Flexible Grid Optical Networks Research Articles

A programmable optical few wavelength source for flexgrid optical networks

Abstract Multi-wavelength (MW) sources will probably replace discrete lasers or laser arrays in next generation multi-carrier transponders (e.g., 1 Tb/s), currently called multi-flow transponders or sliceable bandwidth variable transponders (SBVTs). We present design and experimental demonstration of a few wavelength (FW) source suitable for SBVTs in a flexgrid scenario. We refer to FW instead of MW since for an SBVT just few subcarriers are required (e.g., eight). The proposed FW source does not require optical filtering for subcarrier modulation. The design exploits frequency shifting in IQ modulators by using single side band suppressed carrier modulation. A reasonable number of lines can be provided depending on the chosen architecture, tunable in the whole C-band. The scheme is also capable of providing symmetric (equally spaced) and asymmetric subcarrier spacing arbitrarily tunable from 6.25 GHz to 37.5 GHz. The control on the number of subcarriers (increase/decrease depending on line rate) provides flexibility to the SBVT, being the spacing dependent on transmission parameters such as line rate or modulation format. Transmission performance has been tested and compared with an array of standard lasers considering a 480 Gb/s transmission for different carrier spacing. Additionally, an integrable solution based on complementary frequency shifter is also presented to improve scalability and costs. The impact on transceiver techno-economics and network performance is also discussed.

Computing approximate blocking probability of inverse multiplexing and sub-band conversion in the flexible-grid optical networks

With the rapid growth of data rate, the optical network is evolving from fixed-grid to flexible-grid to provide spectrum-efficient and scalable transport of 100Gb/s services and beyond. Also, the deployment of wavelength converter in the existing network can increase the flexibility of routing and wavelength allocation (RWA) and improve blocking performance of the optical networks. In this paper, we present a methodology for computing approximate blocking probabilities of the provision of multiclass services in the flexible-grid optical networks with sub-band spectrum conversion and inverse multiplexing respectively. Numerical calculation results based on the model are compared to the simulation results for the different cases. It is shown that the calculation results match well with the simulation results for the flexible-grid optical networks at different scenarios.

Link Design and Legacy Amplifier Limitation in Flex-Grid Optical Networks

Flex-grid technology is an effective means to improve the spectral efficiency of optical communication. For a given amplifier spectral bandwidth, it gives rise to the increase in the number of optical channels as it reduces channel spacing. Therefore, in order to reap full benefits from the flex-grid saved spectrum, further amplification power is required with respect to conventional fixed grid. This is a strong limitation if the legacy amplifiers cannot meet this new requirement due to their optical power limits. In this paper, we demonstrate that exploiting the link margins allows the support of this increase while maintaining in use legacy amplifiers.

First Demonstration of Virtual Transport Network Services With Multi-layer Protection Schemes over Flexi-grid Optical Networks

Based on the virtualization technique in optical networks, virtual transport network services (VTNS) has been introduced as a new kind of service in transport networks. With VTNS, transport providers are able to provide their customers entire virtual topologies instead of traditional end-to-end connections. This letter analyzes the new characteristics (i.e., multiple operators, diversified services) aiming at the survivability problem in virtualized optical transport networks, and proposes three novel protection schemes for VTNS against the new features. For the first time, VTNS with the proposed protection schemes are demonstrated on the flexi-grid optical networks testbed with commercial optical transport equipments. Additionally, the performances of the proposed protection schemes have been evaluated via simulations.

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.

Dynamic traffic grooming with Spectrum Engineering (TG-SE) in flexible grid optical networks

Flexible grid has emerged as an evolutionary technology to satisfy the ever increasing demand for higher spectrum efficiency and operational flexibility. To optimize the spectrum resource utilization, this paper introduces the concept of Spectrum Engineering in flex-grid optical networks. The sliceable optical transponder has been proposed to offload IP traffic to the optical layer and reduce the number of IP router ports and transponders. We discuss the impact of sliceable transponder in traffic grooming and propose several traffic-grooming schemes with Spectrum Engineering (TG-SE). Our results show that there is a tradeoff among different traffic grooming policies, which should be adopted based on the network operator’s objectives. The proposed traffic grooming with Spectrum Engineering schemes can reduce OPEX as well as increase spectrum efficiency by efficiently utilizing the bandwidth variability and capability of sliceable optical transponders.

Resource Allocation for Flexible-Grid Optical Networks With Nonlinear Channel Model [Invited

In this paper, we study the joint resource allocation in flexible-grid networks based on a nonlinear physical layer impairment model. An optimization problem is formulated to assign resources and guarantee the signal quality for every channel. Compared with the resource allocation in a fixed-grid wavelength-division multiplexing scenario, our method achieves significant bandwidth reduction and transmission distance extension in flexible-grid networks. The maximum spectrum usage is shown to be insensitive to the ordering of channels. We also analyze the relation between modulation formats and transmission distance based on the results of the proposed method. Finally, we demonstrate the performance and scalability of the proposed algorithm in ring networks.

Differential evolution optimization applied to the routing and spectrum allocation problem in flexgrid optical networks

Flexible optical network (FON) architectures are considered a very promising solution where spectrum resources are allocated within flexible frequency grids. Routing and spectrum allocation (RSA) in FON is an NP-complete problem. So far, this problem has been optimally solved for small instances with integer linear programming and has been suboptimally solved for more realistic instances by heuristic strategies. In this paper, we introduce the application of differential evolution (DE) to the off-line RSA problem in flexible optical networks. To the best of our knowledge, our work is the first application of a DE algorithm to the RSA problem. We develop two DE permutation-based algorithms named DE general approach (DE-GC) and DE relative position index (DE-RPI). Comparative studies show that in many cases, DE outperforms many other well-known evolutionary computational approaches. Furthermore, the method typically requires few control parameters. An illustrative example of the application of the DE-based algorithms is presented, and then, different heuristics are compared against the DE-RSA algorithms. Algorithms are evaluated in different test bench optical networks, such as the NSFnet and the European optical network, and for networks up to 40 nodes, such as the USA and Japan networks. Moreover, the DE-based algorithms save up to 37 % of spectrum utilization for the NSFnet and 13 and 15 % for the USA and Japan networks, respectively.

Baseband unit cloud interconnection enabled by flexible grid optical networks with software defined elasticity

The evolution toward 5G mobile networks is characterized by supporting higher data rate, excellent end-to-end performance and ubiquitous user-coverage with lower latency, power consumption, and cost. To support this, the RANs are evolving in two important aspects. One aspect is “cloudification,” which is to pool baseband units to be centralized for statistical multiplexing gain. The other aspect is to use advanced optical technologies for digital and analog signal transmission in a cloud-based RAN. In this article, we focus on BBU cloud interconnection with optical layer technologies. Flexible grid optical networks with the enabling technologies are introduced to provide elastic, transparent, and virtualized optical paths between the BBU pools. To improve the elasticity and intelligence of C-RAN, we propose a software defined centralized control plane to coordinate heterogeneous resources from three domains: the BBU domain, radio domain, and optical domain. We report an experimental demonstration of elastic lightpath provision for cloud radio-over-flexible grid optical networks in a software-defined-networking-based testbed.

Energy efficiency of elastic frequency grids in multilayer IP/MPLS-over-flexgrid networks

Flexgrid Optical Networks provide higher spectrum efficiency and flexibility in comparison to traditional wavelength switched optical networks (WSON) due to the finer granularity in managing communication channels at the WDM layer, thus allowing beyond 100Gbps transmission capabilities. They also promise allocating part of the traffic aggregation function to the optical layer, thus reducing the use of energy-hungry electronic devices. Accordingly, in this work, we analyzed these architectures from the power consumption perspective by evaluating the energy efficiency derived from introducing the Flexgrid technology in order to allow the shift of grooming and bandwidth aggregation functions at the optical layer, by envisioning a next generation of IP/MPLS-over-flexgrid transport architectures.

Improving Link Spectrum Utilization in Flexgrid Optical Networks

Flexible grid optical networks are emerging as an appealing solution for future high-speed optical transport because they can provide superior flexibility and scalability and cope with the rapid growth of Internet traffic. This work introduces some simple strategies that help improve spectrum usage in flexgrid optical networks based on limiting the bandwidth flexibility of the requested connections. Link performance improvement is demonstrated by adapting fully elastic optical networks to multibitrate optical networks where only some connection bandwidth values are allowed. With the aim of avoiding useless spectrum voids, the set of bandwidth values that connections can take is appropriately adjusted. By limiting the possible connection sizes, it is analytically demonstrated that some gain is obtained at the cost of losing some flexibility. Simulation results obtained at the link as well as at the whole network level assess the advantages anticipated by the theoretical study.

Minimizing Energy and Cost in Fixed-Grid and Flex-Grid Networks

Core networks offer high capacities, thanks mainly to the optical technologies they utilize, but they consume a non-negligible amount of energy. The traffic volume in metro and core networks is forecast to grow at very high rates, exceeding 30% per year for the next five years, and if the corresponding energy requirements grow analogously, they will sooner rather than later form a bottleneck for network communications. Thus, energy efficiency in optical networks is mandatory for the sustainability of the future Internet. The objectives of the current work are to identify the main causes of energy consumption for current fixed-grid wavelength division multiplexing and future flex-grid optical networks, and to propose and compare techniques for improving their energy efficiency. Toward this end, we carried out a comparative study of energy efficiency of flex-grid networks and fixed-grid single-line-rate and mixed-line-rate networks. Under realistic network scenarios, we calculated the energy consumption of the different components of the optical layer and demonstrated that by using energy-aware techniques in planning such networks, we can achieve significant power savings. Since energy prices are location dependent, especially in large networks, e.g., over continents, we show that accounting for such information can increase the cost savings.

Spectrum fragmentation issue in flexible optical networks: analysis and good practices

Flexible grid optical networks allow an efficient utilization of spectrum resources using 12.5-GHz frequency slot multiples instead of a fixed spacing, introducing however spectrum fragmentation (SF). In the literature, SF is often assumed to be a serious problem specifically in a dynamic traffic context. It is mostly related to the bandwidth blocking ratio due to the lack of relevant comparison criteria and efficient metrics. Besides, in operator core network, traffic behavior is instead incremental and it is forecasted for short periods of time in addition to some operational constraints that make of it a specific context. In this work, we present an exhaustive analysis and an accurate evaluation for SF issue in flexible optical networks. We also propose new metric for fragmentation measurements and some approaches to address such a problem.

Spectral Efficiency Optimization in Flexi-Grid Long-Haul Optical Systems

Flexible grid optical networks allow a better exploitation of fiber capacity, by enabling a denser frequency allocation. A tighter channel spacing, however, requires narrower filters, which increase linear intersymbol interference (ISI), and may dramatically reduce system reach. Commercial coherent receivers are based on symbol by symbol detectors, which are quite sensitive to ISI. In this context, Nyquist spacing is considered as the ultimate limit to wavelength-division multiplexing (WDM) packing. In this paper, we show that by employing a limited-complexity trellis processing at the receiver, either the reach of Nyquist WDM flexi-grid networks can be significantly extended, or a denser-than-Nyquist channel packing [i.e., a higher spectral efficiency (SE)] is possible at equal reach. By adopting well-known information-theoretic techniques, we design a limited-complexity trellis processing and quantify its SE gain in flexi-grid architectures where wavelength selective switches over a frequency grid of 12.5 GHz are employed.

Overarching Control of Flexi Grid Optical Networks: Interworking of GMPLS and OpenFlow Domains

Overarching Control of Flexi Grid Optical Networks: Interworking of GMPLS and OpenFlow Domains

Overarching Control of Flexi Grid Optical Networks: Interworking of GMPLS and OpenFlow Domains

Optical transport networks provide transport, multiplexing, routing, management, supervision, and survivability of optical channels. Within a flexible dense wavelength division multiplexing grid, the optical spectrum can be allocated in multiples of a width granularity, depending on the client signal rate and modulation format. A control plane (CP) can be used for efficient and dynamic provisioning and recovery of flexi-grid connections. Two main CP architectures coexist, with common functions like addressing, automatic topology discovery, network abstraction, path computation, and connection provisioning: a distributed generalized multiprotocol label switching CP (with optional path computation element, PCE path computation and instantiation/modification) and a CP based on software-defined networking, with a logically centralized controller and an open protocol, such as the OpenFlow protocol. Both architectures have their own strengths and weaknesses, and are being extended to address the new requirements associated with the aforementioned emerging optical technologies, such as flexible spectrum allocation, efficient corouted connection setup, and configuration of related optical parameters. However, new use cases such as remote data center interconnection highlight the need for multidomain service provisioning, and heterogeneous CP interworking, potentially requiring an overarching control. Different alternatives, with varying degrees of integration and flexibility, are available: straightforward approaches characterized by the adaptation of one control model to the other or more advanced interworking requiring the definition of common models (e.g., a subset of attributes for network elements) and of coordination and orchestration functions. This paper discusses the main relevant interworking architectures and presents a selected set of use cases and proof-of-concepts.

A joint multi-layer planning algorithm for IP over flexible optical networks

We consider the multi-layer network planning problem for IP over flexible optical networks, which consists of three subproblems at two layers: the Routing problem at the IP-layer (IPR), the routing, modulation level (RML), and the spectrum allocation (SA) problems at the optical layer. The input includes the IP end-to-end traffic matrix, the modular model of the IP/MPLS routers, and the feasible transmission configurations of the flexible optical transponders. Demands are served for their requested rates by selecting the IP/MPLS routers modules to be used, the routes in the IP (virtual) topology, and the corresponding paths and spectrum slots in the underlying optical topology, together with the optical transponders' configurations. The proposed algorithm follows a multi-cost approach that solves jointly the IPR, the RML, and the SA problems. It serves demands one-by-one, reusing existing equipment and favoring the deployment of new equipment that could also be reused by subsequent connections, aiming to minimize the total network cost. The problem definition is generic and the proposed algorithm is applicable to both fixed- and flex-grid optical networks. We evaluate the performance gains that can be obtained by the proposed joint multi-layer network planning solution, as opposed to a sequential planning solution that separately plans the IP and optical layers. We also compare a flexible network, using flex-grid optical switches and flexible optical transponders, to a mixed line rate (MLR) network, using fixed-grid or flex-grid optical switches but fixed optical transponders.

Time-aware Software Defined Networking for OpenFlow-based Datacenter Optical Networks

Data center networks are considered to make use of the computing and storage resources in data centers, which include intra-datacenter and inter-datacenter networks. Both of them will depend on the optical networking due to its advantages, such as low latency, high bandwidth, and low energy consumption. Data center interconnected by flexi-grid optical networks is a promising scenario to allocate spectral resources for applications in a dynamic, tunable and efficient control manner. Due to the high burstiness and high-bandwidth characteristics of the services, optical interconnect in intra-datacenter networks has attracted much attention compared with inter-datacenter network. Many datacenter applications in the environment require lower delay and higher availability with the end-to-end guaranteed quality of service. In this paper, we propose a novel time-aware software defined networking (TaSDN) architecture for OpenFlow-based datacenter optical networks, by introducing a time-aware service scheduling (TaSS) strategy. TaSDN can arrange and accommodate the applications with required QoS considering the time factor, and enhance the responsiveness to quickly provide for datacenter demand. The overall feasibility and efficiency of the proposed architecture are experimentally verified on our testbed with OpenFlow-based intra-datacenter and inter-datacenter optical networks.

Reoptimization of Dynamic Flexgrid Optical Networks After Link Failure Repairs

In dynamic flexgrid optical networks, the usage of capacity may not be optimal due to the permanent process of setting up and tearing down connections, which, if not controlled, leads to spectrum fragmentation and, as a result, to increase of connection blocking. On top of this, a restoration mechanism that is launched in reaction to a link failure (cable cut) restores the affected lightpaths. Eventually, when the cable is repaired and its capacity becomes available for new connections, the unbalance between lightly and heavily loaded links increases, thus further decreasing the probability of finding optical paths with continuous and contiguous spectrum for future connection requests. In this paper we study the effects of reoptimizing the lightpath connections after a link failure has been repaired [namely, the after-failure-repair optimization (AFRO) problem] as an effective way for both reducing and balancing capacity usage and, by these means, for improving network performance. To solve AFRO a column generation decomposition method is presented. Illustrative numerical results show that AFRO allows us to significantly decrease the request blocking probability in realistic dynamic network scenarios. Moreover, the proposed column generation algorithm delivers quasi-optimal solutions in reasonable times. Besides, traffic disruptions resulting from lightpath rerouting are practically negligible. Finally, we show that it is sufficient to apply AFRO only for a selected set of link failures in order to achieve high network performance.

Virtual Network Provisioning Over Distance-Adaptive Flexible-Grid Optical Networks [Invited

In software-defined optical networks (SDONs), network services are provided in terms of virtual optical networks (VONs). VON provisioning requires additional considerations regarding their optical characteristics and constraints compared to virtual networks offered in the electrical domain. In this paper, we present a new VON provisioning procedure that is specifically designed for distance-adaptive flexible-grid optical networks. Simulations show up to three times increase in network capacity by leveraging the combined effect of flexible node mapping and distance-adaptive modulation.