The interface between IP and WDM and its effect on the cost of survivability

A summary of research on survivable IP networks overlaid over WDM networks is presented. The WDM networks are part of optical transport service providers, who lease lightpath services to institutions with IP networks. The lightpath services realize IP links for IP networks, and they have different protection grades such as unprotected and protected. The research included considering new network survivability requirements and incorporating them into network design problems. The cost of survivable IP over WDM networks is compared over three scenarios. Each succeeding scenario has the WDM network provide more flexible services, and the IP and WDM networks become more integrated. We consider the problem of setting up lightpaths for an IP network so that the network will remain connected after a fiber link fault. Algorithms to find the lightpaths and minimize cost are given. The network costs under the three scenarios are compared by simulations.     

Logical topology design for IP rerouting: ASONs versus static OTNs

IP-based backbone networks are gradually moving to a network model consisting of high-speed routers that are flexibly interconnected by a mesh of light paths set up by an optical transport network that consists of wavelength division multiplexing (WDM) links and optical cross-connects. In such a model, the generalized MPLS protocol suite could provide the IP centric control plane component that will be used to deliver rapid and dynamic circuit provisioning of end-to-end optical light paths between the routers. This is called an automatic switched optical (transport) network (ASON). An ASON enables reconfiguration of the logical IP topology by setting up and tearing down light paths. This allows to up- or downgrade link capacities during a router failure to the capacities needed by the new routing of the affected traffic. Such survivability against (single) IP router failures is cost-effective, as capacity to the IP layer can be provided flexibly when necessary. We present and investigate a logical topology optimization problem that minimizes the total amount or cost of the needed resources (interfaces, wavelengths, WDM line-systems, amplifiers, etc.) in both the IP and the optical layer. A novel optimization aspect in this problem is the possibility, as a result of the ASON, to reuse the physical resources (like interface cards and WDM line-systems) over the different network states (the failure-free and all the router failure scenarios). We devised a simple optimization strategy to investigate the cost of the ASON approach and compare it with other schemes that survive single router failures.     

Performance and testbed study of topology reconfiguration in IP over optical networks

With the widespread deployment of Internet protocol/wavelength division multiplexing (IP/WDM) networks, it becomes necessary to develop traffic engineering (TE) solutions that can effectively exploit WDM reconfigurability. More importantly, experimental work on reconfiguring lightpath topology over testbed IP/WDM networks is needed urgently to push the technology forward to operational networks. This paper presents a performance and testbed study of topology reconfiguration for IP/WDM networks. IP/WDM TE can be fulfilled in two fashions, overlay vs. integrated, which drives the network control software, e.g., routing and signaling protocols, and selects the corresponding network architecture model, e.g., overlay or peer-to-peer. We present a traffic management framework for IP over reconfigurable WDM networks. Three "one-hop traffic maximization"-oriented heuristic algorithms for lightpath topology design are introduced. A reconfiguration migration algorithm to minimize network impact is presented. To verify the performance of the topology design algorithms, we have conducted extensive simulation study. The simulation results show that the topologies designed by the reconfiguration algorithms outperform the fixed topology with throughput gain as well as average hop-distance reduction. We describe the testbed network and software architecture developed in the Defense Advanced Research Projects Agency (DARPA) Next Generation Internet (NGI) SuperNet Network Control and Management project and report the TE experiments conducted over the testbed.     

Survivable Routing of Mesh Topologies in IP-over-WDM Networks by Recursive Graph Contraction

Failure restoration at the IP layer in IP-over-WDM networks requires to map the IP topology on the WDM topology in such a way that a failure at the WDM layer leaves the IP topology connected. Such a mapping is called survivable. Finding a survivable mapping is known to be NP-complete, making it impossible in practice to assess the existence or absence of such a mapping for large networks, (i) we first introduce a new concept of piecewise survivability, which makes the problem much easier in practice (although still NP-complete), and allows us to formally prove that a given survivable mapping does or does not exist, (ii) secondly, we show how to trace the vulnerable areas in the topology, and how to strengthen them to enable a survivable mapping, (iii) thirdly, we give an efficient and scalable algorithm that finds a survivable mapping. In contrast to the heuristics proposed in the literature to date, our algorithm exhibits a number of provable properties (e.g., it guarantees the piecewise survivability) that are crucial for (i) and (ii)     

在IP/MPLS over WDM网络中基于准入机制的区分服务

在IP over WDM网络中,光层可以通过建立新的光路来为IP层提供带宽.如何利用光层有限的资源使网络服务提供商的利润最大化是流量疏导的一个重要目标.文章基于迭加网络模型,研究IP/MPLS over WDM网络的准入机制,设计算法在不影响整体效率的前提下为网络提供区分服务.     

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 virtualtopology 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 trafficengineering scheme should be deliberately designed for the multi-layer network by judiciously choosing where to put many overlapping functions in the different network layers.     

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.     

Influence of GMPLS Recovery Mechanisms on TCP Performance

Optical networks based on wavelength-division-multiplexing (WDM) techniques are very likely to be omnipresent in future telecommunication networks. Those networks are deployed in order to face the steady growth of traffic, which is for a large part Internet related. In the resulting IP-over-WDM scenario, TCP/IP constitutes an important fraction of the traffic transported over these networks. As IP networks are becoming increasingly mission-critical, it is of the utmost importance that these networks (and hence the supporting transport networks) be able to recover quickly from failures such as cable breaks or equipment outages. To that end, several IP-over-WDM network scenarios and corresponding protection and restoration strategies have been devised. It is clear that some trade-offs will have to be made in order to choose an appropriate strategy. In this paper, we investigate the effects of such recovery actions on the behavior of TCP, being the ubiquitous protocol used by today's network users. We examine the influence of different parameters such as the speed of recovery actions, changing length of the routes followed by the client data (TCP flows), changes in available bandwidth, etc. Thereby, we focus on what the TCP end-users care about, i.e., the number of bytes transported end-to-end within a certain time interval.     

On the Maximum Protection Problem in IP-over-WDM Networks Using IP LSP Protection

In dynamic IP-over-WDM networks efficient fault-management techniques become more difficult since as demands change with time the optimal logical topology varies as well. Changes in the virtual topology should be done with care because working IP LSPs routed on top of a virtual topology should not be interrupted. Reconfiguration of the virtual topology may also affect precomputed backup IP LSPs to be activated in case of failure meaning that backup IP LSPs would need to be recomputed after any change in the virtual topology. A good sense solution can be the dimensioning of the virtual topology for a worst case traffic scenario, having as goal the minimization of the network cost, for example, and then route dynamic IP LSPs on this virtual topology. The virtual topology would remain unchanged as long as possible, that is, until changes in the virtual topology are considered to bring considerable benefits. Since data services over IP are essentially of a best-effort nature, protection could be provided, using IP LSP protection, only when bandwidth is available in the virtual topology. The computation of backup IP LSPs does not interfere with working IP LSPs meaning that no service interruption will exist. Such a strategy, considered in this paper, allows resources to be used efficiently, since free bandwidth is used for backup purposes, while the normal delivery of traffic is guaranteed in peak traffic situations although having no protection guarantees. Our main objective is to quantify the spare capacity, which can be used for restoration (backup) purposes, over a virtual topology designed and optimized to carry a traffic scenario with no survivability and QoS requirements. We analyse the maximum protection (MP) problem in such IP-over-WDM network environment. Protection is provided to IP LSP requests whenever possible through bandwidth reservation in a backup IP LSP on the virtual topology. Besides the mathematical formalization of the MP problem, an upper bound and heuristic algorithms are proposed and evaluated. The traffic considered includes IP LSPs of different granularities and is the worst case traffic scenario for which the network should be dimensioned.     

Lightpath routing for intelligent optical networks

An overview of current issues and challenges in lightpath routing for optical networks is given. An architecture is presented in which optical switches are deployed, usually in the core, to interconnect IP routers at the edges. Lightpath routing within this architecture follows the framework of generalized multiprotocol label switching. Our discussion pays particular attention to the aspects of optical routing that differ from routing in irrational IP networks. Such aspects include physical layer constraints, wavelength continuity, the decoupling of the control network topology from the data network topology, explicit routing with wavelength assignment, and diversity routing for fast protection. We also present an algorithmic framework for lightpath computation, highlighting the issue of wavelength continuity and the differences between lightpath computation and traditional IP route computation     

On the physical and logical topology design of large-scale optical networks

We consider the problem of designing a network of optical cross-connects (OXCs) to provide end-to-end lightpath services to large numbers of label switched routers (LSRs). We present a set of heuristic algorithms to address the combined problem of physical topology design (i.e., determine the number of OXCs required and the fiber links among them) and logical topology design (i.e., determine the routing and wavelength assignment for the lightpaths among the LSRs). Unlike previous studies which were limited to small topologies with a handful of nodes and a few tens of lightpaths, we have applied our algorithms to networks with hundreds or thousands of LSRs and with a number of lightpaths that is an order of magnitude larger than the number of LSRs. In order to characterize the performance of our algorithms, we have developed lower bounds which can be computed efficiently. We present numerical results for up to 1000 LSRs and for a wide range of system parameters such as the number of wavelengths per fiber, the number of transceivers per LSR, and the number of ports per OXC. The results indicate that it is possible to build large-scale optical networks with rich connectivity in a cost-effective manner, using relatively few but properly dimensioned OXCs.     

Design of Logical Topology with Effective Waveband Usage in IP-over-WDM Networks

We deal with the problem of designing the logical topology in IP-over-WDM networks. Many conventional methods for designing the logical topology assume that a constant number of wavelengths will be available on each fiber. But it is not necessary to utilize all wavelengths on each fiber in building an effective logical topology on a WDM network. Instead, several wave-bands may be considered for introduction while deploying additional wave-bands and their corresponding optical amplifiers when additional wavelengths are actually required. In this case, the number of wavelengths available on the respective fibers depends on the number of optical fiber amplifiers deployed on each fiber. In this paper, we propose a heuristic algorithm for the design of a logical topology with as few optical fiber amplifiers as possible. Our results indicate that our algorithm reduces the number of optical fiber amplifiers with a slight increase of average packet delays.     

Comparative analysis of the traffic performance of fiber-impairment limited WDM and hybrid OCDM/WDM networks

Optical code-division multiplexing (OCDM) is a technique that is currently generating considerable research interest. This paper analyzes and compares the traffic performance of wavelength-division multiplexing (WDM) and hybrid OCDM/WDM-based optical networks. The analysis considers the influence of the limitations of fiber-induced signal impairments on traffic performance and comparisons are performed for an example network utilizing different standardized fiber types. Furthermore, comparisons of traffic performance are also made between different lightpath schemes used in WDM and OCDM/WDM networks. The analysis results show that the OCDM/WDM lightpath schemes significantly outperform the WDM lightpath schemes for given blocking probability criteria. Moreover, the analysis indicates that fiber nonlinearity (which limits the minimum channel spacing) affects the traffic performance more severely compared to fiber dispersion (limits code cardinality).     

Passive optical network architecture based on waveguide gratingrouting

We explore an optical network architecture which employs dense wavelength division multiplexing (WDM) technology and passive waveguide grating routers (WGRs) to establish a virtual topology based on lightpath communication. We examine the motivation and the technical challenges involved in this approach, propose and examine the characteristics of a network design algorithm, and provide some illustrative performance results     

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.     

Effects of signal power control strategies and wavelength assignment algorithms on circuit OSNR in WDM networks

Software-defined networking is enabling wavelength-division multiplexed (WDM) networks to be programmable down to individual components. While taking into account typical gain and noise figure profiles of erbium-doped fiber amplifier (EDFA) components, the authors consider a number of signal power control strategies and compare their performance in terms of achievable lightpath optical signal-to-noise ratio (OSNR). These strategies are applied network-wide to concurrently control the gain of each individual amplifier and the signal power equalization at each reconfigurable optical add/drop multiplexer. Simulation and (in part) experimental results show that the lightpath OSNR is affected by three factors: the EDFA gain control strategy, power equalization strategy and wavelength assignment (WA) algorithm. A trade-off between lightpath average OSNR and OSNR variance across the WDM channels is also noted. Experimental work is conducted using a five-node meshed WDM network testbed proving both feasibility and effectiveness of a coordinated use of signal power control strategies and WA algorithms.     

QoS aware traffic grooming and integrated routing on IP over WDM networks

In this article, we consider traffic grooming and integrated routing in IP over WDM networks. The challenges of this problem come from jointly considering traffic grooming, IP routing, and lightpath routing and wavelength assignment (RWA). Due to the high bandwidth of optical fiber, there exists a mismatch between the capacity needed by an IP flow and that provided by a single lightpath. Traffic grooming is therefore used to increase the network utilization by aggregating multiple IP flows in a single lightpath. However, traffic grooming incurs additional delays that might violate Quality-of-Service (QoS) requirements of IP users. In this work, the tradeoff between traffic grooming and IP QoS routing is well-formulated as a mixed integer and linear optimization problem, in which the revenue from successfully provisioning IP paths is to be maximized. Problem constraints include IP QoS, routing, optical RWA, and the WDM network capacity. We propose a novel Lagrangean relaxation (LGR) algorithm to perform constraint relaxation and derive a set of subproblems. The Lagrangean multipliers are used in the proposed algorithm to obtain a solution in consideration of grooming advantage and resource constraints simultaneously. Through numerical experiments and comparisons between the proposed algorithm and a two-phase approach, LGR outperforms the two-phase approach under all experimental cases. In particular, the improvement ratio becomes even more significant when the ratio of IP flow to the wavelength capacity is smaller.     

Some principles for designing a wide-area WDM optical network

We explore design principles for next-generation optical wide-area networks, employing wavelength-division multiplexing (WDM) and targeted to nationwide coverage. This optical network exploits wavelength multiplexers and optical switches in routing nodes, so that an arbitrary virtual topology may be embedded on a given physical fiber network. The virtual topology, which is used as a packet-switched network and which consists of a set of all-optical “lightpaths”, is set up to exploit the relative strengths of both optics and electronics-viz. packets of information are carried by the virtual topology “as far as possible” in the optical domain, but packet forwarding from lightpath to lightpath is performed via electronic switching, whenever required. We formulate the virtual topology design problem as an optimization problem with one of two possible objective functions: (1) for a given traffic matrix, minimize the network-wide average packet delay (corresponding to a solution for present traffic demands), or (2) maximize the scale factor by which the traffic matrix can be scaled up (to provide the maximum capacity upgrade for future traffic demands). Since simpler versions of this problem have been shown to be NP-hard, we resort to heuristic approaches. Specifically, we employ an iterative approach which combines “simulated annealing” (to search for a good virtual topology) and “flow deviation” (to optimally route the traffic-and possibly bifurcate its components-on the virtual topology). We do not consider the number of available wavelengths to be a constraint, i.e., we ignore the routing of lightpaths and wavelength assignment for these lightpaths. We illustrate our approaches by employing experimental traffic statistics collected from NSFNET     

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.     

Performance analysis of multicast routing and wavelength assignment protocol with dynamic traffic grooming in WDM networks

The need for on‐demand provisioning of wavelength‐routed channels with service‐differentiated offerings within the transport layer has become more essential because of the recent emergence of high bit rate Internet protocol (IP) network applications. Diverse optical transport network architectures have been proposed to achieve the above requirements. This approach is determined by fundamental advances in wavelength division multiplexing (WDM) technologies. Because of the availability of ultra long‐reach transport and all‐optical switching, the deployment of all‐optical networks has been made possible. The concurrent transmission of multiple streams of data with the assistance of special properties of fiber optics is called WDM. The WDM network provides the capability of transferring huge amounts of data at high speeds by the users over large distances. There are several network applications that require the support of QoS multicast, such as multimedia conferencing systems, video‐on‐demand systems, real‐time control systems, etc. In a WDM network, the route decision and wavelength assignment of lightpath connections are based mainly on the routing and wavelength assignment (RWA). The multicast RWA's task is to maximize the number of multicast groups admitted or minimize the call‐blocking probability. The dynamic traffic‐grooming problem in wavelength‐routed networks is generally a two‐layered routing problem in which traffic connections are routed over lightpaths in the virtual topology layer and lightpaths are routed over physical links in the physical topology layer. In this paper, a multicast RWA protocol for capacity improvement in WDM networks is designed. In the wavelength assignment technique, paths from the source node to each of the destination nodes and the potential paths are divided into fragments by the junction nodes and these junction nodes have the wavelength conversion capability. By using the concept of fragmentation and grouping, the proposed scheme can be generally applied for the wavelength assignment of multicast in WDM networks. An optimized dynamic traffic grooming algorithm is also developed to address the traffic grooming problem in mesh networks in the multicast scenario for maximizing the resource utilization and minimizing the blocking probability. Copyright © 2011 John Wiley & Sons, Ltd.