Number Of Lightpaths Research Articles

Multi-wavelength transponders for high-capacity optical networks: a physical-layer-aware network planning study

Continued cost- and power-efficient capacity scaling in optical networks is imperative to keep pace with ever-increasing traffic demands. In this paper, we investigate multi-wavelength transponders as a potential way forward. Suitable system architectures and realistic specifications of multi-wavelength transponders are identified and analyzed in terms of transmit optical signal-to-noise ratio penalties and spectral constraints. We investigate the performance for different specifications as compared to single-wavelength transponders in a network planning study on two network topologies, developing guidelines for multi-wavelength transponder specifications and their potential benefits. The studies show a reduction in the number of required lasers of up to 83% at the expense of a slight increase in the number of lightpaths, demonstrating the potential for significant cost savings and efficiency improvements.

Energy-efficient, EDFA lifetime-aware network planning along with virtualized elastic regenerator placement for IP-over-EON

In this paper, we focus on energy-efficient network planning (including traffic provisioning) along with optimal placement of virtualized elastic regenerators (VERs) for IP-over-elastic optical networks based on a static traffic profile, using a mixed integer linear programming-based optimization model. The proposed model judiciously exploits flexibility of IP core routers, sliceable bandwidth variable transponders (SBVTs) and VERs to accommodate the traffic demands with the minimum power consumption (PC). Optical layer traffic grooming allows to simultaneously originate/terminate multiple lightpaths of different capacities, data slots and maximum transparent reach by a single SBVT. The proposed model also allows to use all functionalities of VER, such as simultaneous regeneration, distance-adaptive transmission option selection, frequency slot merging to be used concurrently for the given static traffic profile. In addition, the proposed model includes lifetime awareness of Erbium-doped fiber amplifier (EDFA) to reduce EDFA failure and associated repairing cost in long run. Using the proposed model, we enhance the average EDFA lifetime by restricting average EDFA occupancy, represented by the ratio of the (average) number of lightpaths being amplified in an EDFA and the maximum possible number of lightpaths that can be amplified in it, even though in the process, the overall PC in network may increase, with reference to the scenario with no EDFA occupancy restriction. The variation in PC and average EDFA lifetime for different permissible (user-defined) average EDFA occupancy are studied. We exhaustively study performance of the model under different network conditions.

Exploiting Dual Homing in Access Networks to Improve Resiliency in Core Networks

We propose a network architecture consisting of long-reach passive optical network-based access and a transparent optical core network. The end users are connected to the remote node or the local exchange (LE) through an optical distribution network, and the remote node is connected by disjoint feeder fiber links to central offices located in two distinct metro/core (MC) nodes. This method of connecting a single remote node to two geographically separate MC nodes for dedicated protection in case of feeder fiber failure is referred to as dual homing. In this work, we explore the benefits of dual homing in the access in simultaneously providing better resilience and load balancing in the core network considering connections between LEs. While looking into the benefits of dual homing in terms of network resiliency, we also explore whether the path redundancies added by dual homing play a role in providing efficient distribution of load across the core network and thereby reduce the cost of provisioning capacity in terms of number of lightpaths, transponders, etc. Dual homing at both the source and destination LEs offers more options for paths between LEs through the core network. Our results show that dual-homed access proves to be advantageous over single-homed access in terms of enhancing both core network resiliency and facilitating better load balancing.

Modeling impact of ROADM intra-node Add/Drop contention on a single node lightpath blocking performance

We propose an analytical model to evaluate the lightpath blocking performance for a single ROADM node with intra-node add/drop contention, in which the number of lightpaths that can be added/dropped with the same wavelength is limited by the add/ drop contention factor. Different models of traffic load per nodal degree are considered to validate the effectiveness of the analytical model. The simulation results show that the proposed analytical model is effective in predicting the performance for different values of add/drop contention factor C and for variable offered loads at the node. The add/drop contention factor shows an important impact on the lightpath blocking performance and properly raising the contention factor can significantly improve the lightpath blocking performance. When the add/drop contention factor C exceeds a certain level, the performance of a ROADM with intra-node contention is close to that of a contentionless ROADM.

Add/Drop Contention-Aware RWA With Directionless ROADMs: The Offline Lightpath Restoration Case

Directionless reconfigurable add/drop multi plexers (ROADMs) permit changing the direction of an added/dropped lightpath without manual intervention. If the ROADM is also colorless, its wavelength can be automatically reconfigured. In broadcast-and-select directionless ROADM architectures, the number of lightpaths that can be added/dropped using the same wavelength is limited by the so-called add/drop contention factor C. This is a source of lightpath blocking that reduces the network capacity. A previous work investigated this aspect for the case of unprotected and 1+1 protected lightpaths. In this paper, we address the case in which lightpath restoration is the fault recovery technique in the network. We hypothesize that, using appropriate network planning, the add/drop contention effects can be mitigated. For this, we propose and investigate the add/drop contention-aware routing and wavelength assignment with lightpath restoration offline planning problem, considering colored ROADMs. To solve the problem, an effective heuristic is presented. Extensive results are reported for four reference topologies. In our tests, the reduction in the network capacity caused by add/drop contention is eliminated in practice for factors C=2, or even C=1 in some topologies. These results contribute to the dimensioning of directionless ROADMs.

Stable Routing With Virtual Topology Capacity Adjustment: A Novel Paradigm for Operating Optical Networks

Advances in optical equipment permit network carriers to offer lightpath-on-demand services to Internet service providers (ISPs). These services are the key for constructing cost-efficient virtual topology capacity adjustment (VTCA) schemes, where ISPs dynamically adapt the number of lightpaths contracted between their IP routers, according to time-varying traffic volumes. An existing technique called lightpath bundling offers ISPs the possibility of grouping lightpaths between the same pair of routers into so-called bundles, perceived by the IP layer as single virtual links of aggregated capacity. Consequently, new lightpaths added or removed from bundles are not seen by the IP layer as new links necessary to advertise, but simply as capacity adjustments of already existing links. Adding lightpath bundling to the picture opens the door for developing VTCA schemes which maintain stability in the IP routing tables, a major requirement for ISPs. In this paper, we present and evaluate the merits of the proposed stable routing VTCA (SR-VTCA) paradigm and present algorithms for designing such a scheme. The results clearly show that SR-VTCA gives an advantageous trade-off between the fully static network with no capacity adjustment and the fully dynamic VTCA scheme where both the IP routing and the VT are reconfigured over time.

Clustering Methods for Hierarchical Traffic Grooming in Large-Scale Mesh WDM Networks

We consider a hierarchical approach for traffic grooming in large multiwavelength networks of a general topology. Inspired by similar concepts in the airline industry, we decompose the network into clusters, and select a hub node in each cluster to groom traffic originating and terminating locally. At the second level of the hierarchy, the hub nodes form a virtual cluster for the purpose of grooming intra-cluster traffic. Clustering and hierarchical grooming enables us to cope with large network sizes and facilitates the control and management of traffic and network resources. Yet, determining the size and composition of clusters so as to yield good grooming solutions is a challenging task. We identify the grooming-specific factors affecting the selection of clusters, and we develop a parameterized clustering algorithm that can achieve a desired trade-off among various goals. We also obtain lower bounds on two important objectives in traffic grooming: the number of lightpaths and wavelengths needed to carry the subwavelength traffic. We demonstrate the effectiveness of clustering and hierarchical grooming by presenting the results of experiments on two network topologies that are substantially larger than those considered in previous traffic grooming studies.

Analysis of multi-hop traffic grooming in WDM mesh networks

Traffic grooming is an essential functionality of WDM optical networks to provision multi-granularity subwavelength connections. Depending on the number of lightpaths allowed in a connection route, traffic grooming can be classified as single-hop traffic grooming (SH-TG) and multi-hop traffic grooming (MH-TG). MH-TG is more general and resource-efficient than SH-TG, because it allows connections from different source–destination pairs to share the bandwidth of a lightpath. In this paper, we propose a MH-TG algorithm, namely the fixed-order multi-hop (FOMH) grooming algorithm, based on the fixed-alternate routing approach. We introduce the grooming node selection (GNS) problem in MH-TG and propose three grooming policies, namely exhaustive sequential (ES), limited-hop sequential (LHS) and load sharing (LS) policies, to address the GNS problem. These policies represent different trade-offs among blocking probability, computational complexity and transceiver requirements. Given that the analysis of MH-TG is a relatively unexplored area, we propose an analytical model to evaluate the blocking performance of MH-TG using FOMH and the LS grooming policy. To address the multi-layered routing and multi-rate connection characteristics of traffic grooming, we introduce a novel multi-level decomposition approach in our analytical model which decomposes traffic at four different levels, namely alternate path, connection route, lightpath and link levels. The model also addresses various factors that affect connection blocking probability. These factors include wavelength continuity constraint, channel continuity constraint and route dependence. The Erlang fixed-point approximation method is used to solve the analytical model. Numerical results show that analytical results match well with simulation results. We also evaluate the effect of the grooming policies, the number of virtual hops (lightpaths) within a connection route and the number of alternate paths on the performance of the grooming algorithm.

On minimizing the number of ADMs in a general topology optical network

Minimizing the number of electronic switches in optical networks has been a main research topic in some recent studies. In such networks, we assign colors to a given set of lightpaths, and they are partitioned into unicolor cycles and paths; the switching cost is minimized when the number of paths is minimized. Most approximation and heuristic algorithms for this problem have a preprocessing stage, in which possible cycles are found. Among them, the basic algorithm eliminates cycles of size at most l , and has a performance guarantee of O P T + 1 2 ( 1 + ϵ ) N , where O P T is the cost of an optimal solution, N is the number of lightpaths and 0 ≤ ϵ ≤ 1 l + 2 , for any given odd l . The time complexity of the algorithm is exponential in l . We improve the analysis of this algorithm, by showing that ϵ ≤ 1 3 2 ( l + 2 ) , which implies a reduction of the exponent in the time complexity. We also improve the lower bound by showing that ϵ ≥ 1 2 l + 3 . The results shed more light on the structure of this basic algorithm. In addition, in our analysis we suggest a novel technique–including a new combinatorial lemma–to deal with this problem.

On minimizing the number of ADMs—Tight bounds for an algorithm without preprocessing

Minimizing the number of electronic switches in optical networks is a main research topic in recent studies. In such networks we assign colors to a given set of lightpaths. Thus the lightpaths are partitioned into cycles and paths. The switching cost is minimized when the number of paths is minimized. The problem of minimizing the switching cost is NP-hard, and approximation algorithms have been suggested for it. Many of these algorithms have a preprocessing stage, in which they first find cycles. The basic algorithm eliminates cycles of size at most l, and is known to have a performance guarantee of OPT + 1 2 ( 1 + ε ) N , where OPT is the cost of an optimal solution, N is the number of lightpaths, and 0 ⩽ ε ⩽ 1 l + 2 , for any given odd l. Without preprocessing phase (i.e. l = 1 ), this reduces to OPT + 2 3 N . We develop a new technique for the analysis of the upper bound and prove a tight bound of OPT + 3 5 N for the performance of this algorithm.

Dynamic multicast traffic grooming in WDM networks

Dynamic multicast traffic grooming in wavelength division multiplexing (WDM) networks was analyzed to minimize networkwide costs and to increase the network resource utilization. A network model was developed for dynamic multicast traffic grooming with resource constraints and an algorithm that can provide quality of service (QoS) was proposed. The QoS is measured by the maximum number of lightpaths passing between the source and the destinations. The blocking probability of the algorithm was assessed in simulations. The results show that a higher QoS requirement results in higher blocking probability, and when the QoS requirement is low, changes in the QoS requirements have only small effects on the blocking probability.

Ultra-reliable communication over vulnerable all-optical networks via lightpath diversity

In this paper, we propose using spatial diversity via multiple node-disjointed lightpaths at the optical layer to achieve ultra-reliable communication with low delay between any source-destination pair in all-optical networks. Using a doubly stochastic point process model and a "genie-aided" receiver, we obtain an exponentially tight error probability bound for the lightpath diversity scheme under an independent lightpath failure model. Error probability of the proposed scheme can be designed to be significantly lower than that of a system without lightpath diversity, and system parameters (e.g., the number of lightpaths) can be optimized to achieve efficient utilization of a limited amount of transmitted optical energy. In particular, at the optimum operating point, each lightpath is allocated an optimum average number of signal photons per bit and is biased to have an effective error probability 2f if the decision is based on that path alone, where f is the lightpath failure probability. We also investigate the tradeoff between the error probability and the implementation complexity within the class of all "structured" receivers. We derive receiver architectures for both the optimal receiver, which has the best error performance but complicated receiver architecture, and the equal-gain-combining (EGC) receiver, which has suboptimum error performance but simpler receiver architecture. Closed-form error bounds for both receivers are obtained and compared with the "genie-aided" limit of the lightpath diversity scheme. Performance comparison shows that the simpler equal-gain-combing receiver provides similar performance as the optimal receiver in the regime of high signal-to-noise photon rate ratio (/spl Omega/=/spl lambda//sub s//M/spl lambda//sub n/, where /spl lambda//sub s//M is the signal photon rate per path, /spl lambda//sub n/ is the noise photon rate per path), and performs slightly worse than the optimal receiver in the low and medium signal-to-noise photon rate ratio regimes. It indicates that the simpler EGC receiver is preferred over the complicated optimum receiver in practical receiver design.

A Heuristic Algorithm for Static Wavelength Assignment in WDM Optical Networks

In this paper, we present a heuristic algorithm for assigning wavelengths to static lightpath requests in WDM optical networks. Given the network and lightpath requests, the problem is to assign wavelengths to the lightpaths so as to minimize the number of wavelengths used. The problem is posed as a graph-coloring problem, which is next solved using the heuristic. The proposed heuristic starts with assigning a color to the first node of the auxiliary graph represented by an array of linked list. Subsequently, the algorithm colors other nodes by checking the color of the adjacent nodes through the node-color array and finding the first unused color from the color array. The time complexity of the heuristic is computed as O(n2), where n is the number of lightpaths to be assigned wavelengths.

Dynamic Virtual Topology Reconfiguration Algorithms for Groomed WDM Networks

In a Wavelength Division Multiplexing (WDM) optical network, in which the traffic changes dynamically, the virtual topology designed for an old traffic set needs to be reconfigured for a new demand set in order to route more connections. Though reconfiguration increases the throughput, the resulting disruption in traffic is a cause for concern. We present a simple and flexible framework to evaluate the gain achieved by reconfiguration, based on the two conflicting objectives of increasing throughput and reducing disruption. We present adaptive reconfiguration algorithms which determine the change in the virtual topology with a corresponding change in the demand set. These algorithms incrementally add lightpaths to a given virtual topology and delete a minimum number of lightpaths to facilitate their addition. One of the algorithms improves throughput by making changes to the existing virtual topology and another one reduces disruption by making changes to the virtual topology suited for the new demand set. However, in order to reduce the gap in bandwidths between what a wavelength channel can provide and what an individual connection requires, several low-speed connections need to be groomed onto a single wavelength. As our algorithms aim at increasing the throughput with as few lightpath changes as possible, more connections will be accepted without considerable increase in number of lightpaths. This means, more connections are groomed onto the lightpaths. One nice feature of our approach is that it fits not only for groomed networks where traffic demands are at the sub wavelength level, but also for networks where connection demands are at the wavelength level. The extensive simulation studies, wherein we compared the performance of our algorithms with that of two other possible schemes, demonstrated their flexibility and robustness.

A Study of Path Protection in Large-Scale Optical Networks

We consider the problem of designing a network of optical cross-connects (OXCs) which provides end-to-end lightpath services to large numbers of client nodes, under the requirement that the network will survive any single-link failure. Our main objective is to quantify the additional resource requirements of implementing path protection schemes over a network with no survivability properties. To this end, we present heuristic routing and wavelength assignment algorithms for dedicated path protection and two variants of shared path protection, and integrate them into the physical and logical topology design framework we developed in an earlier study. We apply our heuristics to networks with up to 1000 client nodes, with a number of lightpaths that is an order of magnitude greater than the number of clients, and for a wide range of values for system parameters such as the number of wavelengths per fiber, the number of optical transceivers per client node, and the number of ports per OXC. Our results provide insight into the relative resource requirements of dedicated and shared path protection schemes. We also find that, using shared path protection schemes, it is possible to build cost-effective survivable networks that provide rich connectivity among client nodes with only a modest additional amount of resources over a network with no survivability properties.

Optimal Virtual Topologies for One-To-Many Communication in WDM Paths and Rings

In this paper we examine the problem of constructing optimal virtual topologies for one-to-many communication in optical networks employing wavelength-division multiplexing. A virtual topology is a collection of optical lightpaths embedded in a physical topology. A packet sent from the source node travels over one or more lightpaths en route to its destination. Within a lightpath, transmission is entirely optical. At the terminus of a lightpath the data is converted into the electronic domain where it may be retransmitted on another lightpath toward its destination. Since the conversion of the packet from the optical to the electronic domain introduces delays and uses limited physical resources, one important objective is to find virtual topologies which minimize either the maximum or average number of lightpaths used from the source to all destination nodes. Although this problem is NP-complete in general, we show that minimizing the maximum or average number of lightpaths in path and ring topologies can be solved optimally by efficient algorithms.

Routing and wavelength assignment algorithms for translucent optical networks

Due to transmission impairments, optical regeneration is needed to extend the length of a lightpath beyond its transparent length. Spare transmitter (Tx) and receiver (Rx) modules in an optical network node can be used for optical regeneration. A Max-spare algorithm for locating the regeneration nodes for a lightpath is proposed and compared with a Greedy algorithm in conjunction with two routing algorithms, namely, wavelength weighted (WW) and length weighted (LW) routing algorithms. Through simulation we find that significant number of lightpaths that cannot be established due to the violation of transparent length can be established if the regeneration algorithm is used to select transit nodes that have spare Tx and Rx for 3R regeneration. The translucent optical network with limited number of transceivers can still perform close to a transparent optical network with full set of transceivers. We find that for the same routing algorithm, the Max-spare algorithm yields better results than the Greedy algorithm. In addition; for the same regeneration nodes selection algorithm, the WW algorithm has better performance than the LW algorithm.

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.

Lightpath arrangement in survivable rings to minimize the switching cost

This paper studies the design of low-cost survivable wavelength-division-multiplexing (WDM) networks. To achieve survivability, lightpaths are arranged as a set of rings. Arrangement in rings is also necessary to support SONET/SDH protection schemes such as 4FBLSR above the optical layer. This is expected to be the most common architecture in regional (metro) networks. We assume that we are given a set of lightpaths in an arbitrary network topology and aim at finding a partition of the lightpaths to rings adding a minimum number of lightpaths to the original set. The cost measure that we consider (number of lightpaths) reflects the switching cost of the entire network. In the case of a SONET/SDH higher layer, the number of lightpaths is equal to the number of add-drop multiplexers (ADMs) (since two subsequent lightpaths in a ring can share an ADM at the common node). We prove some negative results on the tractability and approximability of the problem and provide an approximation algorithm with a worst case approximation ratio of 8/5. We study some special cases in which the performance of the algorithm is improved. A similar problem was introduced, motivated, and studied by Liu, Li, Wan and Frieder (see Proc. INFOCOM 2000, p.1020-1025, 2000) Gerstel, Lin and Sasaki, (see Proc. IEEE INFOCOM '98, p. 94-101, 1998)(where it was termed minimum ADM problem). However, these two works focused on a ring topology while we generalize the problem to an arbitrary network topology.

Worst-case analysis of dynamic wavelength allocation in optical networks

This paper proposes algorithms for allocating wavelengths to connections (lightpaths) in optical wavelength division multiplexed networks, predominantly for ring topologies. A worst-case model is considered, where no blocking of lightpaths is allowed, and there are no assumptions made on the traffic arrival and holding times. The traffic is characterized only by its load L, which is the maximum number of lightpaths that can be present on any link, assuming no blocking. A dynamic traffic model is considered where requests to set up lightpaths arrive over time and, must be accommodated without rerouting existing lightpaths, and lightpaths may be terminated over time as well. For networks without wavelength conversion, we show that at least 0.5Llog/sub 2/N wavelengths are required by any dynamic algorithm for rings of N nodes and present an algorithm that uses at most Llog/sub 2/N+L wavelengths for rings and 2(L-1)log/sub 2/N for trees. We also study the worst-case behavior of the well-known first-fit algorithm, and show that it requires at most 2.52Llog/sub 2/N+5L wavelengths (small variants of these constants are proven as well). When limited wavelength conversion is allowed, we first show how to use expanders to insure no blocking in arbitrary topologies. Then, we present conversion patterns for rings with conversion degree d=2, which require Llog/sub 2/L+4L or 2Llog/sub 2/log/sub 2/L+4L wavelengths, thereby eliminating the dependence (that exists without wavelength conversion) between the number of wavelengths and N. We also consider different traffic models where lightpath setup requests arrive over time, but once set up, lightpaths are never taken down. For this model, the number of wavelengths needed is shown to be only max{0,L-d}+L for a conversion degree of d.