Implementing Available Bandwidth Detection and Adaption: A Spectrum-Resource-Diffusion Routing Scheme in Flexible Grid Optical Networks

Spectrum engineering in flexible grid data center optical networks

Service differentiation-based spectrum defragmentation in mixed grid optical networks

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.

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.

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.

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.

We propose a spectrum defragmentation system using maximum ratio combining in wavelength diversity digital coherent transmission. We evaluate the maximum ratio combining characteristics and experimentally show hitless defragmentation of a 128-Gb/s PDM-QPSK signal.

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.

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.

From an algorithmic perspective, planning and operating optical networks falls in the broad category of network optimization problems. We give a short introduction on algorithmic techniques that can be used to solve network optimization problems, emphasizing on linear optimization and heuristics. We present examples of applying these techniques to optimize resource allocation during the planning of optical networks. In particular, we focus on fixed-grid WDM networks, which is the current practice, and on flexible optical networks, considered as the most promising architecture for meeting next generation core and metro network requirements. In doing so, we describe a generic problem definition that can capture both types of networks in a unified manner.

Flexible optical networks, based on bandwidth-variable optical cross-connects (BV-OXCs) and novel flexible transponders, are expected to significantly improve the overall spectrum efficiency with respect to traditional networks where fixed frequency spacing is applied. Flexible optical networks will exploit the BV-OXC capability to dynamically configure the reserved bandwidth as a set of frequency slots. In addition, flexible transponders will be employed to dynamically configure transmission parameters, such as bit-rate and modulation format. To enable these new configuration capabilities, network operation enhancements need to be efficiently introduced and investigated. In this study, we focus for the first time on the Path Computation Element (PCE) architecture for flexible optical networks. PCE architecture and PCE communication protocol are enhanced to maximize the spectral efficiency and to provide indications also on the specific transmission parameters to configure. Experimental demonstration is provided through two different experiments, successfully showing the PCE capability to trigger dynamic rerouting with bit-rate or modulation format adaptation. In particular, the experiments demonstrate, in a real testbed, dynamic frequency slot assignment and format adaptation from DP-16QAM to DP-QPSK at 100 Gb/s, and bit-rate adaptation at DP-16QAM from 200 Gb/s to 100 Gb/s.

Network virtualization over WDM and flexible-grid optical networks

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.

Compared to traditional wavelength division multiplexing (WDM) networks, orthogonal frequency division multiplexing (OFDM)-based flexible optical networks provide better spectral efficiency due to their flexible bandwidth allocation capability and fine granularity. Survivability is a crucial issue in OFDM-based flexible optical networks. Recently we proposed a new survivable multi-path provisioning scheme (MPP) that efficiently supports demands with flexible protection requirement in OFDM-based optical networks and studied the static survivable multipath routing and spectrum allocation (SM-RSA) problem, which aims to accommodate a given set of demands with minimum utilized spectrum. We have showed that the MPP scheme achieves higher spectral efficiency than the traditional single-path provisioning (SPP) scheme. In this paper, we study the dynamic SM-RSA problem, which selects multiple routes and allocates spectrum on these routes for a given demand as it arrives at the network. We develop an integer linear programming (ILP) model as well as a heuristic algorithm for the dynamic SM-RSA problem. We conduct simulations to study the advantage of MPP over SPP for the dynamic traffic scenario in terms of blocking performance and fairness. We also compare the performance of the MPP heuristic algorithm and the ILP model.

Survey of resource allocation schemes and algorithms in spectrally-spatially flexible optical networking