Flexgrid technology is now considered to be a promising solution for future high-speed network design. In this context, we need a tutorial that covers the key aspects of elastic optical networks. This tutorial paper starts with a brief introduction of the elastic optical network and its unique characteristics. The paper then moves to the architecture of the elastic optical network and its operation principle. To complete the discussion of network architecture, this paper focuses on the different node architectures, and compares their performance in terms of scalability and flexibility. Thereafter, this paper reviews and classifies routing and spectrum allocation (RSA) approaches including their pros and cons. Furthermore, various aspects, namely, fragmentation, modulation, quality-of-transmission, traffic grooming, survivability, energy saving, and networking cost related to RSA, are presented. Finally, the paper explores the experimental demonstrations that have tested the functionality of the elastic optical network, and follows that with the research challenges and open issues posed by flexible networks.

Bandwidth fragmentation, a serious issue in elastic optical networks (EONs), can be suppressed by proper management in order to enhance the accommodated traffic demands. In this context, we need an in-depth survey that covers bandwidth fragmentation problems and how to suppress them. This survey paper starts with the basic concept of EONs and their unique properties. The paper then moves to the fragmentation problem in EONs.We discuss and analyze the major conventional spectrum allocation policies in terms of the fragmentation effect in a network. The taxonomies of the fragmentation management approaches are presented along with different node architectures. Subsequently, this paper reviews state-of-the-art fragmentation management approaches. Next, we evaluate and analyze the major fragmentation management approaches in terms of blocking probability. Finally, we address the research challenges and open issues on fragmentation problem in EONs that should be addressed in future research.

One of the challenging issues in optical networks is call blocking and it increases with the number of connection requests due to the limited number of wavelength channels in each fiber link. In this paper, we propose a priority based routing and wavelength assignment scheme with incorporation of a traffic grooming mechanism (PRWATG) to reduce call blocking. In this scheme, the connection requests having the same source–destination (s–d) pair are groomed first to avoid intermediate optical–electrical–optical conversation and then these groomed connection requests are served for routing and wavelength assignment according to their priority order. The priority order of each groomed connection request is estimated based on type of path (direct link physical path or indirect link physical path) first and then the traffic volume. If the priority order of connection requests is estimated using these criteria, blocking of connection requests due to wavelength continuity constraints can be reduced to a great extent, which will in turn lead to better performance of the network in terms of lower blocking probability and congestion. The performance analysis of our proposed scheme is made in terms of blocking probability and congestion and compared with a similar non-priority based routing and wavelength assignment scheme (NPRWATG). It is seen that using the PRWATG scheme, the blocking probability and the congestion of the network are significantly reduced compared to NPRWATG. It is also seen that the performance of the proposed scheme is better compared to NPRWATG when the number of connection requests increases in the network.

Hitless defragmentation has been introduced as an approach to limit the spectrum fragmentation in elastic optical networks without traffic disruption. It facilitates the accommodation of new requests by creating large spectrum blocks as it moves active lightpaths (retuning) to fill in gaps left in the spectrum by expired ones. Nevertheless, hitless defragmentation witnesses limitations for gradual retuning with the conventionally used first-fit allocation. The first-fit allocation stacks all lightpaths to the lower end of the spectrum. This leads to a large number of lightpaths that need to be retuned and are subject to interfering with each other’s retuning. This paper proposes a route partitioning scheme for hitless defragmentation in order to increase the admissible traffic in elastic optical networks. The proposed scheme uses route partitioning with the first-last fit allocation to increase the possibilities of lightpath retuning by avoiding the retuning interference among lightpaths. The first-last fit allocation is used to set a bipartition with one partition allocated with the first fit and the second with the last fit. Lightpaths that are allocated on different partitions cannot interfere with each other. Thus, the route partitioning avoids interference among lightpaths when retuning. We define the route partitioning problem as an optimization problem to minimize the total interference. We then introduce an integer linear programming problem (ILP) that yields an optimal routing and partitioning. We prove that the route partitioning problem is non-deterministic polynomial-time hard (NP-hard). We present a heuristic algorithm for large networks, where the ILP used to represent the route partitioning is not tractable. The simulation results show that the proposed route partitioning scheme using the first-last fit outperforms the conventional first-fit allocation for hitless defragmentation in term of allowable traffic.

For the past several years, traffic especially, Internet traffic has been increased tremendously and this growth demands for a high-speed communication network such as optical network. Routing and Wavelength Assignment (RWA) is one of the key tasks for the optical network, due to its information transparency and wavelength reuse characteristics. In this paper, we discuss different conventional RWA approaches in the wavelength-routed optical networks and compare them with our proposed Priority-based RWA (PRWA) scheme under wavelength continuity constraint. The performance analysis of different conventional RWA approaches including our PRWA scheme is conducted in terms of blocking probability. Finally, we address a number of research issues that appear to us to be both important and challenging.

Traffic in optical backbone networks is evolving rapidly in terms of type, volume, and dynamicity following the rapid growth of cloud-based services, ongoing adoption of 5G communications, and explosion of the Internet of Things (IoT). The elastic optical network (EON), by adopting a flexible grid, can provide the required capacity and flexibility to handle these rapid changes. However, operators rarely perform greenfield deployments, so to limit upfront investment, a gradual migration from fixed-grid to flexible-grid switching equipment is preferable. For gradual migration, switching nodes can be upgraded (starting from bottleneck network links) while keeping the rest of the traditional fixed-grid network operational. We refer to the coexistence of fixed-grid and flex-grid optical equipment as a “mixed-grid” network. Traditional algorithms for dynamic resource assignment in EONs will not effectively be applicable in a mixed-grid network due to interoperability issues among fixed- and flex-grid nodes. In this study, we propose a new algorithm, called Mixed-grid-aware Dynamic Resource Allocation, to solve the route, spectrum, and modulation-format allocation problem in a mixed-grid network while considering interoperability constraints. Our numerical results (on representative network topologies) show that the proposed method achieves 50% less blocking (for 50% offered load) compared to the traditional approach.