Showing papers in "IEEE\/OSA Journal of Optical Communications and Networking in 2016"

On the effects of combined atmospheric fading and misalignment on the hybrid FSO/RF transmission

Abstract: Free-space optical (FSO) communication is known for its various impairments such as atmospheric turbulence and misalignment fading. In this paper, we study the performance of an FSO link operating under combined path loss and atmospheric and misalignment fadings. We derive a closed-form expression for the bit error rate (BER) of the FSO link in such conditions when the on-off keying (OOK) modulation is employed and when the fluctuations of the received signal are modeled by Gamma- Gamma distribution. In addition, we evaluate the effects of the combined fadings on the outage probability of the FSO link for different strengths of turbulence and pointing errors. Furthermore, we investigate the advantages of combining radio frequency (RF) with FSO to form hybrid FSO/ RF systems. The RF link is based on 16 quadrature amplitude modulation (16-QAM) and on Rician channel fading. We study the performance of the hybrid FSO/RF system in terms of outage probability and BER and find that the hybrid FSO/RF system can overcome the weaknesses of FSO links, which are sensitive to atmospheric variations and misalignment fading.

Fast and efficient adaptation techniques for visible light communication systems

Abstract: Beam steering visible light communication (VLC) systems have been shown to offer performance enhancements over traditional VLC systems. However, an increase in the computational cost is incurred. In this paper, we introduce fast computer-generated holograms to speed up the adaptation process. The new, fast, and efficient fully adaptive VLC system can improve the receiver signal-to-noise ratio (SNR) and reduce the required time to estimate the position of the VLC receiver. It can also adapt to environmental changes, providing a robust link against signal blockage and shadowing. In addition, an angle diversity receiver and a delay adaptation technique are used to reduce the effect of intersymbol interference and multipath dispersion. Significant enhancements in the SNR with VLC channel bandwidths of more than 26 GHz are obtained, resulting in a compact impulse response and a VLC system that is able to achieve higher data rates (25 Gbps) with full mobility in the considered realistic indoor environment.

Physical layer aspects of NG-PON2 standards—Part 1: Optical link design [Invited]

Abstract: NG-PON2 is the industry's first multiple wavelength (per direction), standards-based passive optical network system that is compatible with power-split optical distribution networks. The physical media dependent layer recommendation (ITU-T G.989.2) is the result of over three years of collaborative work by members of the FSAN and ITU-T Study Group 15, Question 2 groups. This two-part paper provides the technical insight and rationales behind the recently approved standard. The first part of the paper focuses on optical link design topics, including the optical distribution network characteristics, wavelength plan, Raman fiber nonlinearity related degradation, and interchannel cross-talk tolerance. It also describes the wavelength-tuning capability of optical network units and its impact on the physical media dependent layer specification.

Flex-grid/SDM backbone network design with inter-core XT-limited transmission reach

Abstract: Spatial division multiplexing (SDM) has been presented as a key solution to circumvent the nonlinear Shannon limit of standard single-core fibers. To implement SDM, multi-core fiber (MCF) technology becomes a top candidate that is leveraged by the very low inter-core crosstalk (XT) measurements obtained in real laboratory MCF prototypes with up to 22 cores. In this work, we concentrate on the design of MCF-enabled optical transport networks. To this goal, we present a methodology to estimate the worst-case transmission reach of the optical signals (at different bit rates and modulation formats) across MCFs given real laboratory XT measurements. Next, we present an optimal integer linear programming (ILP) formulation for the design of a flex-grid/SDM optical transport network that makes use of the transmission reach estimations. Additionally, an effective simulated annealing (SA)-based heuristic able to solve large problem instances with reasonable execution times is presented. Once the proposed heuristic is adequately tuned and validated, we use it to compare the resource utilization in MCF-enabled network scenarios against currently available multi-fiber link solutions. Numerical results reveal very close performances with up to 19 cores/fibers in national backbone network scenarios and up to 12 cores/fibers in long-haul continental ones.

Physical layer aspects of NG-PON2 standards–Part 2: System design and technology feasibility [Invited]

Abstract: This is the second of a two-part paper intended to provide technical insight and rationales behind the recently approved ITU-T G.989.2 Recommendation: the physical media dependent layer specification of the 40-gigabit-capable passive optical networks (NG-PON2). While Part 1 of the paper discusses topics related to the optical link design, Part 2 focuses on wavelength control, technology feasibility, management and control channel design, and potential future standardization directions of such a multi-wavelength PON system. As the NG-PON2 system will continue to evolve, technology extensions are also discussed to provide guidance for future research.

Route partitioning scheme for elastic optical networks with hitless defragmentation

Abstract: 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.

Quality of transmission estimation in WDM and elastic optical networks accounting for space–spectrum dependencies

Abstract: We develop a framework for estimating the quality of transmission (QoT) of a new lightpath before it is established, as well as for calculating the expected degradation it will cause to existing lightpaths. The framework correlates the QoT metrics of established lightpaths, which are readily available from coherent optical receivers that can be extended to serve as optical performance monitors. Past similar studies used only space (routing) information and thus neglected spectrum, while they focused on old-generation noncoherent networks. The proposed framework accounts for correlation in both the space and spectrum domains and can be applied to both fixed-grid wavelength division multiplexing (WDM) and elastic optical networks. It is based on a graph transformation that exposes and models the interference between spectrum-neighboring channels. Our results indicate that our QoT estimates are very close to the actual performance data, that is, to having perfect knowledge of the physical layer. The proposed estimation framework is shown to provide up to 4 × 10-2 lower pre-forward error correction bit error ratio (BER) compared to the worst-case interference scenario,which overestimates the BER. The higher accuracy can be harvested when lightpaths are provisioned with low margins; our results showed up to 47% reduction in required regenerators, a substantial savings in equipment cost.

On the filter narrowing issues in elastic optical networks

Abstract: This paper describes the problematic filter narrowing effect in the context of next-generation elastic optical networks. First, three possible scenarios are introduced: the transition from an actual fixed-grid to a flexi-grid network, the generic full flexi-grid network, and a proposal for a filterless optical network. Next, we investigate different transmission techniques and evaluate the penalty introduced by the filtering effect when considering Nyquist wavelength division multiplexing, single side-band direct-detection orthogonal frequency division multiplexing, and symbol-rate variable dual polarization quadrature amplitude modulation. Also, different approaches to compensate for the filter narrowing effect are discussed. Results show that the specific needs per each scenario can be fulfilled by the aforementioned technologies and techniques or a combination of them, when balancing performance, network reach, and cost.

Solving large instances of the RSA problem in flexgrid elastic optical networks

Abstract: We present an optimization procedure that mixes advanced large-scale optimization methods and heuristics to solve large instances (with over 1.7 million integer variables) of the routing and spectrum allocation (RSA) problem — a basic optimization problem in flexgrid elastic optical networks. We formulate the problem as a mixed-integer program for which we develop a branch-and-price algorithm enhanced with such techniques as problem relaxations and cuts for improving lower bounds (LBs) for the optimal objective value, and an RSA heuristic for improving the upper bounds. All these elements are combined into an effective optimization procedure. The results of numerical experiments run on network topologies of different dimensions and with large demand sets show that the algorithm performs well and can be applied to the problem instances that are difficult to solve using commercial solvers such as CPLEX.

High-altitude platform for free-space optical communication: Performance evaluation and reliability analysis

Abstract: High-altitude platforms (HAPs) have been proposed previously for transmitting radio frequency signals from ground stations over large distances of several hundred kilometers. These HAP architectures suffer from the limitations of data rate. The availability of the system has also not been evaluated for these architectures. In this paper, we propose a high-capacity and highly reliable optical wireless multi-hop HAP network configuration with free-space optical links from the ground stations and between the serial HAPs in the stratosphere. In the proposed scheme, communication services for long distance requiring very high bandwidth are transmitted optically from the ground station to the HAP station in the stratosphere, where it can cover distances of several hundreds of kilometers on the free-space optical links with serial multiple hops and be transmitted back optically to the distant ground receiver. In order to increase reliability and availability, particularly with reference to fading of vertical links due to atmospheric conditions, alternate slant links are added as redundant paths. The closed-form expressions for the average bit error rate and average channel capacity are derived assuming the composite channel model, which includes atmospheric attenuation, turbulence, and pointing errors. The gamma-gamma distribution with beam wander effect is used to model the atmospheric turbulence. Beam optimization is also done using the Nelder-Mead simplex algorithm. The effects of beam wandering and random pointing error are mitigated by using multi-hop HAP architecture. Link availability and system downtime of the complete ground-to-ground FSO network and reconfigured architecture has been calculated for component failure and atmospheric turbulence, moderate fog, rain, and snow. The proposed reconfigured architecture has high availability, meeting telecom requirements, and high mean time between failures and lower system downtime.

Elastic optical networking in the microsoft cloud [Invited]

Abstract: To keep pace with the tremendous bandwidth growth in cloud networking, web-scale providers, such as Microsoft, have been quick to adopt elastic features of modern optical networks. In particular, colorless flexible-grid reconfigurable optical add-drop multiplexers, bandwidth-variable transceivers, and the ability to choose a variety of optical source types are integral for cloud network operators to improve network efficiency while supporting a variety of service types. We take an in-depth look at Microsoft's deployed network infrastructure and discuss the impact of elasticity on network capacity and flexibility. As a proof-of-concept, a new elastic open line system (OLS), in which the line system components and the signal sources are disaggregated, was assembled in a laboratory environment, and 4000 km of propagation over primarily nonzero dispersion-shifted fiber using multiple source types is demonstrated. Finally, the long-term goal of unifying the control plane of the OLS, DWDM signal sources, routers, and Ethernet switches under a single software-defined network controller is briefly addressed.

Modeling energy performance of C-RAN with optical transport in 5G network scenarios

Abstract: The deployment of new 5G wireless interfaces based on massive multiantenna transmission and beamforming is expected to have a significant impact on the complexity and power consumption of the transport network. This paper analyzes the energy performance of four radio access network (RAN) architectures, each one utilizing a different option for splitting the baseband processing functions. The radio segment is based on Long-Term Evolution (LTE) and 5G radio access technologies. The transport segment is based on optical wavelength division multiplexing, where coherent and direct detection transmissions are considered. The energy consumption of each RAN architecture is weighted against i) the benefits for the radio segment as a function of the level of centralization of the baseband processing functions and ii) the power consumption levels needed to accommodate the capacity generated at each base station. Results show that, with LTE radio interfaces, the energy consumption of the transport network amounts to only a few percent of the overall network power consumption. As a result, fully centralized LTE radio architectures are a viable option, with energy savings of at least 27% compared with conventional distributed architectures. On the other hand, with advanced 5G radio interfaces, centralized architectures, if not carefully designed, might become impractical due to the excessive energy consumption of the transport network (i.e., as a result of the huge capacity to be accommodated). This aspect can be mitigated via a careful joint design of the radio and the transport network (i.e., leveraging on appropriate optical transmission techniques and compromising where needed on the radio network performance).

Joint link scheduling and brightness control for greening VLC-based indoor access networks

Abstract: Demands for broadband wireless access services are expected to outstrip the spectrum capacity in the near-term "spectrum crunch." Deploying additional femtocells to address this challenge is cost-inefficient due to the backhaul challenge and the exorbitant system maintenance. According to an Alcatel-Lucent report, most mobile Internet access traffic happens indoors. Leveraging power line communication and the available indoor infrastructure, visible light communication (VLC) can be utilized with a small one-time cost. VLC also facilitates the great advantage of being able to jointly perform illumination and communications, and little extra power beyond illumination is required to empower communications, thus rendering wireless access with small power consumption. In this study, we investigate the problem of minimizing total power consumption of a general multiuser VLC indoor network while satisfying users' traffic demands and maintaining an acceptable level of illumination. We utilize the column-generation method to obtain an e-bounded solution. Several practical implementation issues are integrated with the proposed algorithm, including different configurations of light source and ways of resolving the interference among VLC links. Through extensive simulations, we show that our approach reduces the power consumption of the state-of-the-art VLC-based scheduling algorithms by more than 60% while maintaining the required illumination.

Efficient and reliable protection mechanism in long-reach PON

Abstract: Network survivability and protection mechanisms are very important and necessary in optical networks. The long-reach passive optical network (LR-PON) proposed in this paper extends the access network to hundreds of kilometers, and it can support more optical network units by wavelength division multiplexing technology. We propose an efficient protection mechanism for the whole LR-PON. Optical encoders are used to monitor the whole network, and a network management system in the central office will initialize the protection process by monitoring the information it collects. When the network works normally, space division multiplexing technology is adopted to reduce the loss and crosstalk of signals. The designed system propagates the upstream and downstream signals in different fiber rings to mitigate the influence of Rayleigh backscattering. The proposed protection mechanism can protect the network from the multi-faults of fiber links with the designed switches. Furthermore, the loss, availability, costs, and network performance are also studied. The results obtained prove the feasibility of the proposed network. The network is costless and reliable, and it has high availability of up to 99.9992% by the protection mechanism.

Hybrid modulation formats enabling elastic fixed-grid optical networks

Abstract: In this paper, we analyze hybrid modulation formats as an effective technology for the implementation of flexible transponders that are capable of trading off the delivered data rate by the lightpath quality of transmission with fine granularity. Flexible transponders are an enabling technology that can introduce the elastic paradigm in state-of-the-art networks while maintaining compatibility with the installed equipment, including fibers, mux-demux, and reconfigurable optical add-drop multiplexers, as required by telecom operators willing to exploit fixed-grid wavelength-division multiplexed (WDM) transmission. We consider two solutions achieving different levels of flexibility and employing different hybridization approaches: time-division (TDHMF) and quadrature-division (Flex-PAM) hybrid modulation formats. We introduce a comprehensive theoretical assessment of back-to-back performances, analyzing different transmitter operating conditions, and we provide an extensive simulation analysis on the propagation of a Nyquist-WDM channel comb over an uncompensated and amplified fiber link. After assessing the impact of nonlinear propagation on the maximum signal reach, we present simple countermeasures for non-linear mitigation and discuss their effectiveness for both TDHMF and Flex-PAM.

Network virtualization over elastic optical networks with different protection schemes

Abstract: With network virtualization, the physical infrastructure can be partitioned into multiple parallel virtual networks for sharing purposes. However, different transport technologies or quality of service (QoS) levels may impact both the requested amount of resources and the characteristics of different virtual instances that can be built on top of a single physical infrastructure. In this paper we propose a novel mixed integer linear programming (MILP) formulation for different schemes of protection in scenarios where multiple virtual topologies run over an elastic optical network. The proposed MILP formulation uses the concept of bandwidth squeezing to guarantee a minimum bandwidth for surviving virtual topologies. It achieves a high level of survivability for traffic that is subject to a different committed service profile for each virtual topology. Case studies are carried out in order to analyze the basic properties of the formulation in small networks, and three heuristics are proposed for larger networks.

Time-shifted multilayer graph: A routing framework for bulk data transfer in optical circuit-switched networks with assistive storage

Abstract: Increasing bulk data transfers incur peak-hour bandwidth contention between bulky and short flows as well as between bulky flows. To mitigate this contention, storage is introduced into the forwarding path so that bulk data that are delay tolerant can be temporarily stored and forwarded at a later time. However, the storage introduces an additional complexity into the conventional routing process. What was a spatial resource allocation problem becomes a scheduling problem, in which both bandwidth and storage constraints must be considered and both spatial assignments and temporal arrangements must be performed. In this paper, we propose a routing framework for bulk data transfer in optical circuit-switched networks with assistive storage. This framework is based on a multilayer graph built from a set of snapshots (i.e., layers) of the dynamics in a network. By performing shortest path routing on the multilayer graph, "end-to-end" paths over time and space are found for requests, thus greatly simplifying the provisioning process. We study how the number of layers used for routing affects the network blocking performance and how traffic characteristics and link capacity affect the number of layers. We find that the majority of requests can be served with only one additional layer. A trade-off between computational complexity and blocking performance can be reached by limiting the number of layers used for routing. In our simulations, the request blocking probability is reduced from 13.5% to 0.9% by limiting routing to 11 layers. Request blocking is avoided when routing is allowed within 29 layers.

Potentialities and criticalities of flexible-rate transponders in DWDM networks: A statistical approach

Abstract: We propose a novel method to assess physical layer potentialities of core optical networks aimed at finding solutions that better exploit the installed equipment. We focus on the use of flexible-rate transponders for the implementation of the elastic paradigm on the state-of-the-art dense wavelength-division-multiplexed fixed-grid network scenarios. We make use of the waveplane-based routing and wavelength assignment algorithm presented in Dai et al. [J. Lightwave Technol., vol. 33, p. 3815, 2015] to implement a progressive statistical loading of the analyzed network topology, and perform a Monte Carlo analysis delivering a statistical characterization of the average bit rate per lightpath together with the assessment of network blocking. The proposed method allows for the identification of criticalities in terms of link congestion and lightpath quality of transmission, addressing solutions by identifying network bottlenecks. We apply the proposed method to a large pan-European network topology comparing two different transmission techniques for the implementations of flexible-rate transponders: pure PM-m-QAM versus hybrid modulation formats. Over this realistic network example, besides displaying the overall statistics for the average bit rate per lightpath, we show statistics for critical lightpaths and congested fiber links.

Energy efficiency and blocking reduction for tidal traffic via stateful grooming in IP-over-optical networks

Abstract: The growing popularity of high-speed mobile communications, cloud computing, and the Internet of Things (IoT) has reinforced the tidal traffic phenomenon, which induces spatio-temporal disequilibrium in the network traffic load. The main reason for tidal traffic is the large-scale population migration between business areas during the day and residential areas during the night. Traffic grooming provides an effective solution to aggregate multiple fine-grained IP traffic flows into the optical transport layer by flexibly provisioning lightpaths over the physical topology. In this paper, we introduce a comprehensive study on energy efficiency and network performance enhancement in the presence of tidal traffic. We propose and leverage the concept of stateful grooming to apply differentiated provisioning policies based on the state of network nodes. We formulate and solve the node-state-decision optimization problem, which can decide the specific state of network nodes when a certain traffic profile is given, considering the trade-off between energy efficiency and blocking performance with a multi-objective integer linear program (MOILP). Then, we propose an online traffic-aware intelligent differentiated allocation of lightpaths (TIDAL) algorithm, based on stateful grooming and the MOILP, to accommodate the dynamic tidal traffic. Our illustrative numerical results show that the proposed method can achieve a significant performance improvement in an energy-efficient way.

TCP flow classification and bandwidth aggregation in optically interconnected data center networks

Abstract: Optical functionality is being used to realize new data center architectures that minimize electronic switching overheads, pushing the processing to the edge of the network. A challenge in optically interconnected data center networks is to identify the large, bandwidthhungry flows (i.e., elephants) and efficiently establish the optical circuits. Moreover, the amount of optical resources to be provisioned during the network planning phase is a critical design problem. Flow classification accuracy affects the efficiency of optical circuits. Optical channel bandwidth, on the other hand, directly relates to the additive- increase, multiplicative-decrease congestion control mechanism of the transmission control protocol and affects the effective bandwidth allocated to elephant flows. In this paper, we simultaneously investigate the impact of two important mechanisms on data center network performance: traffic flow classification accuracy and optical bandwidth aggregation (i.e., the consolidation of several low-capacity channels into a single high-capacity one by employing advanced modulation formats for short-reach communications). We develop a discrete-event simulator for a hybrid data center network, enabling the tuning of flow classification parameters. Our simulations indicate that data center performance is highly sensitive to the aggregation level.We could observe up to a 74.5% improvement in network throughput only due to consolidating the optical channel bandwidth. We further noticed that the role of flow classification becomes more pronounced with higher bandwidth per wavelength as well as with more hot-spot traffic. Compared to a random classification benchmark, adaptive flow classification could lead to throughput improvements as large as 54.7%.

Traffic-grooming- and multipath-routing-enabled impairment-aware elastic optical networks

Abstract: Traffic grooming and multipath routing are two techniques that are widely adopted to increase the performance of traditional wavelength division multiplexed networks. They have been recently applied in elastic optical networks to increase spectral efficiency. In this study, we investigate the potential gains by jointly employing the two techniques in combination with a realistic physical impairment model. To allocate resources and quantify spectral efficiency gains over existing impairment-aware schemes, we present an analytical optimization formulation for small networks and a heuristic for large networks. Through numerical simulations, we demonstrate that traffic grooming and multipath routing, together, increase spectral efficiency and reduce resource consumption over existing schemes. We show that the proposed scheme offers significant performance improvements in networks with low degrees of connectivity, high traffic loads, and long links.

Survivable multipath routing of anycast and unicast traffic in elastic optical networks

Abstract: In this paper, we focus on the survivability of elastic optical networks (EONs) that jointly support two types of traffic demands: unicast and anycast. To provide network survivability, we apply multipath routing; i.e., we allow the splitting of a demand into a number of routing paths if the paths' combination guarantees the realization of a specific demand volume in the case of a single link failure. We formulate the corresponding optimization problem as an integer linear program (ILP) and propose a survivable multipath allocation (SMA) algorithm to solve the problem in a reasonable amount of time. Next, we perform numerical experiments to compare the efficiency (ability to provide a good-quality solution in a reasonable amount of time) of the ILP model and SMA as well as to evaluate the impact of survivable multipath routing on the objective defined as a maximum spectrum usage in EONs. Our results show that the SMA method finds good-quality solutions in a reasonable amount of time and that survivable multipath routing in EONs requires additional spectrum resources, up to 45%. However, the amount of additional resources depends on the required protection level, amount of anycast traffic, the maximum number of paths used for demand realization, and the considered network topology.

UDWDM-PON using low-cost coherent transceivers with limited tunability and heuristic DWA

Abstract: A new passive optical network (PON) for access, making use of ultra-dense wavelength division multiplexing (UDWDM) by densely spacing channels by a few gigahertz and introducing the "wavelength-to-the-user" concept, is proposed. The key challenge will be developing low-cost coherent transceivers that provide an excellent selectivity while avoiding filters and furnishing high sensitivity, which will allow high splitting ratios, large numbers of users, and long-distance reach. The optical distribution network (ODN) at the outside plant is based on splitters and is kept compatible with legacy systems. Optical network unit (ONU) designs realized with coherent transceivers using one or two lasers are presented, and the corresponding optical line terminal (OLT) architectures are introduced. The ONUs at the customer premises have lasers with limited thermal tenability, and their wavelengths are randomly distributed in a band. By using heuristic dynamic wavelength assignment (DWA) schemes and extending the original working band, the required optical band is obtained and optimized. In activation processes, ONU acceptances up to 99.9% are achieved. Furthermore, in operation scenarios under indoor and also under outdoor environmental conditions, ONU blocking probabilities below 0.1% and ONU availability ratios (OARs) up to 99.9% are demonstrated. The PON is dimensioned according to the number of deployed users and system reach; moreover, power safety and also fiber non-linearities constraints are evaluated, illustrating the characteristics of the projected network. Finally, the coexistence with legacy networks is discussed.

Cost-minimized design for TWDM-PONbased 5G mobile backhaul networks

Abstract: Dense deployment of small cells would be required to provide for high capacity and universal access to future fifth-generation (5G) mobile networks. However, this would require cost-effective and reliable backhaul connectivity between these small cells. Time and wavelength division multiplexed passive optical networks (TWDM-PONs) are considered a promising choice for this purpose. In this paper,we consider the cost-minimized design of a backhaul network for a 5G mobile system using TWDM-PON. For this, equipment and deployment costs are considered, and the design is based on satisfying network constraints such as the maximum number of subscribers per optical line terminal and themaximumnumber of subscribers per wavelength. Considering the fact that many small cell base stations are dispersed over an extensive wireless coverage, a K-means clustering-based algorithm is proposed for the optimal solution. The strategies of using multistage remote nodes and cable conduit sharing are applied to further reduce the labor cost of trenching and laying fibers. Our simulation results show that the proposed approaches can substantially reduce the backhauling cost in comparison with the traditional intuitive random-cut sectoring approach.

Strict-sense nonblocking space- wavelength-space switching fabrics for elastic optical network nodes

Abstract: In elastic optical networks, a connection may occupy a frequency slot that spreads over m adjacent frequency slot units (FSUs). Such connection (called m-slot connection) also must be realized in optical nodes used in these networks. The paper considers two architectures of the three-stage switching fabric that can be used in such network nodes. The switching fabric applies space switching in the first and the third stages and wavelength switching in the second stage [the space-wavelength-space (S-W-S) switching fabric]. For both architectures, denoted by SWS1 and SWS2, we derived and proved the strict-sense nonblocking conditions when m-slot connections are set up. The number of center stage wavelength switches is calculated and evaluated. The switching fabrics are compared with the strict-sense nonblocking wavelength-space-wavelength (W-S-W) switching fabric. It is shown that the number of center stage switches does not depend on the number of FSUs available in input and output fibers but only on the maximum number of FSUs, which can be used by one connection and the number of input/output fibers in one input/ output space switch. When the number of FSUs in one connection is, for instance, limited to 5, only 11 center stage switches are needed for nonblocking operation in one of the architectures. Moreover, for switching fabrics with a greater number of maximum FSUs in one connection and a greater number of FSUs in one input or output fiber, the S-W-S switching fabrics can be practically realized, unlike the W-S-W switching fabric.

Demonstration of dynamic resource sharing benefits in an optical C-RAN

Abstract: The next generation of mobile communication (i.e., 5G) will bring new challenges for the transport infrastructure, e.g., in terms of flexibility and capacity. The joint orchestration of radio and transport resources can help to address some of these challenges. One example is the possibility of reconfiguring the use of the transport network resources according to the spatial and temporal variations of the wireless traffic patterns. Using the concept of dynamic resource sharing, a limited pool of transport resources can be shared among a large number of radio base stations (RBSs), thus reducing considerably the overall deployment cost of the transport infrastructure. This paper proposes a provisioning strategy for a centralized radio access network with an optical transport whose wavelength resources can be dynamically shared among multiple RBSs. The proposed strategy utilizes a hierarchical software-defined networking control plane where a global orchestrator optimizes the usage of radio and transport resources. The benefits of the proposed strategy are assessed both by simulation and by experiment via an optical data plane emulator developed for this purpose. It is shown that the dynamic resource sharing can save up to 31.4% of transport resources compared to a conventional dimensioning approach, i.e., based on the overprovi-sioning of wavelength resources.

Energy efficient baseband unit aggregation in cloud radio and optical access networks

Abstract: A cloud radio access network enabled by optical transport technology has been recently proposed as a next-generation mobile access network. It separates the legacy base station into remote radio heads (RRHs) and centralized baseband unit (BBU) pools. "Any-to-any" connections between RRHs and BBUs can be realized through a flexible optical fronthaul network. In this paper, we focus on the energy consumption of a BBU pool under a tidal traffic scenario, and an energy-efficient BBU aggregation is proposed by dynamically sleeping and awaking BBU cards. We discuss a BBU capacity model and transform BBU aggregation into an evolved 2D bin-packing problem. An integer linear programming formulation and two heuristic algorithms are developed to minimize the number of active BBU cards. Simulation comparisons are performed among different cell sites (i.e., macro cell, micro cell, pico cell, and heterogeneous cell) and network coverage. Numerical evaluations show that BBU aggregation can achieve significant energy savings, especially for a small cell scenario.

Restoration in optical cloud networks with relocation and services differentiation

Abstract: Optical cloud networks allow for the integrated management of both optical and IT resources. In this paradigm, cloud services can be provisioned in an anycast fashion; i.e., only the source node asking for a service is specified, while it is up to the cloud control/management system to select the most suitable destination data center (DC) node. During the cloud service provisioning process, resiliency is crucial in order to guarantee continuous network operations also in the presence of failures. On the one hand, a survivability strategy needs to be able to meet the availability requirements of each specific cloud service, while on the other hand it must be efficient in using backup resources. This paper proposes a restoration-based survivability strategy, which combines the benefits of both cloud service relocation and service differentiation concepts. The former is used to enhance the restorability performance (i.e., the percentage of successfully restored cloud services) offered by restoration, while the latter ensures that critical services are given the proper consideration while backup resources are assigned. The paper proposes both an integer linear programming (ILP) formulation, which guarantees optimal results, and a heuristic, which trades the optimality of the solution achieved by the ILP for faster processing times. Simulation results show that the average service availability and restorability performance obtained by both the ILP and the heuristic are very close to that achievable using a protection-based strategy, but with the inherent benefit, in terms of efficient use of resources, offered by a restoration-based approach.

Approximation methods for estimating the availability of optical ground networks

Abstract: Optical communications are a key technology enabler to return increasing amounts of data from space exploration platforms such as robotic spacecraft in Earth orbit or across the solar system. However, several challenges have hindered the deployment and utilization of this technology in an operational context, most notably its sensitivity to atmospheric impairments such as cloud coverage. To mitigate this problem, building a network of interconnected and geographically disperse ground stations has been proposed as a possible solution to ensure that, at any point in time, at least one space-to-ground optical link is available to contact the space-based platforms. In this paper, we present a new approach for quantifying the availability of an optical ground network that is both computationally inexpensive and suitable for high-level architectural concept studies. Based on the cloud fraction data set, several approximation methods are used to estimate the probability of having a certain number of space-to-ground links fail due to cloud coverage. They are developed in order to capture increasingly complex atmospheric factors, from sites with independent weather conditions, to stations that are both temporally and spatially correlated. Then, the proposed approximation methods are benchmarked and recommendations on how to utilize and implement them are finally summarized.

Networking aspects for next-generation elastic optical interfaces

Abstract: Next-generation elastic optical interfaces will support a wide range of line rates and modulation formats. Such transmission schemes enable doubling of the channel capacity at the expense of a lower reach, whereas a flexible DWDM grid supports the transport of multiple optical signals within a single frequency slot by packing them closely, thus saving spectral resources. The resulting multitude of options on the line side provides network planners the capability to derive the most suitable one for each individual path inside the network. However, planning and operational complexity should not be overlooked, since aspects such as added spectrum fragmentation can hamper the expected network improvements. This paper investigates how to obtain the best compromise between harnessing the main benefits of next-generation optical interfaces, while keeping the complexity of the underlying system at a reasonable level. In particular, we define a novel network spectral efficiency (SE) metric that enables, on one hand, to highlight the relevance of supporting a flexible grid to improve SE and, on the other hand, to demonstrate that the grid granularity can be based on coarser 50 GHz increments without major penalties. The effectiveness of this parameter has been verified by considering two reference network topologies. Finally, the paper also discusses how future optical interface technology developments will shape network design.