Lightpath Re-optimization in mesh optical networks

Intelligent mesh optical networks deployed today offer unparalleled capacity, flexibility, availability, and, inevitably, new challenges to master all these qualities in the most efficient and practical manner. More specifically, demands are routed according to the state of the network available at the moment. As the network and the traffic evolve, the lightpaths of the existing demands becomes sub-optimal. In this paper we study two algorithms to re-optimize lightpaths in resilient mesh optical networks. One is a complete re-optimization algorithm that re-routes both primary and backup paths, and the second is a partial re-optimization algorithm that re-routes the backup paths only. We show that on average, these algorithms allow bandwidth savings of 3% to 5% of the total capacity in scenarios where the backup path only is re-routed, and substantially larger bandwidth savings when both the working and backup paths are re-routed. We also prove that trying all possible demand permutations with an online algorithm does not guarantee optimality, and in certain cases does not achieve it, while for the same scenario optimality is achieved through re-optimization. This observation motivates the needs for a re-optimization approach that does not just simply look at different sequences, and we propose and experiment with such an approach. Re-optimization has actually been performed in a nationwide live optical mesh network and the resulting savings are reported in this paper, validating reality and the usefulness of re-optimization in real networks.     

Distributed computation of shared backup path in mesh optical networks using probabilistic methods

We assess the benefits of using statistical techniques to ascertain the shareability of protection channels when computing shared-mesh restored lightpaths in optical mesh networks. These optical networks support wavelength conversion everywhere as a byproduct of the electronic nature of the switching in the optical-electronic-optical optical cross connect used. Current deterministic approaches require a detailed level of information proportional to the number of active lightpaths. Although this is not an issue for good sized networks in the foreseeable future, these approaches are not practicable for distributed route computation involving larger networks. On the other hand, distributed approaches that do not make use of shareability information require a significant amount of additional capacity compared to a centralized approach with access to complete shareability information. With the proposed approach we show that even with less information, independent of the amount of traffic demand, it is possible to predict the shareability of protection channels with remarkable accuracy. In addition, we propose a local distributed channel assignment scheme that is used in conjunction with our distributed route computation proposal to assign shared channels when provisioning the backup path. This channel assignment scheme can also be used to further optimize capacity usage in individual links upon certain events or at regular intervals. Experiments are provided that demonstrate that our approach yields faster computation times with no significant penalty in terms of capacity usage than a centralized approach using complete information.     

Branch-exchange sequences for reconfiguration of lightwave networks

Some of today's telecommunications networks have the ability to superimpose some form of logical connectivity, or virtual topology, on top of the underlying physical infrastructure. According to the degree of independence between the logical connectivity and the physical topology, the network can dynamically adapt its virtual topology to track changing traffic conditions, and cope with failure of network equipment. This is particularly true for lightwave networks, where a logical connection diagram is achieved by assignment of transmitting and receiving wavelengths to the network stations that tap into, and communicate over, an infrastructure of fiber glass. Use of tunable transmitters and/or receivers allow the logical connectivity to be optimized to prevailing traffic conditions. With rearrangeability having thus emerged as a powerful network attribute, this paper discusses the reconfiguration phase which is the transition between the current logical connection diagram and a target diagram. We consider here an approach where the network reaches some target connectivity graph through a sequence of intermediate connection diagrams, so that two successive diagrams differ by a single branch-exchange operation. This is an attempt at logically reconfiguring the network in a way that is minimally disruptive to the traffic. We propose and compare three polynomial-time algorithms that search for “short” sequences of branch-exchange operations, so as to minimize the overall reconfiguration time. For networks made of up to 40 stations, theoretical and simulation results show that, when a randomly selected diagram is to be changed to another randomly chosen diagram, the average number of branch-exchange operations required grows linearly with the size of the network     

A traffic-handling comparison of centralized and distributed ATMswitching systems

This paper describes a geographically distributed ATM switching architecture that exploits new possibilities offered by lightwave technology. Small modules that provide both an access and an ATM switching function are distributed over some extended geographical area and connected to a passive optical medium. Each module is equipped with some small number of electrooptic transmitters and receivers that provide access to the medium. Assignment of wavelengths to transmitter/receiver pairs create logical channels that are wavelength-multiplexed onto the medium, thus creating a logical connection diagram among the modules. Advantages of the lightwave-based distributed architecture are the huge bandwidth of the medium, a high degree of modularity to facilitate growth, high reliability, and the ability to provide “clear channels” among modules. On the other hand, if the externally offered flow of ATM cells among modules is too uniform, then each module may be capable of supporting only a fraction of the load that could be carried by any port of a fully connected centralized ATM switch with output queueing. Fortunately, under the much more realistic assumption of nonuniform traffic, the independence between the logical connection diagram and the physical topology of the medium can be exploited by reconfiguring the connection diagram to “match” the nonuniformity of the cell traffic. Simulation results show that, as the nonuniformity becomes larger, the reconfiguration technique allows the traffic-handling capability of a distributed switch to match that of a centralized switch     

The proliferation of mature oligodendrocytes in vitro is stimulated by basic fibroblast growth factor and inhibited by oligodendrocyte-type 2 astrocyte precursors

Basic fibroblast growth factor (bFGF) and platelet-derived growth factor (PDGF) are mitogens for bipotential oligodendrocyte-type 2 astrocyte (O-2A) progenitor cells. We investigated the mitogenic effect of these growth factors on quiescent mature oligodendrocytes (OL) expressing myelin basic protein (MBP) in OL cultures that were treated for 3 days with cytosine arabinoside (ARA-C) in order to kill O-2A precursors which divide in chemically defined medium. After treatment with ARA-C proliferation decreased and O-2A precursors identified with A2B5 monoclonal antibody were nearly undetectable. After exposure of mature OL to bFGF, cell proliferation increased markedly within 24 hr. PDGF had a much weaker effect. Cultures treated with ARA-C for 3 days and then with bFGF for the next 24 hr and incubated with BrdU for the last 2 hr before the end of the experiment were immunolabeled with anti-MBP or A2B5 and anti-bromodeoxyuridine (BrdU) antibodies. Eighty-seven percent of the cells were MBP+, 10% were both MBP+ and BrdU+, and none was A2B5+ BrdU+, showing that at least a part of the population of mature MBP+ OL retains the ability to reenter the cell cycle in vitro. Since mature OL did not proliferate in response to bFGF in the cultures not treated with ARA-C, i.e., in the presence of O-2A progenitors, we assumed that these precursors were responsible for the lack of mitogenic effect of bFGF on MBP+ OL in such conditions. Conditioned medium from O-2A precursors almost halved the bFGF-induced OL proliferation after treatment with ARA-C, suggesting that O-2A progenitors control the proliferation of a subpopulation of mature OL (possibly young mature OL) via the secretion of active molecule(s).     

Invited: Routing Strategies for Capacity-Efficient and Fast-Restorable Mesh Optical Networks

Wavelength division multiplexed (WDM)-based mesh network infrastructures that route optical connections using intelligent optical cross-connects (OXCs) are emerging as the technology of choice to implement the next generation core optical networks. In these architectures a single OXC is capable of switching tens of terabits of traffic per second. With such data transfer rates at stake, it becomes increasingly challenging for carriers to (1) efficiently and cost-effectively operate and manage their infrastructure, and (2) cope with network failures while guaranteeing prescribed service level agreements (SLAs) to their customers. Proper routing of primary and backup paths is a critical component of the routing and restoration architecture required to meeting these challenges. In this paper we review some of the various strategies and approaches proposed so far to intelligently route connections while at the same time providing guaranteed protection against various types of network failures. We explore the tradeoffs associated with these approaches, and investigate in particular different, sometimes competing aspects, such as cost/capacity required, level of protection (link vs. node failure), restoration time, and complexity of route computation.     

3D SEDIMENTARY MODELLING OF A MIOCENE DELTAIC RESERVOIR UNIT, SINCOR FIELD, VENEZUELA: A NEW APPROACH

The “C2” unit in the Morichal Member of the Miocene Oficina Formation at the Sincor field (East Venezuela Basin) is characterised by a succession of superposed deltaic cycles which control vertical and horizontal reservoir connectivity. In order to model this reservoir in three dimensions, a workflow was developed which addressed a series of specific challenges. First, the deltaic nature of the succession required that both distributary channel and mouth bar sandbodies had to be modelled according to the defined sequence stratigraphic framework. Second, the relationship between distributary channels and mouth bars had to be honoured. A third issue was that individual distributary channel sandbodies were on occasion thicker than the cycle to which they belonged (i.e. they eroded down into the underlying cycle), and the relationship between mouth bar and channel sandbodies in adjacent sequences was therefore broken. Although conceptually simple from a geological viewpoint, this aspect proved particularly difficult during 3D modelling. This paper discusses the construction of the stratigraphic model, as well as that of the channel and mouth bar models. The workflow is based on existing stochastic approaches, arranged in nested steps dependent on the stratigraphic framework and on defined depositional processes in order to simulate reservoir distribution and partitioning. By integrating sedimentological observations and interpretations with existing modelling procedures, a reliable reservoir model can be built. The model is based on the observed sequence stratigraphic framework, and its infill takes into account the relationship between distributary channels and mouth bar deposits, derived from depositional processes. The model provides realistic distributions of the channel fills and mouth bar deposits at Sincor field using a multi‐realisation scheme. Improved local vertical connectivity between individual sequences, caused by erosion of highstand mudstone seals and baffles, can effectively be simulated; this is of particular significance at Sincor where planned second‐phase heavy oil recovery will depend on the use of steam‐assisted gravity drainage.     

Logically rearrangeable multihop lightwave networks

The optimization problem of rearrangeable multihop lightwave networks is considered. The authors formulate the flow and wavelength assignment problem, when minimizing the maximum flow in the network, as a mixed integer optimization problem subject to linear constraints. The problem is decomposed into two independent subproblems, the wavelength assignment (or connectivity problem) and the flow assignment (or routing problem). A simple heuristic provides a meaningful formulation to the connectivity problem, in a form similar to a transportation problem. An algorithm is then proposed which finds a heuristic initial logical connectivity diagram and the corresponding routing, and then iterates from that solution by applying branch-exchange operations to the connectivity diagram. The algorithm was tested on illustrative traffic matrices for an 8 node network with two transmitters and two receivers per node, and an improvement in achievable throughput over the Perfect Shuffle interconnection pattern was shown in all cases     

Energy release rate computation by an inverse flexibility method

This paper presents the “Inverse Flexibility Method” which allows substantial gain in time when computing energy release rates (or stress intensity factors) for various lengths of a growing crack. Reduction ratio is of order of ten compared with classical methods. Despite the relative coarseness of meshes used, when practicing finite element analysis, accuracy of results remains good as shown by examples presented (metallic cracked plate and composite specimens).