Samir Sebbah

Efficient and Scalable Design of Protected Working Capacity Envelope

The Protected Working Capacity Envelope (PWCE) concept was proposed by Grover (2004) in order to simplify network and operation management in survivable WDM networks. In this paper, we focus on PCWE with p-cycles and investigate a new design method, highly efficient and scalable, for designing survivable WDM networks. Traditional design methods proceed in two steps: A first step where a large (sometimes huge) number of cycles is enumerated followed by a second step where the selection of the most promising p-cycles is made with the help of combinatorial optimization tools. We develop a new (single step) method based on large scale optimization tools, i.e., column generation techniques, where the generation of cycles is dynamic and embedded within the optimization process. The key advantage of column generation (CG) techniques is that no a priori cycle enumeration step is required ahead of the optimization process: The generation of the relevant cycles, only one or few at a time, is embedded in the optimization process. We conducted intensive computational experiments. Not only do we considered several network instances with quite different topology characteristics, but we also compared our CG-based model and solution method with several existing models and methods from the literature. Results obtained in the experiments on five different network instances, show that the CG-based model and method outperform by far the results of all previous studies, both with respect to the scalability (much smaller computing times for large network instances) but also with respect to the quality of the solutions.

Differentiated quality-of-recovery in survivable optical mesh networks using p-structures

This paper investigates design methods of protection schemes in survivable WDM networks that use preconfigured protection structures (p-structures) in order to provide different quality-of-recovery (QoR) classes within 100% resilient single-link protection schemes. QoR differentiation is a practical and effective approach in order to strike different balances among protection cost, recovery delay, and management complexity. Based on the degree of pre-cross connectivity of the protection structures, we develop three design approaches of shared protection capacity schemes based on the following: 1) fully pre-cross-connected p-structures (fp-structures); 2) partially pre-cross-connected p-structures (pp-structures); and 3) dynamically reconfigured p -structures (dp -structures). In order to identify the optimal combinations of protection structures to meet the requirements of the three QoR classes, we use a column generation (CG) model that we solve using large-scale optimization techniques. Our CG decomposition approach is based on the separation processes of the design and selection of the protection structures. In the design process of the protection structures, the shape and protection capability of each p-structure is decided dynamically during the selection process depending on the network topology and the targeted QoR parameters. Extensive experiments are carried out on several data instances with different design constraints in order to measure the protection capacity cost and the recovery delay for the three QoR classes.

A new framework for efficient shared segment protection scheme for WDM networks

This work introduces a new shared segment protection scheme that ensures both node and link protection in an efficient manner in terms of cost and bandwidth, while taking full advantage of the optical hop endpoints of the primary logical hops (induced by the routing) without adding extra ones for protection. As opposed to the link or path protection schemes, the segment protection scheme has been less studied although it offers an interesting compromise between those two protection schemes, attempting to encompass all their advantages. We investigate two different Shared Segment Protection (SSP) schemes: Basic Shared Segment Protection (BSSP) and Shared Segment Protection with segment Overlap (SSPO), and propose design of 100% single segment protections. In SSPO, we study the extra protection capabilities, node failure and dual link failure survivability, offered by the single 100% segment protection. For both BSSP and SSPO schemes, we propose two novel efficient ILP formulations, based on a column generation mathematical modeling. While (SSPO) offers the advantage over (BSSP) to ensure both node and link protection, it is not necessarily much more costly. Indeed, depending on the network topology and the traffic instances, it can be shown that none of the two SSP schemes dominates the other one. Therefore, the SSPO protection scheme should be favored as it offers more protection, i.e., it adds the node protection to the link protection at the expense of a minor additional cost.

Survivable WDM Networks Design with Non-Simple p-Cycle-Based PWCE

We propose a new design approach of survivable WDM networks using simple and non-simple p-cycles-based protected working capacity envelope (PWCE). We investigate the added-value of the non-simple p-cycles over simple p-cycle protection method in terms of capacity efficiency and reliability. As opposed to the prevalent two-step design approach where an explicit enumeration of all/set of p-cycles step is performed ahead of an optimization step, we develop a new optimization approach using a large scale optimization tool named column generation technique, where only few globally optimal cycles are generated dynamically during the optimization process. We conduct performance evaluation experiments on various network topologies, using different metrics in order to measure the added- value of the non-simple p-cycles over simple p-cycles in the design of survivable WDM networks based on PWCE. It is shown that depending on the network topologies, and in particular of their connectivity, significant protection improvement can be achieved when using non simple p-cycles.

A Resilient Transparent Optical Network Design with a Pre-Configured Extended-Tree Scheme

We propose a new design scheme of resilient Wavelength Division Multiplexing (WDM) networks by extending and reshaping pre-configured protection tree (p-tree) structures. The resulting protection scheme relies on optimized pre-cross connected structures that span all previously proposed protection patterns. p-Tree-based protection schemes offer the advantages of scalability, local restoration capabilities, and failure impact restriction, but at the same time suffer from capacity inefficiency. While keeping these advantages, we propose an extension (reshaping) of the p-tree protection pattern that imposes no restriction on the shapes of the protection building blocks. Not only the resulting protection scheme remains scalable and highly flexible, but it also leads to pre-configured protection structures that improve much further on capacity efficiency and recovery delay. We establish some new integer linear programming models, and use a large scale optimization tool, named column generation (CG) to solve them. Our CG-based solution method is highly scalable as it does not require an a priori explicit enumeration of the protection structures, but an efficient dynamic enumeration of only the most promising ones. Comparison are made with three other protection schemes, i.e, simple and non-simple p-cycles (fully pre-cross connected structures) as well as p-trees. Results show a clear advantage of the proposed extended-tree scheme with respect to flexibility, capacity efficiency, and restoration delay.

Towards Scalable Traffic Management in Cloud Data Centers

Cloud Computing is becoming a mainstream paradigm, as organizations, large and small, begin to harness its benefits. This novel technology brings new challenges, mostly in the protocols that govern its underlying infrastructure. Traffic engineering in cloud data centers is one of these challenges that has attracted attention from the research community, particularly since the legacy protocols employed in data centers offer limited and unscalable traffic management. Many advocated for the use of VLANs as a way to provide scalable traffic management, however, finding the optimal traffic split between VLANs is the well known NP-Complete VLAN assignment problem. The size of the search space of the VLAN assignment problem is huge, even for small size networks. This paper introduce a novel decomposition approach to solve the VLAN mapping problem in cloud data centers through column generation. Column generation is an effective technique that is proven to reach optimality by exploring only a small subset of the search space. We introduce both an exact and a semi-heuristic decomposition with the objective to achieve load balancing by minimizing the maximum link load in the network. Our numerical results have shown that our approach explores less than 1% of the available search space, with an optimality gap of at most 4%. We have also compared and assessed the performance of our decomposition model and state of the art protocols in traffic engineering. This comparative analysis proves that our model attains encouraging gain over its peers.

Design of Resilient Ethernet Ring Protection (ERP) Mesh Networks With Improved Service Availability

Ethernet Ring Protection (ERP) has recently emerged to provide protection switching for Ethernet ring topologies with sub-50 ms failover capabilities. ERPs promise to provide protection in mesh packet transport networks positions Ethernet as a prominent competitor to conventional SONET/SDH and as the technology of choice for carrier networks. Higher service availability, however, in ERP has been challenged by the issue of network partitioning and contention for shared capacity caused by concurrent failures. In this paper, we show that in a network designed to withstand single-link failure, the service availability, in the presence of double link failures, depends on the designed ERP scheme, i.e., the RPL placement as well as the selection of ring hierarchy. Therefore, we present a study for characterizing service outages and propose a design method which strikes a balance between capacity requirement and service availability (i.e., the number of service outages resulting from concurrent failures). We observe that through effective design, remarkable reduction in service outages is obtained at a modest increase in capacity deployment.

Optimal capacity planning and RPL placement in carrier Ethernet mesh network design

Ethernet Ring Protection (ERP) has recently emerged to provide protection switching for Ethernet ring topologies with sub-50 ms failover capabilities. ERPs promise to also provide protection in multi-ring mesh packet transport networks will position Ethernet as a serious competitor to conventional SONET/SDH and the technology of choice for carrier networks. Operating ERP in multi-ring mesh networks however comes with unique challenges. This paper presents an optimal ERP capacity design formulation by jointly solving the problem of routing, RPL placement, and ring hierarchy selection. When prior work relied on exhaustive enumeration, our current design is formulated as an integer linear program (ILP) which is shown to be both more capacity and computationally efficient.

Design of survivable WDM networks using pre-configured protection structures with unrestricted shapes

We propose a novel protection approach for the design of link-protection schemes in survivable Wavelength Division Multiplexing mesh networks by merging the well-known p-cycle- and p-tree-protection structures. So doing, we aim at gathering the advantages of p-cycles in terms of protection capabilities, and of p-trees in terms of protection flexibilities (local re-routing, scalability) in a single protection scheme. As opposed to existing protection schemes based on protection structures with a pre-defined shape, the building blocks of the new scheme are protection structures with unrestricted shapes. Thus, they allow more flexibility in provisioning spare capacity, and provide higher capacity efficiency when compared to the shaped-protection schemes that have been proposed so far. In order to cope with the size of the solution space which includes all the possible protection structures, we propose an efficient and scalable optimization technique in large-scale systems named column generation (CG). In our CG-based optimization approach, the shape of a candidate protection structure is dynamically decided during the optimization process according to a link spare capacity budget. Experimental results on different network instances show that the protection plan resulting from the merging of p-cycle and p-tree structures is, on average, ~15% less capacity redundant and ~15% more reliable than the pure p-cycle one. It also requires, on average, ~30% less protection structures. In addition, those structures provide backup paths ~30% smaller than those of the p-cycle-based scheme.

Design of flexible protection plans in survivable WDM networks: An application to PWCE

We propose a new flexible design approach of protection plans in survivable WDM networks by using protection structures with no predefined shapes in order to maximize the protected working capacity in an end-to-end basis. Previous design approaches of survivable WDM based on Protected Working Capacity Envelope (PWCE) have looked at the optimization problem of maximizing the protected capacity on a link basis, independently of the source and destination nodes of the potential traffic. Moreover, those approaches have only investigated the design problem with pre-configured protection cycles (p-cycles).