Matthieu Clouqueur

Availability analysis of span-restorable mesh networks

The most common aim in designing a survivable network is to achieve restorability against all single span failures, with a minimal investment in spare capacity. This leaves dual-failure situations as the main factor to consider in quantifying how the availability of services benefit from the investment in restorability. We approach the question in part with a theoretical framework and in part with a series of computational routing trials. The computational part of the analysis includes all details of graph topology, capacity distribution, and the details of the restoration process, effects that were generally subject to significant approximations in prior work. The main finding is that a span-restorable mesh network can be extremely robust under dual-failure events against which they are not specifically designed. In a modular-capacity environment, an adaptive restoration process was found to restore as much as 95% of failed capacity on average over all dual-failure scenarios, even though the network was designed with minimal spare capacity to assure only single-failure restorability. The results also imply that for a priority service class, mesh networks could provide even higher availability than dedicated 1+1 APS. This is because there are almost no dual-failure scenarios for which some partial restoration level is not possible, whereas with 1+1 APS (or rings) there are an assured number of dual-failure scenarios for which the path restorability is zero. Results suggest conservatively that 20% or more of the paths in a mesh network could enjoy this ultra-high availability service by assigning fractional recovery capacity preferentially to those paths upon a dual failure scenario.

New options and insights for survivable transport networks

This article is devoted to a selection of recent topics in survivable networking. New ideas in capacity design and ring-to-mesh evolution are given, as well as a systematic comparison of the capacity requirements of several mesh-based schemes showing how they perform over a range of network graph connectivity. The work provides new options and insights to address the following questions. How does one evolve from an existing ring-based network to a future mesh network? If the facilities graph is very sparse, how can mesh efficiency be much better than rings? How do the options for mesh protection or restoration rank in capacity requirements? How much is efficiency increased if we enrich our network connectivity? We also outline p-cycles, showing this new concept can realize ring-like speed with meshlike efficiency. The scope is limited to conveying basic ideas with an understanding that they could be further adapted for use in IP or DWDM layers with GMPLS-type protocols or a centralized control plane.

Strategies for enhanced dual failure restorability with static or reconfigurable p-cycle networks

We suggest methods for achieving high dual-failure restorability in p-cycle networks that are optimally designed only to withstand all single failures, or have minimized amounts of additional capacity for dual failure considerations. In one set of circumstances we consider static p-cycles (that cannot be rearranged once established) and propose strategies to enhance the dual-failure restorability based on concepts of failure spreading and limiting the maximum number of protection relationships of any p-cycle. We then consider the case of reconfigurable p-cycles, in which the spare capacity can be reconfigured dynamically, creating a new set of p-cycles that are optimized to withstand possible second failures. Results show significant improvements of the dual-failure restorability obtained by limiting the number of protection relationships in static design. For 100% dual-failure restorability, p-cycle reconfiguration reusing most of the existing p-cycles and adding some new ones appears to be the most promising approach.

Availability Analysis and Enhanced Availability Design in p-Cycle-Based Networks

Abstractp-Cycle protection is a fairly new survivability scheme that has the interesting properties of offering restoration speeds essentially the same as those offered by ring protection while requiring almost as little redundant capacity as adaptive mesh restoration [D. Stamatelakis, W.D. Grover, IEEE Transactions on Communications, vol. 48, no. 8, (August 2000), pp. 1262–1265]. This paper presents the first analytical consideration of the availability of paths in a network protected by p-cycles. Results confirm the importance that cycle sizes play in terms of availability and suggests principles or strategies for achieving high availability of paths in a network protected by p-cycles. Based on these insights, two new formulations for joint service path provisioning and capacity planning are proposed. The first one offers a way to improve the availability of selected service paths by using a different routing strategy for them than for regular service paths. The second formulation enables a new class of service paths that are offered two protection options instead of just one. That class of service paths is expected to see its availability improved in a quantum step way relative to the availability of paths having only one protection option.

Mesh-Restorable Networks with Enhanced Dual-Failure Restorability Properties

We consider extensions of the most common mesh-restorable network capacity design formulation that enhance the dual-failure restorability of the designs. A significant finding is that while design for complete dual-failure restorability can require up to triple the spare capacity, dual failure restorability can be provided for a fairly large set of priority paths with little or no more spare capacity than required for single-failure restorability. As a reference case we first study the capacity needs under complete dual-failure restorability. This shows extremely high spare capacity penalties. A second design model allows a user to specify a total capacity (or budget) limit and obtain the highest average dual-failure restorability possible for that investment limit. This formulation, and a relationship between dual-failure restorability and availability, can be used to trace-out the capacity-versus-availability trade-off curve for a mesh network. A third design strategy supports multiple-restorability service class definitions ranging from best-efforts-only to an assurance of complete single and dual-failure restorability on a per-demand basis. This lets a network operator tailor the investment in protection capacity to provide ultra-high availability on a service-selective basis, while avoiding the very high investment that would be required to support complete dual-failure restorability of the network as a whole.

Span-Restorable Mesh Networks with Multiple Quality of Protection (QoP) Service Classes**

In the modern business environment there is considerable interest in being able to support a range of different transport service classes in an optical network, and charge accordingly. In this work we consider the capacity design problem for a mesh-restorable optical network supporting any mixture of four basic “quality of protection” (QoP) classes. The service definitions are (gold): assured restorability, (silver): best efforts, (bronze): non-protected and (economy): preemptible service. We give design models for optimal capacity design of span-restorable (or corresponding link-protected) mesh networks having any particular mixture of these service classes. We also apply and test the design models under several multi-QoP test case scenarios to gain insights about various strategies and options possible in a multi-QoP design environment. An interesting finding is that in some test cases, 15 to 30% of all demand can be in the gold class enjoying 100% restorability solely through preemption of economy class service capacity. This suggests the potential to design and operate mesh-based networks that have no spare capacity at all in the conventional sense: all capacity is bearing service of some paying type. The resulting frequency of preemption in the economy class services is also studied. Results also show typically high levels of best-efforts restorability in the silver class occurring in networks that are strictly designed only for the restorability of the gold class services. High restorability of best effort services, however, requires the preemption of economy services. These methods and findings can be used by network and business planners to evaluate a number of different service structuring, pricing, and capacity-design strategies that may offer advantages to them and new options for their customers.

Span-Restorable Mesh Network Design to Support Multiple Quality of Protection (QoP) Service-Classes

In the modern business environment for optical mesh networking, there is considerable interest in being able to support a range of service classes, and to charge accordingly. In this work we consider the capacity design problem for different combinations of four protection priority classes in a span-restorable mesh network. Most significantly we have test-case findings that 15% to 30% of all demand can be in the gold class and enjoy 100% restorability, solely through preemption of economy class service capacity. This indicates the potential to design and operate mesh-based restorable networks which have no spare capacity at all in the conventional sense: all capacity is bearing service of some paying type. The methods provided could be the basis for future mesh-based optical transport networks that simultaneously support multiple QoP services with optimal capacity.

Design of Survivable Networks Based on p-Cycles

p-Cycles are a recently discovered and promising new paradigm for surviv-able networking. p-Cycles simultaneously provide the switching speed and simplicity of rings with the much greater capacity-efficiency and flexibility for reconfiguration of a mesh network. p-Cycles also permit shortest-path routing of working paths (as opposed to ring-constrained working path routing), which adds further to network capacity efficiency. Operationally p-cycles are similar to BLSRs in that, upon failure, switching actions are required at only two nodes and both those nodes are fully pre-planned as to the actions that are required for any failure detected at their sites. With the optimization models in this chapter, entire survivable transport networks can be easily designed with essentially the same spare to working capacity (redundancy) ratios as optimized span-restorable mesh networks. p-Cycles thus bridge the ring versus mesh debate that dominated work in survivable networks through the 1990s and provide the best of both worlds: the efficiency of mesh with the speed of rings.

CAPEX Costs of Lightly Loaded Restorable Networks under a Consistent WDM Layer Cost Model

Shared protection promises the benefits of lower network capacity utilization without sacrificing the availability level of dedicated protection. Shared protection is often evaluated in terms of its spare capacity efficiency, but seldom in terms of real-world CAPEX (capital expenditure) costs. In this paper, we investigate the design of several shared protection architectures under a standardized cost model for the WDM layer, developed recently within the European NOBEL project. This paper presents a comparative study of the implementations and costs of these architectures under this model. Findings show a counterintuitive relationship between network capacity utilization and design cost when the network is lightly loaded.