Table of Contents:
This section presents the related works on software-defined optical networks (SDONs), network virtualization, and different resource allocation schemes related to SD-EONs. The surveyed approaches are analyzed by clarifying their strong and weak points.
Software-Defined Optical Networks and Network Virtualization
To enhance the performance of optical networks, researchers have been incorporating SDN in optical networks. In this direction, Channegowda et al.  presented a combined OpenFlow based control plane architecture for optical SDN networks, including an abstraction technique for enabling OpenFlow devices. Their work focuses on implementing OpenFlow protocol extensions for emerging optical transport technologies. The work in  summarized the optical network models and their application for the SDN management purpose.
A.S. Thyagaturu et al.  presented a comprehensive survey on optical networks with SDN technology. The survey begins with the introduction of software-defined optical networks (SDONs), and then focuses on different layers of the SDON. They discuss network virtualization along with the orchestration of multi-layer and multi-domain networks. The work in  summarized the different virtual network allocation algorithms in EONs. A survey on control plane architectures of EONs was presented in .
The work in  presents a tutorial on EONs by focusing on different research areas, which are physical layer issues, network optimization, and control plane. An in-depth survey on the current developments of SDN technology, including its three-layer architecture, is provided in .
The performance of optical networks with or without SDN mainly depends on resource allocation schemes. The work in  provided a tutorial on RSA in EONs and its several aspects, which are modulation based quality-of-transmission, traffic grooming, fragmentation, networking cost related to RSA, energy saving, and survivability. The work in  presented an in-depth study on different existing RSA algorithms, and compared them in terms of resource management and computational complexity. S. Talebi et al.  presented a comprehensive study on spectrum allocation approaches for EONs, which analyzes and categorizes a variety of spectrum management policies, as well as fragmentation-aware RSA, distance-adaptive RSA, survivability, and traffic grooming. An exhaustive survey on spectrum management policies in EONs is available in , which deals with spectrum fragmentation issues. A comprehensive survey of resource allocation schemes that address different issues, namely the RSA problem in spatial mode, how to deal with crosstalk, and resource fragmentation, in space division multiplexing based optical networks was presented in [241 ].
A. Alyatama et al.  introduced an RSA algorithm using a learning approach, which is based on estimated call net gains, for EONs to enhance the network performance with respect to normalized lost revenue. The work in  introduced a dynamic impairment-aware spectrum allocation scheme for EONs to improve the traffic admissibility in the network.
Several studies [244-246] on the provisioning of multicast requests in WDM- based optical networks have been carried out to improve the resource utilization. Provisioning of multicast requests is still under investigation for EONs. Taking this direction, M. Moharrami et al.  presented a resource allocation and multicast routing scheme in EONs to suppress the call blocking. An integer linear programming (ILP) is formulated to execute both multicast routing and spectrum allocation.
The related work on SDN technologies and research issues and challenges for SD-EONs were presented in . The authors in  emphasized the latest deployment of elastic technology for core optical networks, where generalized multiprotocol label switching (GMPLS) is executed . An OpenFlow-based control plane for multilayer optical networks is presented by L. Liu et al.  to reduce the overall end-to-end delay.
The authors in  presented an optimization approach considering an SDN architecture for migrating data center services in EONs. In their scheme, the cross layer optimization and resource utilization are managed by SDN controllers according to physical layer parameters, like bandwidth demand and modulation technique. Lightpath setup and release are experimentally demonstrated through a testbed that consists with four OpenFlow-enabled EONs nodes in order to observe the call blocking and resource occupation rate.
The work in [251 ] described and experimentally verified online defragmentation by implying OpenFlow-assisted RSA for a single-domain SD-EON. To realize effective online defragmentation, the authors in  first designed the overall system by extending OpenFlow protocol and then experimented the de- fragmentation process that involved RSA reconfiguration. The effect of multi- domain fragmentation-aware RSA is evaluated in SD-EONs with the incorporation of OpenFlow controllers. They studied how fragmentation is managed on inter-domain links with fragmentation-aware RSA for SD-EONs.
An OpenFlow based EON architecture is demonstrated by N. Cvijetic et al. . The authors in  extended OpenFlow 1.0 to manage optical line terminal side, which allows the instantaneous downstream communication over bandwidth-variable flex-grid channels. The introduced scheme provides low latency, high-speed, and high quality services over fiber substructures.
To maximize the admissible traffic in the network, a control-plane framework that performs online defragmentation in SD-EONs is presented by S. Ma et al. . The work in  experimentally verified SDN over EONs for migrating data center services.
The authors in  demonstrated an optical channel monitoring scheme in SDN based EONs. To maintain the signal quality, the authors  select and adjust optical parameters dynamically.
An EON with a real-time adaptive control plane is validated by D.J. Geisler et al. ; an appropriate modulation format is adapted to maintain the signal quality. An SDN-based control scheme with defragmentation functionality was demonstrated to authenticate the overall possibility of extended OpenFlow messages and signal performance .
Note that the real deployment of the EON considering SDN is under development. We expect that fully deployed SD-EONs will be available in the near future.
Table 10.1: Summary of existing spectrum management approaches in SD-EONs.
Several studies on the different spectrum management approaches in SD- EONs have been stated in the literature, which are summarized in Table 10.1.