WDM optical interconnects with recirculating buffering and limited range wavelength conversion

All-optical communication, in particular, wavelength-division-multiplexing (WDM) technique, has been proposed as a promising candidate to meet the ever-increasing demands on bandwidth from emerging bandwidth-intensive computing/networking applications. However, with current technology, the cost of optical communication, especially the cost of optical buffering and wavelength conversion, remains a major concern for such applications. In this paper, we study WDM optical interconnects that utilize low cost recirculating buffering and limited range wavelength conversion. We first consider the packet scheduling problem in this type of interconnect, and formalize the problem of maximizing throughput and minimizing packet delay as a matching problem in a bipartite graph. We give an optimal parallel algorithm for this problem that runs in O(Bk/sup 2/) time, compared to O((N+B)/sup 3/k/sup 3/) time if directly applied to existing matching algorithms for general bipartite graphs, where N is the number of input/output fibers of the interconnect, B is the number of fiber delay lines, and k is the number of wavelengths. We also consider efficient switching fabric designs for this type of interconnect. We distinguish between the switching fabric connecting the input fibers to the output fibers and the switching fabric connecting the input fibers to the delay lines and show that by adopting the idea of concentration, the cost of the latter can be reduced significantly in terms of the number of crosspoints.     

Cost-effective designs of WDM optical interconnects

Optical communication, in particular, wavelength-division-multiplexing (WDM) technique, has become a promising networking choice to meet ever-increasing demands on bandwidth from emerging bandwidth-intensive computing/communication applications, such as data browsing in the World Wide Web, multimedia conferencing, e-commerce, and video-on-demand services. As optics becomes a major networking media in all communications needs, optical interconnects will inevitably play an important role in interconnecting processors in parallel and distributed computing systems. We consider a cost-effective design of WDM optical interconnects for current and future generation parallel and distributed computing and communication systems. We first categorize WDM optical interconnects into two different connection models based on their target applications: the wavelength-based model and the fiber-link-based model. Most of existing WDM optical interconnects belong to the first category. We then present a minimum cost design for WDM optical interconnects under wavelength-based model by using sparse crossbar switches instead of full crossbar switches in combination with wavelength converters. For applications that use the fiber-link-based model, we show that network cost can be significantly reduced, and present such a minimum cost design for WDM optical interconnects under this model. Finally, we generalize the idea used in the design for the fiber-link-based model to WDM optical interconnects under the wavelength-based model, and obtain another new design that can trade off switch cost with wavelength converter cost in this type of WDM optical interconnect. The results in this paper are applicable to any emerging optical switching technologies, such as SOA-based and MEMS-based technologies.     

Routing wavelength and time-slot reassignment algorithms for TDM based optical WDM networks

In this paper, we consider the problem of maximizing the time of first lightpath request rejection, T in the circuit-switched time division multiplexed (TDM) wavelength-routed (WR) optical WDM networks. TDM is incorporated into WDM, to increase the channel utilization when the carried traffic does not require the entire channel bandwidth. In TDM–WDM network, multiple sessions are multiplexed on each wavelength by assigning a sub-set of the TDM slots to each session. Thus, given a session request with a specified bandwidth, a lightpath has to be established by using the routing, wavelength and time-slot assignment (RWTA) algorithms. If the lightpath cannot be established, lightpath request rejection or call blocking occurs. As each lightpath is substantial revenue and long-lived, lightpath request rejection is highly unfavourable in the optical backbone networks. In this paper, we are proposing an intelligent routing, wavelength and time-slot reassignment algorithm for multi-rate traffic demands, where, when a call gets blocked, the already established calls in the network are rerouted, wavelength and time-slot reassigned so as to accommodate the blocked call. Since we are talking of slow arrivals and long holding times for the lightpaths, it is possible to do this reassignment while provisioning a new call. Simulation based analyses are used to study the performance of the proposed reassignment algorithm. The results show that the proposed reassignment algorithm can be used to maximize the time of first call blocking, thereby accommodating more calls in the network before upgrading the network capacity.     

Restoration in all-optical GMPLS networks with limited wavelength conversion

A new generation of optical components and the advance of the Generalized Multi-Protocol Label Switching (GMPLS) control plane supporting dynamic provisioning and restoration of optical connections (i.e., lightpaths), brings the vision of the dynamic all-optical network closer to reality. An emerging technology is the conversion between wavelengths, which removes the wavelength continuity constraint, thus allowing an easier and more flexible connection allocation. A limitation in the number of wavelength converters impairs their benefits especially during the restoration phase, when many simultaneous recovery attempts must share residual resources.This paper investigates the restoration performance of GMPLS-controlled all-optical networks with limited wavelength converter deployment. We investigate how different restoration methods, namely span restoration, segment restoration, and end-to-end restoration are affected by the availability of a limited number of wavelength converters at each node. For this purpose an enhanced wavelength assignment scheme compliant with GMPLS signaling is exploited, aiming at saving converters by assigning a higher preference to wavelengths not requiring conversion. An extensive simulation study has been conducted comparing the performance of this scheme to the most advanced scheme based on standard GMPLS signaling for the three restoration methods.Simulation results show that the enhanced wavelength assignment scheme significantly reduces the number of wavelength converters (WCs) necessary to achieve good recovery performance. The enhanced scheme especially improves span restoration performance, where the matching between the stubs’ and recovery segment wavelength may require a WC. End-to-end restoration is the least affected, due to a higher degree of freedom in the route choice, while segment restoration performance lies in between.     

Ant colony optimization for dynamic routing and wavelength assignment in WDM networks with sparse wavelength conversion

Since optical WDM networks are becoming one of the alternatives for building up backbones, dynamic routing, and wavelength assignment with delay constraints (DRWA-DC) in WDM networks with sparse wavelength conversions is important for a communication model to route requests subject to delay bounds. Since the NP-hard minimum Steiner tree problem can be reduced to the DRWA-DC problem, it is very unlikely to derive optimal solutions in a reasonable time for the DRWA-DC problem. In this paper, we circumvent to apply a meta-heuristic based upon the ant colony optimization (ACO) approach to produce approximate solutions in a timely manner. In the literature, the ACO approach has been successfully applied to several well-known combinatorial optimization problems whose solutions might be in the form of paths on the associated graphs. The ACO algorithm proposed in this paper incorporates several new features so as to select wavelength links for which the communication cost and the transmission delay of routing the request can be minimized as much as possible subject to the specified delay bound. Computational experiments are designed and conducted to study the performance of the proposed algorithm. Comparing with the optimal solutions found by an ILP formulation, numerical results evince that the ACO algorithm is effective and robust in providing quality approximate solutions to the DRWA-DC problem.     

On burst rescheduling in OBS networks with partial wavelength conversion capability

The core nodes in an optical burst switching (OBS) network are normally equipped with wavelength converters (WCs) to reduce the burst loss probability. Since WCs are expensive and still immature technologically, it is desirable to reduce the number of WCs in the network and to have partial wavelength conversion capability at the core nodes. Nevertheless, a majority of algorithms in the literature are proposed under the full wavelength conversion assumption. As a result, they do not consider the burst loss caused by insufficient WCs, i.e., bursts dropped due to the unavailability of free WCs to convert them to unused wavelengths. In this paper, we demonstrate how to use burst rescheduling to decrease the burst loss due to insufficient WCs and hence cut down on the overall burst loss probability in OBS networks. Two burst rescheduling algorithms are proposed. Their effectiveness in reducing the overall burst loss probability is verified through simulation experiments.     

Optimal control data scheduling with limited controller-plant communication

This paper considers optimal control data scheduling for finite-horizon linear quadratic regulation (LQR) control of scalar systems with limited controller-plant communication. Both the single-system and multiple-system scenarios are studied. For the first scenario, we derive the necessary and sufficient condition for a comparison function to be positive. Using this condition, the optimality of an explicit schedule is extended from unstable systems in the existing work to general systems. For the second scenario, we are able to construct explicit optimal scheduling policies for three particular classes of problems. Numerical examples are provided to illustrate the proposed results.     

Optimal scheduling of contract algorithms with soft deadlines

A contract algorithm is an algorithm which is given, as part of its input, a specified amount of allowable computation time, and may not return useful results if interrupted prior to that time. In contrast, an interruptible algorithm will always output some meaningful (albeit suboptimal) solution, even if interrupted during its execution. Simulating interruptible algorithms by means of schedules of executions of contract algorithms in parallel processors is a well-studied problem with significant applications in AI. In the standard model, the interruptions are hard deadlines in which a solution must be reported immediately at the time the interruption occurs. In this paper, we study the more general setting of scheduling contract algorithms in the presence of soft deadlines. In particular, we address the setting in which if an interruption occurs at time t, then the system is given an additional window of time \(w(t)\le c \cdot t\), for constant c, within which the simulation must be completed. We formulate extensions to performance measures of schedules under this setting and derive schedules of optimal performance for all concave functions w.     

Routing and wavelength assignment for hypercube in array-based WDM optical networks

Hypercube is one of the most versatile and efficient communication patterns shared by a large number of computational problems. As the number of edges in hypercube grows logarithmically with the size of networks, the complexity of network topologies can be significantly reduced to realize hypercube in optical networks by taking advantage of the parallel transmission characteristic of optical fibers. In this paper, we study the routing and wavelength assignment for realizing hypercube on WDM optical networks including linear arrays and rings with the consideration of communication directions. Specifically, we analyze this problem for both bidirectional and unidirectional hypercubes. For each case, we identify a lower bound on the number of wavelengths required, and design the embedding scheme and wavelength assignment algorithm that uses a provably near-optimal number of wavelengths. In addition, we extend the results to meshes and tori. By our embedding schemes, many algorithms, originally designed based on hypercubes, can be applied to optical networks, and the wavelength requirements can be easily derived using our obtained results.     

Adaptive scheduling for integrated traffic on WDM optical networks

One of the important issues in the design of future generation of high-speed networks is to provide differentiated service to different types of traffic with various time constraints. In this paper, we study the problem of providing real-time service to either hard or soft real-time messages and normal transmission service to variable-length messages without time constraints in WDM optical networks. We propose an adaptive scheduling algorithm for scheduling message transmissions in order to improve the network performance when both real-time and non real-time messages are transmitted in one topology. We have analyzed the complexity of the algorithm to show its feasibility. We have conducted extensive discrete-event simulations to evaluate the performance of the proposed algorithm. The study suggests that when scheduling message transmission in WDM networks differentiated services should be considered in order to meet time constraints of real-time messages while non real-time messages are being served so that the overall performance of the network could be improved.     

Optimal algorithms for the batch scheduling problem in OBS networks

This paper introduces optimal batch scheduling algorithms for the problem of scheduling batches of bursts in optical burst switching networks. The problem is modelled as a job scheduling problem with identical machines. The consideration of previously scheduled bursts in the scheduling allows such modeling. Two optimal algorithms with polynomial time complexity are derived and evaluated. Results show that the algorithm that allows re-scheduling of previously scheduled bursts leads to preferred solutions.Moreover, an extended version of the JET reservation protocol is proposed for efficient handling of batches of bursts. Results obtained via simulation prove the superior performance of the BATCHOPT algorithm.     

Optimal on-line algorithms for the uniform machine scheduling problem with ordinal data

In this paper, we consider an ordinal on-line scheduling problem. A sequence of n independent jobs has to be assigned non-preemptively to two uniformly related machines. We study two objectives which are maximizing the minimum machine completion time, and minimizing the l_p norm of the completion times. It is assumed that the values of the processing times of jobs are unknown at the time of assignment. However it is known in advance that the processing times of arriving jobs are sorted in a non-increasing order. We are asked to construct an assignment of all jobs to the machines at time zero, by utilizing only ordinal data rather than actual magnitudes of jobs. For the problem of maximizing the minimum completion time we first present a comprehensive lower bound on the competitive ratio, which is a piecewise function of machine speed ratio s. Then, we propose an algorithm which is optimal for any s ⩾ 1. For minimizing the l_p norm, we study the case of identical machines (s = 1) and present tight bounds as a function of p.     

Optimal semi-online algorithms for scheduling problems with reassignment on two identical machines

In this paper, we consider semi-online minimum makespan scheduling problem with reassignment on two identical machines. Two versions are discussed. In the first version, one can reassign the last job of one machine that is based on the problem proposed by Tan and Yu (2008) [1], in which case the last job of each machine is allowed to be reassigned. An optimal algorithm which has the same competitive ratio is presented. In the second version we consider the combination of the next two conditions: the total size of all jobs is known in advance and one can reassign the last job of one machine. For this problem an optimal algorithm with competitive ratio is also given.     

Microelectromechanical vibration sensor with optical interconnects

A resonant vibration sensor realized using silicon micromachining for wear monitoring of rotating machinery is described. It comprises a mechanical resonator, piezoresistive bridge, and components for fiber-optical signal readout by an infrared light-emitting diode. The performance of the microsystem is demonstrated by its static and dynamical behavior. The results confirm that the described sensor has the potential for on-line vibration control of rotating machinery operated under the conditions of industrial production     

Wavelength decomposition approach for computing blocking probabilities in WDM optical networks without wavelength conversions

We present an approximate analytical method to evaluate the blocking probabilities in Wavelength Division Multiplexing (WDM) networks without wavelength converters. Our approach assumes fixed routing with Random or First-Fit wavelength assignment. The new approach views the WDM network as a set of different layers (colors) in which, blocked traffic in one layer is overflowed to another layer. Analyzing blocking probabilities in each layer of the network is derived from an exact approach. A moment matching method is then used to characterize the overflow traffic from one layer to another. The results indicate that our approach is more accurate than previous works.     

Utilize 100% redundancy to offer higher-level multiple fault restoration in WDM networks without wavelength conversion

This study addresses the problem of achieving higher-level multi-fault restoration in wavelength division multiplexing (WDM) networks with no wavelength conversion capability. A heuristic scheme, designated as the Directional Cycle Decomposition Algorithm (DCDA), is developed to maximize the number of tolerable faults utilizing only 100% redundancy in WDM networks without wavelength conversion. The redundancy is calculated as the required spare capacity over the given working capacity. The process of identifying the maximum number of tolerable faults is modeled as a constrained ring cover set problem. DCDA decomposes this problem into three steps and has an overall computational complexity of O(∣E∣∣V∣(C + 1) + ∣E∣(C² + 1)), where ∣V∣, ∣E∣ and C represent the number of vertices, the number of edges in the graph and the number of cycles in the cycle cover, respectively. The evaluation results reveal that the average number of tolerable simultaneous faults increases considerably under DCDA and the maximum number of tolerable simultaneous faults approaches the optimal solution provided by the brute-force method. DCDA facilitates an improved best-effort multi-fault restorability for a variety of planar and non-planar network topologies. An analytical method is proposed to facilitate a rapid estimation of the multi-fault restorability in a network using DCDA without the need for experimental evaluations. In addition, an approximation method is developed to obtain an estimate of the multi-fault restorability directly from DCDA without the requirement for a detailed knowledge of the network topology and restoration routes. The results show that the average errors in the approximated restorability values obtained using this method range from 0.12% (New Jersey) to 1.58% (Cost 239).     

Two processor scheduling with limited preemption

We introduce deterministic task system scheduling with limited preemption—that is, a preempted task can resume execution only on the processor where it originally executed. Two classes of limited preemptive schedules are introduced, corresponding to whether or not unforced idle time is allowed in the schedule. Our main result is to show that, on two processors, these two classes are equivalent with respect to optimal schedule lengths. We use this result to establish a hierarchy of optimal schedule lengths.     

Optimal algorithms for semi-online preemptive scheduling problems on two uniform machines

This paper investigates two preemptive semi-online scheduling problems to minimize makespan on two uniform machines. In the first semi-online problem, we know in advance that all jobs have their processing times in between p and rp . In the second semi-online problem, we know the size of the largest job in advance. We design an optimal semi-online algorithm which is optimal for every combination of machine speed ratio and job processing time ratio for the first problem, and an optimal semi-online algorithm for every machine speed ratio for the second problem.Received: 2 December 2003, Published online: 16 January 2004This research is supported by the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of MOE, China, and National Natural Science Foundation of China (10271110).     

Novel configuration for wavelength conversion based on cross-gain modulation in semiconductor optical amplifiers

Novel configuration for cross-gain modulation (XGM) wavelength conversion based on the single-port-coupled semiconductor optical amplifier (SOA), in which the input and the output share one port, has been demonstrated with the output extinction ratio as high as 15 dB, even in the 2.5 Gbit/s wavelength up conversion with a 12.8 nm span. Owing to the existence of double-pass gain and the transmission loss of the rear facet, the novel scheme can be achieved with simpler implementation and higher output extinction ratio, compared with the conventional schemes based on double-port-coupled SOA either with identical long chip or double long chip.