The emerging software defined networking (SDN) paradigm separates the data plane from the control plane and centralizes network control in an SDN controller. Applications interact with controllers to implement network services, such as network transport with quality of service. SDN facilitates the virtualization of network functions so that multiple virtual networks can operate over a given installed physical network infrastructure. Due to the specific characteristics of optical (photonic) communication components and the high optical transmission capacities, SDN-based optical networking poses particular challenges, but holds also great potential. In this article, we comprehensively survey studies that examine the SDN paradigm in optical networks; in brief, we survey the area of software defined optical networks (SDONs). We mainly organize the SDON studies into studies focused on the infrastructure layer, the control layer, and the application layer. Moreover, we cover SDON studies focused on network virtualization, as well as SDON studies focused on the orchestration of multilayer and multidomain networking. Based on the survey, we identify open challenges for SDONs and outline future directions.

Software Defined Optical Networks (SDONs): A Comprehensive Survey

Modulation formats for 100G and beyond

This study investigates the survivable traffic grooming problem for elastic optical networks with flexible spectrum grid employing new transmission technologies. In such networks, instead of following the traditional fixed ITU-T wavelength grid, optical transponders are capable of properly tuning their rates, and consequently their spectrum occupation, by introducing the fine-granular spectrum unit, called a frequency slot. The number of contiguous frequency slots allocated to an optical path (i.e., lightpath) is adjusted to the current network flow. In this study, we propose a novel shared protection specific to elastic networks, namely, elastic separate-protection-at-connection (ESPAC). It not only provides traditional backup sharing, but also offers a new opportunity of spectrum sharing enabled by the elasticity of the transponders: 1) if the working paths of two connections are link disjoint physically, and 2) if their backup paths traverse two lightpaths which are adjacent on a fiber link, then the two backup lightpaths can share spectrum. The new opportunity of spectrum sharing is realized by using First-Fit to assign working traffic and Last-Fit to assign backup traffic, and allowing spectrum overlap between adjacent backup wavelengths. The elasticity of the transponder enables the expansion and contraction of the lightpaths, thus when a single failure occurs in the network, lightpaths carrying backup flows can be tuned to appropriate rates in such a way that the overlap spectrum is used by only one of the adjacent lightpaths. The results show ESPAC is very spectrum efficient in elastic network setting.

Survivable Traffic Grooming in Elastic Optical Networks—Shared Protection

We discuss the emerging rate-adaptable optical transmission technology and how this new technology may be employed to further reduce the transport network cost to meet ever growing bandwidth demand in the core network Two different types of transponders are considered: those adjusting either the transported bit rate (ie, client data rate) or the symbol rate (with a fixed bit rate) We propose a methodology for calculating the (normalized) cost to build out an entire long-haul transport network with several options for bit-rate-adaptable transponders By using link-length demands from an exemplary distance-diverse network, we demonstrate that time-domain hybrid-QAM-enabled fine-grain rate-adaptable transponders can reduce network cost by more than 20 percent within a traditional, fixed-bandwidth, wavelength-division-multiplexed grid We also argue that the total transponder expense using symbol-rate-adaptable technology will be greater than when using bit-rate-adaptable technology, as well as requiring more costly flex-grid ROADMs for channel routing

Rate-adaptable optics for next generation long-haul transport networks

This article provides motivation for the elastic optical network (EON) approach, an efficient and cost-effective solution for provisioning of cloud computing traffic. As opposed to wavelength switched optical networks (WSONs), the capabilities of which are limited by the use of rigid frequency grids, EON architectures allow for both scalable bandwidth provisioning and flexible resource allocation. The deployment cost, energy consumption, and bandwidth usage for both EON and classical WSON transport networks are compared in pan-European and U.S. backbone networks for 2012-2020 using Cisco traffic predictions. Results show that the EON concept significantly outperforms WSON in all examined criteria, and the gap between the two architectures increases in subsequent years. Moreover, potential advantages of anycast routing in transport networks with data center traffic are demonstrated.

On the advantages of elastic optical networks for provisioning of cloud computing traffic

Software-defined optics is used to configure the transponders in high-capacity communication systems. Reach, data rate, and spectral efficiency are three parameters which are traded-off to achieve an optimized network capacity.

Programmable modulation for high-capacity networks

A combined DPSK and 3-ASK modulation format for 100-Gb/s optical transport is reported as realizable at relatively low cost with currently available components. It is shown that metro and regional network transmission requirements are met.

Requirements for 100-Gb/s Metro Networks

Adaptation of the signal bandwidth or the data rate to the channel reach in optical networks is discussed. Network improvement is considered, and an implementation option is presented.

Recent progress in network equipment allows trade-offs to be made between the optical bandwidth, distance-reach, and bit-rate for given optical data channels. We discuss methods to increase the utilization of network resources and thereby to lower the network's cost-per-bit of transport.

Flexible bandwidth and bit-rate programmability in future optical networks

Spectrum-efficient optical transmission with bitrates of 400 Gb/s and beyond can be achieved using flexible modulation with advanced DSPs. The technology options include modulation format, signal baud rate, number of subcarriers, and spectral bandwidth. A fine-granular spectral bandwidth requires a flexible WDM grid as recently defined by ITU-T. Transmission of a signal with multiple optical carriers, each potentially with their own set of modulation options, allows bandwidth-variable multi-flow transceivers. This can reduce the spectrum continuity constraint in the network. At the network layer, these new degrees of freedom create additional levels of complexity and constraints during network design, planning and operation. Which modulation constellation should be chosen for a new optical connection? What are the impacts on transmission reach, spectrum continuity constraints, and network utilization? Routing and spectrum assignment is becoming more complex, and the inevitable spectrum fragmentation reduces the spectral efficiency gained though efficient modulation schemes. Dynamic spectrum defragmentation requires transceivers supporting hitless defragmentation, or it is traffic affecting for the reallocated signals. The sheer number of technology options will increase the operational complexity of the network. In this paper, we give an overview of technology options for software-defined transceivers for fixed-grid and flex-grid optical transport networks, and their impact for network planning and operation. We evaluate the spectral network efficiency, and operational complexity of selected technology options, such as multi-carrier transmission in fixed WDM grid, bandwidth-variable transponders with multiple subcarriers in a flexible WDM grid, and fully flexible multi-flow transponders. The evaluation is done based on a network planning study on a national European and US reference network. Based on the evaluation result, a guideline is given for a technology strategy with a good balance between flexibility, spectrum efficiency, and network cost.

Brian Teipen

Papers

Programmable modulation for high-capacity networks

Requirements for 100-Gb/s Metro Networks

Programmable modulation for high-capacity networks

Flexible bandwidth and bit-rate programmability in future optical networks

Evaluation of Technology Options for Software-Defined Transceivers in Fixed WDM Grid versus Flexible WDM Grid Optical Transport Networks