The possibility of flexibly assigning spectrum resources with channels of different sizes greatly improves the spectral efficiency of optical networks, but can also lead to unwanted spectrum fragmentation. We study this problem in a scenario where traffic demands are categorized in two types (low or high bit-rate) by assessing the performance of three allocation policies. Our first contribution consists of exact Markov chain models for these allocation policies, which allow us to numerically compute the relevant performance measures. However, these exact models do not scale to large systems, in the sense that the computations required to determine the blocking probabilities—which measure the performance of the allocation policies—become intractable. In order to address this, we first extend an approximate reduced-state Markov chain model that is available in the literature to the three considered allocation policies. These reduced-state Markov chain models allow us to tractably compute approximations of the blocking probabilities, but the accuracy of these approximations cannot be easily verified. Our main contribution then is the introduction of reduced-state imprecise Markov chain models that allow us to derive guaranteed lower and upper bounds on blocking probabilities, for the three allocation policies separately or for all possible allocation policies simultaneously.
A rich body of research work on flexi-grid optical networks has appeared in the last few years. The reader is referred to @cite_0 and @cite_18 for a thorough overview. More specifically, both the static and the dynamic Routing and Spectrum Assignment (RSA) problem in flexi-grid networks has been extensively addressed; for a survey of the most relevant literature we refer to @cite_27 . In the present contribution, we focus on a single-link dynamic scenario. The arrival and departure processes of traffic requests are random, and requests are either accommodated in a spectrum portion of the link or rejected in real-time.
@cite_0 @cite_27 @cite_18
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Orthogonal frequency-division multiplexing (OFDM) is a modulation technology that has been widely adopted in many new and emerging broadband wireless and wireline communication systems. Due to its capability to transmit a high-speed data stream using multiple spectral-overlapped lower-speed subcarriers, OFDM technology offers superior advantages of high spectrum efficiency, robustness against inter-carrier and inter-symbol interference, adaptability to server channel conditions, etc. In recent years, there have been intensive studies on optical OFDM (O-OFDM) transmission technologies, and it is considered a promising technology for future ultra-high-speed optical transmission. Based on O-OFDM technology, a novel elastic optical network architecture with immense flexibility and scalability in spectrum allocation and data rate accommodation could be built to support diverse services and the rapid growth of Internet traffic in the future. In this paper, we present a comprehensive survey on OFDM-based elastic optical network technologies, including basic principles of OFDM, O-OFDM technologies, the architectures of OFDM-based elastic core optical networks, and related key enabling technologies. The main advantages and issues of OFDM-based elastic core optical networks that are under research are also discussed. Optical networks are moving from opaque and translucent architectures towards all-optical (transparent) architectures. In translucent architectures a small amount of regeneration (e.g. optical-electronic-optical conversion) is available in the network. The incorporation of the physical impairments in the routing and wavelength assignment (RWA) problem in transparent optical networks has recently received some attention from the research communities. This work compiles a comprehensive survey of the proposed algorithms that address this issue. The physical layer impairments and related classification in optical networks are initially presented followed by physical layer impairments (PLI) constrained and aware RWA algorithms. Algorithmic approach, current PLI-RWA proposals, impact of wavelength conversion on these algorithms, protection and resilience considerations, and proposed extensions to control planes are covered in this work. Further research topics are presented in this study. There is a growing awareness that the utilized bandwidth of deployed optical fiber is rapidly approaching its maximum limit. Given the possibility for such capacity crunch, the research community has focused on seeking solutions that make the most out of the scarce network resources (such as the fiber bandwidth) and allow accommodating the ever-increasing traffic demands. In such context, new spectrum efficient optical networking techniques have been introduced as a way to offer efficient utilization of the available optical resources. "Flexible", "elastic", "tunable", "gridless" or "adaptive" are few examples of the terms used in literature to describe solutions that migrate from the fixed WDM single line rate systems to systems that provide support for the most efficient bandwidth utilization. In this paper, we review the recent developments on the research topic of flexible elastic networking and we highlight the future research challenges.