Forthcoming capacity scaling requirements of optical networks and advances in optical fiber communications beyond the omnipresent single-mode fiber operating over the conventional band introduces new opportunities and challenges for exploiting the expanded spectral and spatial resources available for fiber-optic network designs. The spectral and spatial degrees of freedom introduce utilization tradeoffs that can be leveraged under different applications. After briefly reviewing the supporting network building elements (emerging fiber types, multiplexers, optical switches, and amplifiers), we consider networking scenarios such as intra/inter datacenter, terrestrial, undersea, and wireless 5G/6G hauling networks and identify the best utilization plan and key attributes that can be harnessed by the offered spectral and spatial degrees of freedom for each scenario and how optical switching can be employed therein for traffic management. Networks supporting these scenarios are experiencing tremendous traffic growth pushing their existing implementations to their physical capacity-distance limits. New network builds must offer significant capacity multipliers in an efficient and cost-effective manner, requiring new switching solutions for supporting the unabated traffic growths foreseen in the years to come. We discuss how space-division multiplexed networks can help address this challenge in key communication scenarios.

In 5G and beyond telecommunication infrastructures a crucial challenge in achieving the strict Key Performance Indicators (KPIs) regarding capacity, latency, and guaranteed quality of service, is the efficient handling of the fronthaul bottleneck. This part of the next generation networks is expected to comprise the New Radio (NR) access and the Next Generation Passive Optical Network (NGPON) domains. Latest developments load the fronthaul with computing tasks as well (e.g., for AI-based processes) in the context of Mobile Edge Computing (MEC). Towards efficient management of all resource types, this paper proposes a joint allocation scheme with three optimization phases for radio, optical, and MEC resources. This scheme, which has been developed in the context of the blueSPACE 5G Infrastructure Public Private Partnership (5G PPP) project, exploits cutting-edge technologies, such as radio beamforming, spatial-spectral granularity in optical networks, and Network Function Virtualization (NFV), to provide dynamic, adaptive, and energy efficient allocation of resources. The devised model is mathematically described and the overall solution is evaluated in a realistic simulation scenario, demonstrating its effectiveness.

Dynamic network slicing has emerged as a promising and fundamental framework for meeting 5G’s diverse use cases. As machine learning (ML) is expected to play a pivotal role in the efficient control and management of these networks, in this work we examine the ML-based Quality-of-Transmission (QoT) estimation problem under the dynamic network slicing context, where each slice has to meet a different QoT requirement. We examine ML-based QoT frameworks with the aim of finding QoT model/s that are fine-tuned according to the diverse QoT requirements. Centralized and distributed frameworks are examined and compared according to their accuracy and training time. We show that the distributed QoT models outperform the centralized QoT model, especially as the number of diverse QoT requirements increases.

In this work, efficient routing, spectrum, and core allocation (RSCA) techniques are proposed for spatial division multiplexed (SDM) elastic optical networks (EONs) utilizing multicore fibers (MCFs). These techniques provide efficient resource utilization with minimal computational complexity, while also taking into consideration physical layer effects. Initially, a crosstalk- aware approach is presented that accounts for the crosstalk effects in the network that can have detrimental impact on the connections established in the network. This approach is subsequently enhanced with a feedback-based procedure that takes into account all physical layer impairments, aiming to minimize the number of connections that cannot be established in the network due to quality-of-transmission (QoT) considerations.

The traffic carried by core optical networks as well as the per-channel interface rates required by IP routers are growing at a remarkable pace year-over-year. This trend is due to the widespread deployment of fixed and wireless broadband networks as well as huge growth of video-based traffic supported via internet and social media applications. Optical transmission and switching advancements have so far satisfied this huge traffic growth by delivering the content over the network infrastructure in a cost and energy efficient manner utilizing to the maximum extend the capabilities of optoelectronic and photonic subsystems and the available bandwidth of deployed optical fibers. However, we are approaching fast fundamental spectral efficiency limits of single-mode fibers and the scientific and industrial telecommunications community foresees that the growth capabilities of conventional WDM networks operating on a fixed frequency grid are quite limited.

Elastic optical network (EON) technology arises as a promising solution for future high-speed optical transport networks, inasmuch as it can provide superior flexibility and scalability in the spectrum allocation for seamlessly supporting a disparity of services, while coping with the rapid growth of Internet traffic. Spectrally efficient lightpath establishment in EONs is enabled by recent developments in flex-grid transceivers and wavelength selective switches (WSS), which are the key players in determining the amount of spectrum that needs to be reserved for a connection as well as the guard band (GB) required between adjacent channels, since the imperfect shape of the filters degrades the spectrum utilization efficiency of the network. In this paper, we focus on the impact of the filter sharpness on the performance of EONs from the networking perspective. In this regard, we initially quantify the effect of the sharpness of a transmitter Nyquist shaping filter and an LCoS-based WSS filter on the amount of spectrum required for efficient connection establishment. Subsequently, we investigate the practicality of moving to finer frequency slot sizes and, based on extensive simulation results, evaluate the improvement in terms of network blocking probability that can be obtained by using sharper filters.

Rigid fixed-grid wavelength division multiplexing (WDM) optical networks can no longer keep up with the emerging bandwidth-hungry and highly dynamic services in an efficient manner. As the available spectrum in optical fibers becomes occupied and is approaching fundamental limits, the research community has focused on seeking more advanced optical transmission and networking solutions that utilize the available bandwidth more effectively. To this end, the flexible/elastic optical networking paradigm has emerged as a way to offer efficient use of the available optical resources. In this work, we provide a comprehensive view of the different pieces composing the “flexible networking puzzle” with special attention given to capturing the occurring interactions between different research fields. Only when these interrelations are clearly defined, an optimal network-wide solution can be offered. Physical layer technological aspects, network optimization for flexible networks, and control plane aspects are examined. Furthermore, future research directions and open issues are discussed.

Low Density Parity Check(LDPC) codes are a special class of error correction codes widely used in communication and disk storage systems, due to their Shannon limit approaching performance and their favorable structure. In this paper, a methodology for optimized hardware multiplication by constant matrices in GF(2) is introduced and then applied to the Quasi-Cyclic LDPC encoding algorithm. Taking advantage of the fact that the parity check matrix rarely changes, the signals in many cases are hard-wired into the LUTs and thus the cyclic-shifters and block-memories conventionally used are eliminated. Therefore, the proposed framework leads to less complex, mapped to reconfigurable logic designs, whereas it combines the performance of hard-wired solutions (high throughput, low latency) and the flexibility of the software and its hardware counterparts. These advantages in terms of hardware savings and throughput prove that the proposed encoder scheme is suitable for high-speed applications, such as long-haul optical transmission, where speed and resources utilization are a major issue.

We consider the problem of serving traffic in a spectrum-flexible optical network, where the spectrum allocated to an end-to-end connection can change so as to adapt to the time-varying required transmission rate. In the proposed framework, each connection is assigned a route and is allocated a reference frequency over that route, using an appropriate Routing and Spectrum Allocation (RSA) algorithm, but the spectrum it utilizes around the reference frequency is allowed to expand and contract to match source rate fluctuations. We propose and analyze three spectrum expansion/contraction (SEC) policies for modifying the spectrum allocated to each connection. The first policy, named the Constant Spectrum Allocation (CSA) policy, allocates a number of spectrum slots for exclusive use by each connection. We also present two policies that enable the dynamic sharing of spectrum slots among connections, named the Dynamic High Expansion-Low Contraction (DHL) and the Dynamic Alternate Direction (DAD) policy. We give exact formulas for calculating the blocking probability for a connection and for the whole network under the CSA policy and provide corresponding approximate analyses under the DHL and DAD policies. We also present a simple iterative RSA algorithm that uses the developed blocking models so as to minimize the average blocking of the network.

The six papers in this special issue are devoted to the topic of next generation spectrum efficient and elastic optical transport systems.

Next generation data centers will have to face an increased amount of network traffic due to emerging applications such as cloud computing. Processing larger data volume leads to higher energy consumption, which in turn causes higher operation expense. Optical interconnects have gained attention recently as a promising solution due to its large bandwidth and high energy efficiency compared to the current networks based on commodity electrical switches. In this paper, we evaluate the energy efficiency of a novel data center network architecture based on optical multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) technology. By utilizing passive optical switch (cyclic arrayed waveguide grating) and parallel signal detection technology to detect multiple optical channels simultaneously with a single photodetector, this architecture offers high energy efficiency. The comparison with a reference design based on commodity electrical switches shows that the proposed architecture can reduce the power consumption and operation cost significantly while providing higher throughput.

C3 - Journal Articles Unrefereed Letters or Notes

This paper defines a novel MAC framework for OFDMA-PONs and proposes two scheduling algorithms which operate in a hybrid OFDMA/TDMA manner. The trade-offs of the proposed schemes in terms of delay performance as well as computational complexity are analyzed and comparisons are performed using computer simulations.

Orthogonal Frequency Division Multiplexing (OFDM) has been recently proposed as a modulation technique for optical networks, due to its good spectral efficiency and impairment tolerance. Optical OFDM is much more flexible compared to traditional WDM systems, enabling elastic bandwidth transmissions. We consider the planning problem of an OFDM-based optical network where we are given a traffic matrix that includes the requested transmission rates of the connections to be served. Connections are provisioned for their requested rate by elastically allocating spectrum using a variable number of OFDM subcarriers. We introduce the Routing and Spectrum Allocation (RSA) problem, as opposed to the typical Routing and Wavelength Assignment (RWA) problem of traditional WDM networks, and present various algorithms to solve the RSA. We start by presenting an optimal ILP RSA algorithm that minimizes the spectrum used to serve the traffic matrix, and also present a decomposition method that breaks RSA into two substituent subproblems, namely, (i) routing and (ii) spectrum allocation (R+SA) and solves them sequentially. We also propose a heuristic algorithm that serves connections one-by-one and use it to solve the planning problem by sequentially serving all traffic matrix connections. To feed the sequential algorithm, two ordering policies are proposed; a simulated annealing meta-heuristic is also used to obtain even better orderings. Our results indicate that the proposed sequential heuristic with appropriate ordering yields close to optimal solutions in low running times.

Two general approaches have been followed for solving the problem of upstream grant scheduling and wavelength assignment in hybrid WDM/TDMA EPON networks, i.e. the offline and the online one. The latter boasts significantly lower frame delay performance in all cases. Nevertheless, we show that simplistic online schemes do not utilize wavelength resources as efficiently as possible, especially in the case of large differential distances of ONUs from the OLT. We propose and analyze several low- and higher-complexity solutions to overcome those inefficiencies, leading to improved utilization of network capacity and reduced frame delay. All schemes are evaluated and compared using computer simulations.

We design and implement a multicost impairment- aware routing and wavelength assignment algorithm for online traffic. In transparent optical networks the quality of a transmission degrades due to physical layer impairments. To serve a connection, the proposed algorithm finds a path and a free wavelength (a lightpath) that has acceptable signal quality performance by estimating a quality of transmission measure, called the Q factor. We take into account channel utilization in the network, which changes as new connections are established or released, in order to calculate the noise variances that correspond to physical impairments on the links. These, along with the time invariant eye impairment penalties of all candidate network paths, form the inputs to the algorithm. The multicost algorithm finds a set of so called non-dominated Q paths from the given source to the given destination. Various objective functions are then evaluated in order to choose the optimal lightpath to serve the connection. The proposed algorithm combines the strength of multicost optimization with low execution time, making it appropriate for serving online connections.

In future transparent optical networks, it is important to consider the impact of physical impairments in the routing and wavelengths assignment process, to achieve efficient connection provisioning. In this paper, we use classical multi-objective optimization (MOO) strategies and particularly genetic algorithms to jointly solve the impairment aware RWA (IA-RWA) problem. Fiber impairments are indirectly considered through the insertion of the path length and the number of common hops in the optimization process. It is shown that blocking is greatly improved, while the obtained solutions truly converge towards the Pareto front that constitutes the set of global optimum solutions. We have evaluated our findings, using an Q estimator tool, that calculates the signal quality of each path analytically.

In this talk we will present state-of-the-art routing and wavelength assignment algorithms considering as constraints physical impairments that arise in transparent core network infrastructures utilized to support Grid computing applications. These algorithms can be incorporated into network planning tools and the control plane of future optical networks with the goal to support the realization of the vision of full transparency in core network design and implementation while achieving efficient resource utilization and strict quality of service guarantees based on certain service level agreements (SLAs). It will be shown that such cross-layer optimized optical network designs will be major enablers for the network of the future that will support advanced grid computing applications. In addition, we will review recent proposals for introduction to current GMPLS signaling/routing protocols of extensions which encompass the physical layer parameters to enable support for grid computing.

This paper presents a novel hybrid optical burst/circuit switched (OBCS) ingress edge router solution towards ubiquitous photonic Grid networking. It is based on the deployment of Grid application-aware packet classification and burst aggregation algorithms as well as agile and intelligent optical resource allocation. The proposed solution utilises a generic and highly scalable multi-dimension classification mechanism able to provide wire-speed classification at high bit rates up to 40 Gbps. The mechanism is responsible for per application switching (OBS/OCS) service selection and Grid Differentiated Service (GridDiffServ) provisioning. A CoS-Traffic-Time-LEngth-Service-oriented aSembly (COST2LESS) algorithm has been proposed to smooth the incoming Grid traffic and provide some CoS differentiation and initial results are presented. An agile optical data transmission mechanism has been also implemented to map Grid traffic asynchronously into optical bursts or wavelength channels (data plane) based on user/application-specific requirements. Furthermore an optical burst Ethernet switched (OBES) transport mechanism has been implemented to transport out-of- band control plane signalling information.

Fault and attack management has become a very important issue for network operators that are interested to offer a secure and resilient network capable to prevent and localize, as accurately as possible, any failure (fault or attack) that may occur. Hence, an efficient failure location method is needed. To locate failures in opaque optical networks, existing methods which allow monitoring of the optical signal at every regeneration site can be used. However, to the best of our knowledge, no method exists today that performs failure location for transparent optical networks. Such networks are more vulnerable to failures than opaque networks since failures propagate without being isolated due to optoelectronic conversions. In this paper, we present a failure location algorithm that aims to locate single and multiple failures in transparent optical networks. The failure location algorithm developed in this paper can cope with ideal scenarios (i.e., no false and/or lost alarms), as well as with nonideal scenarios having false and/or lost alarms.

This paper demonstrates the use of computer simulation for topological design and performance engineering of transparent wavelength-division multiplexing metropolitan-area networks. Engineering of these networks involves the study of various transport-layer impairments such as amplifier noise, component ripple, chirp/dispersion, optical crosstalk, waveform distortion due to filter concatenation, fiber nonlinearities, and polarization effects. A computer simulation methodology composed of three main simulation steps is derived and implemented. This methodology obtains performance estimations by applying efficient wavelength-domain simulations on the entire network topology, followed by time-/frequency-domain simulations on selected paths of the network and finally Q-budgeting on an identified worst case path. The above technique provides an efficient tool for topological design and network performance engineering. Accurate simulation models are presented for each of the performance impairments, and the computer simulation methodology is used for the design and engineering of a number of actual metro network architectures.