Hitless spectrum defragmentation in flexible grid optical network using maximum ratio combining in wavelength diversity transmission

Spectrum engineering in flexible grid data center optical networks

Service differentiation-based spectrum defragmentation in mixed grid optical networks

With the emergence of cloud services based on data centers, demands for bandwidth-intensive applications have increased dramatically, and application services have transferred to a more diversified direction. Management as well as capacity of the backbone network needs further development to catch up with rapidly evolved application demands. Optical network virtualization can facilitate the sharing of physical infrastructure among multiple network applications. Virtual Network Embedding (VNE), the main implementation of network virtualization, determines how to map a virtual network request onto physical substrate. To expand the network capacity, flexible-grid elastic optical networks have been considered as a promising supporting technology for the future infrastructure of the next-generation Internet. However, due to the expense of key enabling equipment for flexible grid optical networks, the brown-field migration from a fixed grid to a flexible grid gave birth to the co-existing fixed/flexible grid. Based on the co-existing fixed/flexible grid optical networks, we investigate the problem of Virtual Optical Network (VON) provisioning, and present a flexible-grid-aware virtual network embedding algorithm to map the virtual networks onto the substrate network. In addition, the performance of the algorithm was evaluated under four different network scenarios. Simulation results show that the proposed algorithm can achieve better performance in all four scenarios.

We propose a novel distributed path-vector routing scheme in flexible grid optical networks. This scheme further exploits the flexibility of flexible grid optical networks through the detection and adaptation of available bandwidth with polynomial complexity.

We propose a novel distributed path-vector routing scheme in flexible grid optical networks. This scheme further exploits the flexibility of flexible grid optical networks through the detection and adaptation of available bandwidth with polynomial complexity.

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 orderto 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.

Using Optical Orthogonal Frequency Multiplexing (O-OFDM), variable bandwidth channels can be created in Elastic Optical Networks (EON). This allows the use of spectrum more efficiently by allocating integral multiple of basic bandwidth slots to the lightpath requests. Consequently, such networks are also called flexible grid optical networks. It also adds a constraint of keeping all the allocated slots together when deciding the routes for the requests. This constraint called the contiguity constraint makes the routing and spectrum algorithms more challenging. In any network, the lightpath requests will arrive and depart dynamically and will invariably lead to spectrum fragmentation, and hence network will have a reduction in maximum possible utilization due to increased blocking probability. In this paper, we have presented an improvised RSA algorithm that leads to lesser fragmentation. It is evident from the results that the presented RSA algorithm uses adaptive parameters to reduce the blocking probability and fragmentation compared to other algorithms reported in the recent past.

Due to crosstalk-induced interactions among different connections, malicious high-power jamming signals can potentially spread widely in a transparent optical network. Moreover, due to imperfect port isolation in wavelength selective switches (WSSs), present within optical switching nodes, crosstalk also affects the quality of the transmitted signal. Therefore, it is necessary to design an optical network in a way that the effect of crosstalk is minimized, while at the same time keeping the cost and the power consumption of the network low. This is achieved in this work by the design of appropriate WSS placement and crosstalk-aware Routing and Spectrum Assignment (RSA) algorithms in flexible grid optical networks, in the form of an Integer Linear Program (ILP) formulation and a heuristic algorithm analogous to vertex coloring. The objective of the optimization algorithms is to minimize the impact of the crosstalk effect, thus minimizing the impact to the normal operation of the network. The optimization objective is enhanced with proper functions in order to minimize the capital expenditure (CAPEX) and the operational expenditure (OPEX) of the networks investigated in terms of cost and power consumption respectively. Performance results indicate that the proposed algorithms minimize the number of WSSs required to compensate for the crosstalk effect, while only slightly increasing the spectrum utilization.

Flexible spectrum ROADMs facilitate networks to support channels operating at heterogeneous line rates by allocating spectral resources in a dynamic and flexible manner leading to better Spectral Efficiency. In a realistic network, traffic scenario is very dynamic in nature leading to continuous tear down of existing demands and set up of new demands. This dynamic tear down and set up process leads to fragmentation of the network, in terms of spectral resources, called “Spectral fragmentation” resulting in the blocking of new demands. Therefore, an optimization mechanism handling spectrum fragmentation, called “Spectrum Defragmentation”, is of vital importance in flexgrid optical networks. Defragmentation of the spectral resources can be obtained by either changing wavelength of the demand or route of the demand or both. This paper discusses a novel defragmentation algorithm considering only the wavelength tunability criterion and performs defragmentation by moving as minimal existing traffic demands as possible in order to support the new demand.

Network virtualization over WDM and flexible-grid optical networks

The rapid escalation of Internet traffic and increasingly heterogeneous service demands impose stringent requirements on optical networks for dynamic resource scheduling, efficient spectrum utilization, and automated operation. Elastic optical networks (EONs) are regarded as a promising solution, yet their performance remains constrained by two critical challenges: routing, modulation format, and spectrum allocation (RMSA) and spectrum defragmentation (SD). Existing approaches predominantly focus on optimizing one of these tasks, which may lead to limited adaptability and suboptimal network efficiency. To address this gap, we propose an asynchronous cooperative multi-agent deep reinforcement learning framework, termed MADRL-JRASD, for the joint optimization of RMSA and proactive SD. The framework incorporates an RMSA agent with dynamic allocation capability and an SD agent with autonomous decision-making ability, coordinated through an asynchronous architecture that enables adaptive responses to environmental changes. Invalid-action masking and carefully designed reward functions are further integrated to enhance training stability and convergence. Comprehensive evaluations over three representative topologies demonstrate that MADRL-JRASD reduces the blocking probability by up to 81% compared with RMSA heuristics without SD and achieves an 85% reduction in overhead relative to heuristic algorithms combining RMSA and SD that attain similar blocking performance. Moreover, the sensitivity analysis shows that the SD agent improves spectrum utilization and that multi-agent cooperation enhances global decision coordination, while action masking and reward design jointly strengthen the convergence and efficiency of MADRL-JRASD.

In future optical networks, versatile functionalities will be required for the optical network subsystems to fully utilize the spectral resources with low energy consumption. The key technologies are the spectral efficient MUX/DEMUX technique and flexible control of optical channels with high frequency granularity. An orthogonal frequency division multiplexing (OFDM) and Nyquist wavelength division multiplexing (N-WDM) are the most promising candidates of spectral efficient multiplexing techniques, and all-optical (AO) processing is expected to reduce the energy consumption. In an AO-OFDM systems, discrete Fourier transform (DFT) and inverse DFT (IDFT) are performed in optical domain by specially designed arrayed waveguide gratings (AWGs). In our experiment, 12.5 GHz spaced AO-OFDM system has been successfully demonstrated with no guard interval. In N-WDM systems, the Nyquist signal is generated by using carrier-suppressed return-to-zero (CS-RZ) signal and optical Nyquist filtering, which is achieved with two flat-top AWGs and optical interleaver, and the 25 Gbaud signals are successfully multiplexed in the experiment. Although both AO-OFDM and N-WDM can achieve the highest spectral efficiency, N-WDM is more suitable for flexible optical networks. This is because the N-WDM channels have less spectral overlap with the other channels than AO-OFDM, owing to its rectangular shaped compact spectrum. Therefore, N-WDM channel can be easily multiplexed and demultiplexed by optical filters. At an optical network node, channel defragmentation is indispensable technology to flexibly control the optical channels. We have experimentally demonstrated a format independent optical channel defragmentation with N-WDM signal. We believe these technologies are promising for future flexible optical networks.

Emerging services, such as high-definition Internet Protocol TV (IP-TV) or data center migration, are going to increase the amount of multicast traffic in the Internet. The support of multicast directly in the optical domain, instead of at the IP layer, is a target for reducing the amount of optical–electronic–optical conversions (thus, the network operational and capital expenditure) and energy consumption. In parallel, flex-grid technology (e.g., bandwidth variable wavelength selective switches) is emerging as a candidate solution to be adopted in future optical transport networks given its capacity of improving spectrum efficiency. This paper is focused on optical multicast in flex-grid optical networks and on its control through the Path Computation Element (PCE). First, we present two node architectures supporting optical multicast. The first node architecture achieves optical multicast through passive light split and requires that the multicast connection satisfies the spectrum continuity constraint. The second node architecture achieves optical multicast with frequency conversion. In particular, a specific implementation of the second architecture is proposed in this paper exploiting a periodically poled lithium niobate (PPLN) waveguide. Then, a PCE architecture to control optical multicast (with and without frequency conversion) is proposed. Optical multicasting, based on the proposed node architectures, at 100 and 200  Gb/s is experimentally demonstrated in a flex-grid network testbed. In particular, multicasting is demonstrated with 112  Gb/s polarization multiplexing 16 quadrature amplitude modulation (PM-16QAM) and polarization multiplexing quadrature phase shift keying (PM-QPSK), and with 224  Gb/s PM-16QAM considering the light-split node architecture. Moreover, optical multicast with two frequency conversions, achieved in a single PPLN device, is demonstrated for the first time with a 224  Gb/s PM-16QAM signal. The testbed also includes the PCE, which is extended to control optical multicast in flex-grid optical networks.

The routing and spectrum assignment (RSA) is one of the key problems in flexible optical networks. When in a gridless fashion, the shortest-path RSA algorithms have exponential computational complexity, and are thus not suitable to be employed in real networks. On the other hand, today most optical components cannot support fully gridless tunability, which also limits the application of gridless RSA schemes. In this paper, we propose a novel grid-based spectrum-scan routing (SSR) scheme in flexible optical networks. The SSR scheme achieves optimal routing with a polynomial computational complexity. Compared with other RSA schemes, SSR has shorter computation time, lower blocking probability, and higher resource utilization.

Flexgrid optical networks are attracting huge interest due to their higher spectrum efficiency and flexibility in comparison with traditional wavelength switched optical networks based on the wavelength division multiplexing technology. To properly analyze, design, plan, and operate flexible and elastic networks, efficient methods are required for the routing and spectrum allocation (RSA) problem. Specifically, the allocated spectral resources must be, in absence of spectrum converters, the same along the links in the route (the continuity constraint) and contiguous in the spectrum (the contiguity constraint). In light of the fact that the contiguity constraint adds huge complexity to the RSA problem, we introduce the concept of channels for the representation of contiguous spectral resources. In this paper, we show that the use of a pre-computed set of channels allows considerably reducing the problem complexity. In our study, we address an off-line RSA problem in which enough spectrum needs to be allocated for each demand of a given traffic matrix. To this end, we present novel integer lineal programming (ILP) formulations of RSA that are based on the assignment of channels. The evaluation results reveal that the proposed approach allows solving the RSA problem much more efficiently than previously proposed ILP-based methods and it can be applied even for realistic problem instances, contrary to previous ILP formulations.