Alejandra Beghelli

Migration cost analysis for upgrading WDM networks

We present a generic step-by-step methodology for evaluating the total cost of migrating from a capacity-exhausted WDM network to different upgraded alternatives. The presented methodology is the first effort to provide a generic evaluation framework (allowing the evaluation of scenarios with different traffic growth rates, optical technologies, network architectures, and resource allocation algorithms) that considers both capital and operational expenditures of the upgraded alternatives to then identify the lowest-cost option. Previous works have just evaluated specific scenarios or only CapEx or OpEx (not both). As a way of illustration, the proposed methodology was applied to compare the migration cost of two upgrading scenarios. The methodology allowed identifying the lowest-cost alternative, categorizing the key cost factors of CapEx and OpEx, and evaluating its impact on the migration cost. Surprisingly, results for the study case presented show that migrating to an automatically provisioned network does not necessarily lead to cost savings.

A sub-graph mapping-based algorithm for virtual network allocation over flexible grid networks

A new algorithm for dynamic network virtualization over flexible-grid networks is proposed. The algorithm, called Hamming-Fit, aims to map each request in a zone of the physical network resembling as much as possible the topology of the virtual network (sub-graph mapping). To do so, every possible node set (with as many nodes as the virtual network) of the physical topology is analyzed and its suitability evaluated using a metric analogous to the Hamming distance. In this way, a minimum usage of network resources is expected due to the use of 1-hop routes to establish the virtual links. The virtual link allocation also deals with the physical impairments of flexible grid networks by limiting the optical reach of the different optical signals and selectively adding guard bands when such optical reach must be surpassed. Simulation results show that Hamming-Fit exhibits a significant lower blocking than a previous proposal that uses exactly the same physical impairment model. The blocking improvement comes mainly from the fact that by performing a sub-graph mapping, shorter routes are used to map the virtual links. Thus, rejection due to the optical reach violation is minimized, which was the main drawback of the previous proposal.

Deadlock-Avoiding vs. greedy spectrum allocation algorithms in dynamic flexible optical networks

Greedy spectrum allocation algorithms assign the required number of slots to a connection request as long as there are enough contiguous slots available for it. Instead, a deadlock-avoidance algorithm only assigns slots if future connections can be fit in the spectrum void left after assigning the current connection. That is, a deadlock-avoidance algorithm avoids leaving available slots that cannot be used by a future connection. The mean time required to exhaust the spectrum of a deadlock-avoidance approach has been studied in the context of a single flexible-grid link under incremental traffic. However, such algorithm has not been studied under dynamic traffic, where the blocking ratio is relevant, neither considering routes longer than 1 hop. In this paper we evaluate the blocking performance of greedy and deadlock-avoidance algorithms under dynamic traffic, in routes from 1-hop to 3-hops. Simulation results show that - with respect to a greedy approach - there is no benefit from deadlock-avoidance algorithms in 1-hop routes. However, the deadlock-avoidance strategy achieves up 2 orders of magnitude lower blocking than the greedy algorithm when longer routes are considered. This result highlights the importance of using algorithms that prevent spectrum fragmentation for efficient resource utilization in dynamic flexible grid networks.

A joint RSA algorithm for dynamic flexible optical networking

We propose a novel algorithm to solve the Routing and Spectrum Allocation (RSA) problem in dynamic flexible grid optical networks. Unlike most previous proposals, the algorithm solves the R and SA problems jointly by exhaustively searching the solution space and taking the network state into account. As a result, the shortest possible path with enough spectrum availability is allocated to establish the connections. Simulation results show that, in terms of blocking ratio, our proposal significantly outperforms previously proposed algorithms. In some cases, the performance is better by more than one order of magnitude.

Does fragmentation avoidance improve the performance of dynamic spectrum allocation in elastic optical networks

Most spectrum allocation algorithms in elastic optical networks apply a greedy approach: A new connection is allocated as long as there are enough spectrum slots to accommodate it. Recently, a different approach was proposed. Named Deadlock–Avoidance (DA), it only establishes a new connection if the portion of spectrum left after allocating it is zero (full-link utilization) or is big enough to accommodate future requests. Otherwise, the connection request is blocked as a way to avoid fragmentation. The performance of DA has been evaluated in a single-link scenario, where its performance is not affected by the slot continuity constraint. In this paper, we evaluate for the first time the blocking performance and fragmentation level of DA in a fully dynamic network scenario with different bitrates and number of slots for a single link, a 4-node bus and a mesh topology. The performance was evaluated by simulation, and a lower bound was also derived using a continuous Markov chain model. Results are obtained for DA and three greedy algorithms: First Fit, Exact Fit and First–Last Fit. Results show that DA significantly decreases fragmentation, and thus, it exhibits a much lower blocking due to fragmentation than the greedy algorithms. However, this decrease is compensated by a new type of blocking due to the selective acceptance of connections. As a result, the extra computational complexity of DA does not compensate a gain in performance.

Correctness assessment of a crowdcoding project in a computer programming introductory course

Crowdcoding is a programming model that outsources a software project implementation to the crowd. As educators, we think that crowdcoding could be leveraged as part of the learning path of engineering students from a computer programming introductory course to solve local community problems. The benefits are twofold: on the one hand the students practice the concepts learned in class and, on the other hand, they participate in real‐life problems. Nevertheless, several challenges arise when developing a crowdcoding platform, the first one being how to check the correctness of students code without giving an extra burden to the professors in the course. To overcome this issue, we propose a novel system that does not resort to expert review; neither requires knowing the right answers beforehand. The proposed scheme automatically clusters the students codes based solely on the output they produce. Our initial results show that the largest cluster contains the same codes selected as correct by the automated and human testing, as long as some conditions apply.

Routing, code, modulation level and spectrum assignment (RCMLSA) algorithm for elastic optical networks

To date, most resource allocation proposals in elastic optical networks have focused either on routing and spectrum assignment (RSA, assuming fixed coding scheme and modulation format) or routing, modulation level and spectrum assignment (RMLSA, assuming a fixed coding scheme). Recent research has studied the impact of adaptive assignment of the code rate, without adapting the modulation format (RCSA algorithm). In this paper, we report for the first time the impact that adapting both the code rate as well as the modulation format has on the blocking performance. To do so, we present a new RCMLSA algorithm and evaluate it in terms of blocking by means of simulation. We compared the performance of our RCMLSA algorithm to that of RMLSA and RCSA algorithms. Results obtained for the NSFNet topology show that our RCMLSA algorithm performs better than the RCSA and RMLSA proposals, with a decrease in blocking of more than one order of magnitude (for traffic loads lower than 0.7) with respect to the RCSA.

Generic techno-economic evaluation methodology: the case of migration to flexible grid networks

Current fixed grid DWDM networks are expected to be insufficient to accommodate expected demands in few years. Hence, operators need to compare different alternatives to cope with future demands. This paper presents a step-by-step techno-economic methodology to compare and identify the most convenient upgrade alternative(s). One of the possible migration scenarios is towards flexible grid optical networks. The comparative evaluation is made in terms of investment and operational expenditures considering the cost per spectral unit. The proposed methodology is applied to an operator having as initial scenario a statically operated network using a fixed grid with connections at 10 Gbps. Three different upgrade alternatives are evaluated: (i) static operation using the fixed grid is maintained but the bit rate of connections is increased to 100 Gbps; (ii) the network is operated dynamically (lightpaths established/released on demand) in a fixed grid with 100 Gbps connections and finally, (iii) a dynamic network operating with flexible grid. Results for this particular case, show that dynamic operation with flexible grid is not the most convenient migration alternative, given the high cost of the required equipment. This methodology is expected to be useful for network operators evaluating different upgrade alternatives for their infrastructure, and manufacturers aiming to evaluate the potential of their equipment.

A Joint RSA Algorithm for Dynamic Flexible Optical Networking

We propose a novel algorithm to solve the Routing and Spectrum Allocation (RSA) problem in dynamic flexible grid optical networks. Unlike most previous proposals, the algorithm solves the R and SA problems jointly by exhaustively searching the solution space and taking the network state into account. As a result, the shortest possible path with enough spectrum availability is allocated to establish the connections. Simulation results show that, in terms of blocking ratio, our proposal significantly outperforms previously proposed algorithms. In some cases, the performance is better by more than one order of magnitude.