Eleni Palkopoulou

A tutorial on the flexible optical networking paradigm: State of the art, trends, and research challenges

Rigid fixed-grid wavelength division multiplexing (WDM) optical networks can no longer keep up with the emerging bandwidth-hungry and highly dynamic services in an efficient manner. As the available spectrum in optical fibers becomes occupied and is approaching fundamental limits, the research community has focused on seeking more advanced optical transmission and networking solutions that utilize the available bandwidth more effectively. To this end, the flexible/elastic optical networking paradigm has emerged as a way to offer efficient use of the available optical resources. In this work, we provide a comprehensive view of the different pieces composing the “flexible networking puzzle” with special attention given to capturing the occurring interactions between different research fields. Only when these interrelations are clearly defined, an optimal network-wide solution can be offered. Physical layer technological aspects, network optimization for flexible networks, and control plane aspects are examined. Furthermore, future research directions and open issues are discussed.

Quantifying spectrum, cost, and energy efficiency in fixed-grid and flex-grid networks [Invited]

Single and multi-carrier networks offering channel rates up to 400 Gb/s are evaluated under realistic reach parameters. It is found that efficient spectrum utilization and fine bit-rate granularity are essential to achieve cost and energy efficiency. Additionally, the break-even cost of flexible orthogonal frequency division multiplexing transponders is examined under different settings. The break-even cost of a flexible transponder corresponds to the cost value for which the total cost of the network is equal to that of the related single-line-rate network. The impact of the traffic load, the additional cost required for flex-grid optical cross connects, the cost of spectrum, as well as the cost of fixed-grid transponders is examined.

Energy efficiency and CAPEX minimization for backbone network planning: Is there a tradeoff?

Energy efficiency and the minimization of capital expenditures (CAPEX) for network equipment are studied for two network architectures: IP over WDM and IP over OTN over WDM. Case study results from mixed integer linear programming (MILP) formulations indicate that a CAPEX optimized network yields solutions having minimum energy consumption for both architectures. However, it is shown that the most cost efficient architecture is not always the least energy consuming. We quantify the occurring tradeoff and observe that network equipment CAPEX are dominating in terms of cost. Additionally, it is observed that the relative power contribution of different network layers is independent of the average inter-node traffic demand for both studied architectures.

Nyquist-WDM-Based Flexible Optical Networks: Exploring Physical Layer Design Parameters

We focus on Nyquist-WDM-based flexible optical networks and examine the impact of introducing flexibility in physical layer design parameters on the network level performance. The considered physical layer parameters include the modulation format, the symbol-rate, the inter-subcarrier spacing, the inter-superchannel spacing, and the launch power. In order to examine the impact of allowing the symbol rate to be tuned on a super channel basis, three alternative launch power schemes are considered: (i) optimum launch power, (ii) constant launch power, and (iii) constant power spectral density. Case study results from a realistic reference core network highlight the identified trade-offs with respect to the utilized spectrum and the required transponders. It is shown that significant savings can be achieved by allowing the symbol rate to be dynamically tuned under different launch power options.

Traffic models for future backbone networks – a service-oriented approach

In this paper we present a novel approach to assess the impact of new and existing services on traffic volume in current and future backbone networks. Several proposals to model traffic load in access and backbone networks exist in the literature. These proposals consider current Internet traffic like http, smtp, ftp and Peer-to-Peer (P2P). We expect, however, that there will be a change in traffic load for future networks caused by services like IP Television (IPTV), Video on Demand (VoD) and Virtual Private Networks (VPN). Additionally, population-based models may no longer be applicable due to the widespread use of service-providers and hierarchical routing through network peering points. Therefore, it is important to reassess future traffic volumes and traffic patterns and to identify those services that have the most impact on the networks. We model todays traffic volume of each of the described services and estimate future traffic volumes taking peering points into account. To illustrate the different traffic flows and to characterise the traffic distribution we apply our results to a Germany reference network. Copyright

Dynamic cooperative spectrum sharing and defragmentation for elastic optical networks

Flexible optical networks appear as the most prevalent candidates for next-generation core transport networks. The transition from a fixed to a flexible frequency grid calls for novel spectrum allocation techniques. This work focuses on the notion of dynamic cooperative spectrum sharing and addresses the issue of dynamic cooperative spectrum allocation for flexible optical networks. In order to cope with the dynamicity of the traffic demands, spectrum expansion/contraction (SEC) policies attempt to accommodate incoming requests by means of expanding/contracting the allocations of connections. When spectrum expansion is limited by neighboring connections, appropriate spectrum defragmentation (SD) policies undertake the task of reallocating connections in order to free up the spectrum. In this work, a novel cooperative SEC policy is examined, which takes into consideration the spectrum allocation of neighboring connections. Additionally, a class of cooperative SD policies is proposed, based on the capabilities of current technology. Simulation results from two reference core networks quantify the benefits that can be reaped in terms of blocking rate. Trade-offs with respect to the achieved reduction in blocking rate and the number of reallocated connections during the SD procedure are examined.

Quantifying CAPEX savings of homing architectures enabled by future optical network equipment

Alternative homing architectures can lead to significant cost reductions. These reductions are quantified using linear programming under a multi-layer consideration. In this study we focus on savings with respect to network equipment capital expenditures (CAPEX) achieved by transitioning to different homing architectures in an IP over optical data unit (ODU) setting. The impact of traffic demand is examined for different reference networks. In all conducted case studies we find that network equipment costs follow an approximately linear relationship with the traffic demand. Additionally, case study results assuming up to an aggressive 50% cost decrease in IP layer equipment, quantify the impact of variations in the cost of network elements.

Combining multi-period and multi-layer network planning: Ignored potential?

Multi-layer and multi-period network planning can lead to significant cost reductions. Using linear programming we study the effect of their joint consideration for IP-over-WDM networks. Case studies show cost savings surpassing 20% by including forecast knowledge in multi-layer optimization.

CAPEX and availability tradeoffs of homing architectures in multi-layer networks

A generic multi-layer model and a linear programming (LP) formulation are developed enabling the calculation of the optimal solution in terms of network equipment capital expenditures (CAPEX) in multi-homing design. Alternative homing architectures to traditional dual homing are studied not only with respect to network equipment costs but also regarding the achieved availability gain. Case study results show that significant CAPEX savings can be obtained by deploying a shared router strategy, which are independent from the traffic demand and deployed equipment.

Cognitive, Heterogeneous and Reconfigurable Optical Networks: The CHRON Project

High degree of heterogeneity of future optical networks, stemming from provisioning of services with different quality-of-transmission requirements, and transmission links employing mixed modulation formats or switching techniques, will pose a challenge for the control and management of the network. The incorporation of cognitive techniques can help to optimize a network by employing mechanisms that can observe, act, learn and improve network performance, taking into account end-to-end goals. The EU project CHRON: Cognitive Heterogeneous Reconfigurable Optical Network proposes a strategy to efficiently control the network by implementing cognition. In this paper we present a survey of different techniques developed throughout the course of the project to apply cognition in future optical networks.