP restoration

We consider an IP-over-WDM network in which network nodes employ optical crossconnects and IP routers. Nodes are connected by fibers to form a mesh topology. Any two IP routers in this network can be connected together by an all-optical wavelength-division multiplexing (WDM) channel, called a lightpath, and the collection of lightpaths that are set up form a virtual topology. In this paper, we concentrate on single fiber failures, since they are the predominant form of failures in optical networks. Since each lightpath is expected to operate at a rate of few gigabits per second, a fiber failure can cause a significant loss of bandwidth and revenue. Thus, the network designer must provide a fault-management technique that combats fiber failures. We consider two fault-management techniques in an IP-over-WDM network: (1) provide protection at the WDM layer (i.e., set up a backup lightpath for every primary lightpath) or (2) provide restoration at the IP layer (i.e., overprovision the network so that after a fiber failure, the network should still be able to carry all the traffic it was carrying before the fiber failure). We formulate these fault-management problems mathematically, develop heuristics to find efficient solutions in typical networks, and analyze their characteristics (e.g., maximum guaranteed network capacity in the event of a fiber failure and the recovery time) relative to each other.

Efficient routing and wavelength assignment for multicast in WDM networks

The next generation multimedia applications such as video conferencing and HDTV have raised tremendous challenges on the network design, both in bandwidth and service. As wavelength-division-multiplexing (WDM) networks have emerged as a promising candidate for future networks with large bandwidth, supporting efficient multicast in WDM networks becomes eminent. Different from the IP layer, the cost of multicast at the WDM layer involves not only bandwidth (wavelength) cost, but also wavelength conversion cost and light splitting cost. It is well known that the optimal multicast problem in WDM networks is NP-hard. In this paper, we develop an efficient approximation algorithm consisting of two separate but integrated steps: multicast routing and wavelength assignment. We prove that the problem of optimal wavelength assignment on a multicast tree is not NP-hard; in fact, an optimal wavelength assignment algorithm with complexity of O(NW) is presented. Simulation results have revealed that the optimal wavelength assignment beats greedy algorithms by a large margin in networks using many wavelengths on each link such as dense wavelength-division-multiplexing (DWDM) networks. Our proposed heuristic multicast routing algorithm takes into account both the cost of using wavelength on links and the cost of wavelength conversion. The resulting multicast tree is derived from the optimal lightpaths used for unicast.

Constrained multicast routing in WDM networks with sparse light splitting

As wavelength division multiplexing (WDM) technology matures and multicast applications become increasingly popular, supporting multicast at the WDM layer becomes an important and yet challenging topic. In this paper, we study constrained multicast routing in WDM networks with sparse light splitting, i.e., where some switches are incapable of splitting light (of copying data in the optical domain) due to evolutional and/or economical reasons. Specifically, we propose four WDM multicast routing algorithms, namely, re-route-to-source, re-route-to-any, member-first, and member-only. Given the network topology, multicast membership information, and light splitting capability of the switches, these algorithms construct a source-based multicast "light-forest" (consisting one or more multicast trees) for each multicast session. While the first two algorithms can build on a multicast tree constructed by IP (which does not take into consideration the splitting capability of the WDM switches), the last two algorithms attempt to address the joint problem of optimal multicast routing and sparse splitting in WDM networks. The performance of these algorithms are compared in terms of the average number of wavelengths used per forest (or multicast session), average number of branches involved (bandwidth) per forest as well as average number of hops encountered (delay) from a multicast source to a multicast member. The results obtained from this research should present new and exciting opportunities for further theoretical as well as experimental work.

Fault Localization at the WDM Layer

A single failure in a communication network may trigger many alarms. When the communication network uses optical fibres as transmission medium and increases its capacity by using Wavelength Division Multiplexing (WDM), the number of alarms and the difficulty to locate the failure are considerably higher. In this case, a single failure may interrupt several channels which causes a large information loss. We propose an Alarm Filtering Algorithm (AFA) for the fault management of an optical network that supports multiple failures and works in the presence of passive elements, that is, network elements which may fail but never generate an alarm (e.g. optical fibres). The algorithm provides a list of components whose failure explain the observed alarms. It avoids the use of failure probabilities, which are difficult to estimate, and does not need a global knowledge of the network topology. Moreover it also tolerates alarm losses and false alarms. The algorithm is tailored to the specific behaviour of the hardware components of an optical network when a failure occurs. The classification of the network components according to the alarm signals they generate enables a formalisation of the alarm-filtering problem and results in an efficient algorithm for localising the failure(s). This algorithm is applied to the WDM rings of the COBNET network \cite{cobnet} (COBNET is a European ACTS project) and to a meshed optical network with the ARPA2 topology. Keywords: Alarm filtering, optical communication networks, fault diagnosis