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## Über dieses Buch

This handbook is an authoritative, comprehensive reference on optical networks, the backbone of today’s communication and information society. The book reviews the many underlying technologies that enable the global optical communications infrastructure, but also explains current research trends targeted towards continued capacity scaling and enhanced networking flexibility in support of an unabated traffic growth fueled by ever-emerging new applications.

The book is divided into four parts: Optical Subsystems for Transmission and Switching, Core Networks, Datacenter and Super-Computer Networking, and Optical Access and Wireless Networks.

Each chapter is written by world-renown experts that represent academia, industry, and international government and regulatory agencies. Every chapter provides a complete picture of its field, from entry-level information to a snapshot of the respective state-of-the-art technologies to emerging research trends, providing something useful for the novice who wants to get familiar with the field to the expert who wants to get a concise view of future trends.

## Inhaltsverzeichnis

### 1. The Evolution of Optical Transport Networks

This introductory chapter describes the role of optics in networks, their capabilities, and their scaling limitations (different multiplexing techniques, i. e., time-division multiplexing (TDM) (time-division multiplexing), code-division multiplexing (CDM) (code-division multiplexing), frequency-division multiplexing (FDM) (frequency-division multiplexing), space-division multiplexing (SDM) (space-division multiplexing)). Types of optical networks installed around the globe are summarized, as well as their impact on society, market structure, and future perspectives.Optical fiber transmission links were first deployed in the mid 1970s to provide $${\mathrm{45}}\,{\mathrm{Mb/s}}$$ 45 Mb / s capacity in metropolitan networks at a time when most traffic was wireline telephone communication. Few would have imagined then, when bandwidth demand on the telephone network was growing only as rapidly as population growth, that this new technology would radically alter business and everyday life by enabling the worldwide Internet. As the Internet grew in popularity and extended to all parts of the world, new devices and transmission technologies were invented and developed to cost-effectively achieve the required higher capacity transmission, and fiber was laid across continents and under the oceans to cover the globe. Thus began a virtuous cycle of higher capacity optical transmission systems and, ultimately, reconfigurable optical networks enabled by new technologies to meet increased demand, which resulted in new applications and services, such as video, which drove ever greater capacity and flexibility demand, which was, again, achieved via new technology innovation at significantly lower costs/capacity.First-era systems grew the capacity of optical links by increasing the bit rate of information carried on the fiber. The second era, which was achieved cost-effectively by the optical fiber amplifier, increased capacity by multiplexing many wavelengths, each carrying independent information, onto a single fiber. The result was an increase in single fiber transmission equal to the number of wavelength channels and long spans over which all the wavelength channel signals could be periodically boosted with a single optically-powered optical fiber amplifier. The next era took the giant step of moving optics from transmission links only to fully reconfigurable, wavelength-channel-based networks to achieve higher efficiency (and, therefore, capacity), flexibility, and restorability by allotting and managing network capacity at the optical layer level.This step required cost-effective optical switching components to provide the reconfigurable wavelength add/drop and cross connect functions. Today’s optical networks provide superhighways of information bandwidth of the order 100 wavelength-defined lanes each providing hundreds of $$\mathrm{Gb/s}$$ Gb / s information capacity. These networks provide network optimization to address changing capacity needs under software control via configurable wavelength on-and-off ramps and route switching centers. Without these optical networks, the global internet, cloud computing, and high bandwidth mobile services, including video, would not be possible. This chapter provides a view of the evolution of these optical networks—the market drivers, network architectures, transmission system innovations and enabling devices, and module technologies—which was a result of the efforts of a global community of researchers, developers, and, ultimately, manufacturers.

Rod C. Alferness

### 2. Optical Fiber and Cables

This chapter gives an overview and introduces application scenarios for optical fibers and cables in optical communications. The use of single-mode optical fibers for both short-reach and long-haul applications is growing due to continually increasing demand for higher bandwidth optical communication systems. To better understand fiber-based optical communications the chapter first focuses on the design of the single-mode fiber while the latter half focuses on the design of optical fiber cable. A wide variety of optical fiber cables have been designed and installed to meet the needs of various applications and this chapter reviews the many types of cables for fiber to the home as well as for datacenter connectivity.We will start with a section showing the history of optical loss improvement. Then, the categories of optical fibers and their cross-sectional structure are explained. Next, the main features of single-mode fibers such as standard single-mode fiber (standard single-mode fiber (SSMF)), bend-insensitive fiber (bend-insensitive fiber (BIF)), cutoff-shifted fiber (cutoff-shifted fiber (CSF)), dispersion-shifted fiber (dispersion-shifted fiber (DSF)), and nonzero dispersion-shifted fiber (nonzero dispersion-shifted fiber (NZDSF)) are summarized with their major optical characteristics. Then, features of each fiber type are explained. In particular, the cutting-edge application of low-loss fiber in ultralong-haul systems such as subsea systems are discussed and the importance of their system impact is also described. Thereafter, the key characteristics of multimode fiber are explained. In the final part of the optical fiber section, emerging fiber types are introduced such as fiber for space-division multiplexing (space-division multiplexing (SDM)) systems, for example multicore fiber, few-mode fiber, or coupled multicore fiber.Thereafter, in the optical fiber cable section, we start with the classification of use cases such as indoor or outdoor cables and their features. Next, we introduce the optical fiber unit, a basic element used to bundle the fiber into cable, such as an optical fiber ribbon or loose tube. Following this we present many examples of optical fiber cables and their features, such as the slotted-rod cable, loose-tube cable, central-tube cable, layered fiber core cable, and direct-jacketed cable. Next, we present key considerations in optical cable design, such as fiber density, environment conditions, temperature change, water durability, biological attacks, and mechanical durability (bend, impact, torsion, crush). Finally, specific fiber cable use cases such as air-blown fiber cables and ultrahigh-density fiber cables for datacenter applications are introduced.For more information about the basics of optical fibers and cables, we refer the reader to text books such as [2.1, 2.2] 1. D. Marcuse: Theory of Dielectric Optical Waveguides, Academic, New York, (1974) 2. Barry Elliott, Mike Gilmore: Fiber Optic Cabling, Elsevier Science, Netherlands, (2002)

Takashi Sasaki, Takemi Hasegawa, Hiroki Ishikawa

### 3. Optical Amplifiers

Theopticalamplifier principles, design, and operation of erbium-doped and Raman amplifiers, two of the most important classes used in modern lightwave communication, are described. Developed over two decades, erbium-doped fiber devices act as lumped optical gain elements in terrestrial, submarine, and access networks, underpinning nearly all commercial data traffic today. Raman amplifiers have allowed significant reach and capacity increases and, unlike erbium-doped devices, are not confined to a specific lightwave band. In contrast to alternatives such as parametric and semiconductor amplification technology, erbium-doped and Raman amplifiers have been commoditized and can be readily designed and constructed from a mature set of components that includes specialty fibers, semiconductor pumps, dedicated filters, and passive elements. The design of both types of amplifiers is described, along with the most important engineering rules that allow for optimal device construction. Mitigation of noise and distortion mechanisms is detailed for both types of amplifiers when operating with commercial fiber plants.

### 4. Optical Transponders

The first commercial $$\mathrm{10}$$ 10 - $$\mathrm{Gb/s}$$ Gb / s transponders, deployed in the mid 1990s, were based on a very simple modulation technique, i. e., a binary light intensity modulation with envelope detection by a single photodiode. To extend the fiber capacity, bandwidth-efficient modulation techniques such as duobinary line coding and multilevel intensity-modulation formats gained popularity in optical communications in the late 1990s. In the following years, the use of differential phase modulation in combination with interferometric detection allowed the transponder data rates to be increased up to $${\mathrm{40}}\,{\mathrm{Gb/s}}$$ 40 Gb / s . However, despite all improvements, the system performance of these $$\mathrm{40}$$ 40 - $$\mathrm{Gb/s}$$ Gb / s solutions was still not on par with state-of-the art $$\mathrm{10}$$ 10 - $$\mathrm{Gb/s}$$ Gb / s systems at that time. With the advent of coherent detection, things suddenly changed and transmission rates of $${\mathrm{100}}\,{\mathrm{Gb/s}}$$ 100 Gb / s and beyond could soon be achieved, thanks to the use of high-order modulation formats and advanced digital signal-processing techniques.In this chapter, the configuration and performance of the most common transmitter and receiver combinations that are currently used in optical transmission systems will be described, including an overview of transponder types and their hardware architectures. Finally, relevant standards will be discussed and pluggable optical transceiver modules used in modern transponder implementations will be explained.

Gabriella Bosco, Jörg-Peter Elbers

### 5. Optical Transponder Components

This chapter introduces the general architecture of an optical transponder and describes the three critical optical components that comprise the transponder, the laser, the optical modulator, and the photodetector. Following this, the subsystem consisting of the optical transmitter and the coherent receiver that are typically used for generating and detecting dual-polarization complex-modulated signals are explained. The typical characteristics of the components used in the transponder are presented both from current-generation products, as well as from more recent research demonstrations.In this chapter, we focus on the line-side optics, as they are the key to understanding the components in a transponder. The transmitter at the line side consists of a number of high-performance optical components, such as narrow-linewidth lasers, and high-speed modulators with linear driver amplifiers. The receiver at the line side consists of $$90^{\circ}$$ 90 ∘ optical hybrids, high-dynamic-range linear balanced photodetectors (balanced photodetector (BPD)s), and high-gain low-noise transimpedance amplifiers (transimpedance amplifier (TIA)s). The digital electronic functions are in application-specific integrated circuits (application-specific integrated circuit (ASIC)s), which include digital-to-analog converters (digital-to-analog converter (DAC)s) for the transmitter to generate optical signals, analog-to-digital converters (analog-to-digital converter (ADC)s) for the receiver to recover the optical signals. Both ends need digital signal processing (digital signalprocessing (DSP)) engines to condition and process the signals. The client side optics, which are not covered in this chapter, are usually a subset or a simpler version of those used in the line-side part of the transponder.

### 6. DSP for Optical Transponders

This chapter outlines the principles of the digital signal processing (digital signalprocessing (DSP)) used in modern optical transceivers. The historic developments that have led to the emergence of DSP being applied in optical transceivers is reviewed, including the high-speed complementary metal oxide semiconductor (complementary metal–oxide–semiconductor (CMOS)) analog to digital converters (analog-to-digital converter (ADC)) that have facilitated the creation of the application-specific integrated circuit (application-specific integrated circuit (ASIC)) which underpins digital coherent transceivers. Following on from this, the mathematics associated with finite impulse response (finite impulse response (FIR)) filters is reviewed, including the Wiener and least-squares design of FIR filters. The mathematics associated with the adaptive multiple-input-multiple-output (multiple-input multiple-output (MIMO)) filter employed in the receiver is also discussed, including derivation of the stochastic descent algorithm based on differentiation with respect to a complex vector. Subsequently, we provide an overview of DSP algorithms, before detailing both those required for equalization and synchronization. Following a summary of error correction used in a digital transceiver, we reflect on the current research trends and future opportunities for DSP in optical transceivers.

Seb J. Savory, David S. Millar

### 7. Forward Error Correction for Optical Transponders

Forward error correction (forward error correction (FEC)) is an essential technique required in almost all communication systems to guarantee reliable data transmission close to the theoretical limits. In this chapter, we discuss the state-of-the-art FEC schemes for fiber-optic communications. Following a historical overview of the evolution of FEC schemes, we first introduce the fundamental theoretical limits of common communication channel models and show how to compute them. These limits provide the reader with guidelines for comparing different FEC codes under various assumptions. We then provide a brief introduction to the general basic concepts of FEC, followed by an in-depth introduction to the main classes of codes for soft-decision decoding and hard-decision decoding. We include a wide range of performance curves, compare the different schemes, and give the reader guidelines on which FEC scheme to use. We also introduce the main techniques to combine coding and higher-order modulation (coded modulation), including constellation shaping. Finally, we include a guide on how to evaluate the performance of FEC schemes in transmission experiments. We conclude the chapter with an overview of the properties of some state-of-the-art FEC schemes used in optical communications, and an outlook.

Alexandre Graell i Amat, Laurent Schmalen

### 8. Optical Node Architectures

Brandon Collings, Mark Filer

### 9. Fiber Nonlinearity and Optical System Performance

This chapter aims to provide a comprehensive picture of the impact of fiber nonlinear effects on modern coherent wavelength division multiplexing (wavelength-division multiplexing (WDM)) systems' performance. First, the main nonlinearity models currently available are introduced and discussed in depth. Then, various specific aspects are addressed, such as the interplay of polarization mode dispersion (polarizationmode dispersion (PMD))/polarization dependent loss (polarization-dependentloss (PDL)) and nonlinearity, or the dependence of nonlinear effects on modulation format. The important topic of nonlinear effects mitigation is then dealt with. Finally, system performance metrics and capacity are discussed extensively, as to how they are fundamentally influenced and limited by fiber nonlinearity.

Alberto Bononi, Ronen Dar, Marco Secondini, Paolo Serena, Pierluigi Poggiolini

### 10. Space-Division Multiplexing

Fiber-based optical communication networks are reaching a point where the capacity required on a single link can significantly exceed the capacity of a single-mode fiber, and at the same time conventional network architectures can no longer be scaled cost-effectively. Space-division multiplexing (space-division multiplexing (SDM)) addresses the capacity bottleneck imposed by the use of single-mode fibers within a completely new approach that relies on new fiber types, optical amplifiers, and optical switches capable of supporting multiple spatial channels.The aim of this chapter is, on one hand, to provide an overview of the components that are necessary for the implementation of SDM transmission and, on the other hand, to review the modeling of the main propagation effects that occur in multimode and multicore fibers. The chapter also includes a description of the techniques that are used in SDM transmission experiments and an update on transmission records reported from around the globe. The chapter ends with the description of potential architectures supporting SDM networks.

Roland Ryf, Cristian Antonelli

### 11. Carrier Network Architectures and Resiliency

In this chapter we investigate carrier network architectures, and how and where resilience is provided by commercial telecommunications carriers in today’s optical networks. Much of the content is generally unpublished and we provide the reader a unique insight into this topic through our privilege of working for decades on the inside of different telecommunications carriers. To provide a fuller understanding of this complex topic, we first describe the typical partitioning of terrestrial networks into their metro-access, metro-core, and intermetro segments and then describe the multilayered structure within each of these segments (Sect. 11.2). Within these constructs, we describe where and how network resiliency is provided against a modeled set of potential outages and other network impacting events (Sect. 11.3). To better understand how the resiliency techniques deployed in various layers and segments are engineered, we discuss how end-to-end services are pieced together across these segments to provide their needed network quality of service and availability. Finally, to even better understand why today’s network resiliency techniques have been deployed, we take the reader through a historical evolution of how and why key resiliency technologies and methodologies were developed and deployed, including why some phased out (Sect. 11.4).

Robert D. Doverspike

### 12. Routing and Wavelength (Spectrum) Assignment

Routing a connection from its source to its destination is a fundamental component of network design. The choice of route affects numerous properties of a connection, most notably cost, latency, and availability, as well as the resulting level of congestion in the network. This chapter addresses various algorithms, strategies, and tradeoffs related to routing.At the physical optical layer, connections are assigned a unique wavelength on a particular optical fiber, a process known as wavelength assignment (wavelength assignment (WA)). Together with routing, the combination of these two processes is commonly referred to as routing and wavelength assignment (RWA). In networks based on all-optical technology, WA can be challenging. It becomes more so when the physical properties of the optical signal need to be considered. This chapter covers several WA algorithms and strategies that have produced efficient designs in practical networks.A recent development in the evolution of optical networks is flexible networking, where the amount of spectrum allocated to a connection can be variable. spectrumassignment (SA)Spectrum assignment is analogous to, though more complex than, wavelength assignment; various heuristics have been proposed as covered in this chapter. Flexible (or elastic) networks are prone to more contention issues as compared to traditional optical networks. To maintain a high degree of capacity efficiency, it is likely that spectral defragmentation will be needed in these networks; several design choices are discussed.

Jane M. Simmons, George N. Rouskas

### 13. Standards for Optical Transport Networks

Most larger optical networks are built using a combination of standardized and proprietary technology.This chapter provides information on how a combination of complete and functionally standardized optical interfaces are used to build an optical transport network optical transportnetwork (OTN).

Stephen J. Trowbridge

### 14. Traffic Grooming

A particular thread of research in optical networking that is concerned with the efficient assignment of traffic demands to available network bandwidth became known as traffic grooming in the mid-1990s. Initially motivated by the distinctly different network characteristics of optical and electronic communication channels, the area focused on how subwavelength traffic components were to be mapped to wavelength communication channels, such that the need to convert traffic back to the electronic domain at intermediate network nodes, for the purpose of differential routing, was minimized. Over time, it broadened to include joint considerations with other network design goals and constraints. It was influenced in turn by existing technology limitations, and in turn served to influence continuing technology trends. Traffic grooming has had a significant effect on both the research and practice of transport networking. It continues to be a meaningful area not just in historical terms, but as a wealth of techniques that can be called upon for considering the traffic engineering problem afresh as each new development at the optical layer, or change in economic realities of networking equipment or traffic requirements, redefines the conditions of that problem.

Rudra Dutta, Hiroaki Harai

### 15. Dynamic Control of Optical Networks

Significant developments in technologies such as distributed/cloud computing, mobile applications and the Internet of things (Internetof things (IoT)) are driving demands for lower costs and higher throughput from the underlying network infrastructure. Optical networks have evolved accordingly (flex-grid, all-optical switching, etc.) to accommodate increasing demand and reduce capital expenditure (capital expenditure (CAPEX)). In order to further lower costs, automation of control and management processes is essential to reduce human intervention over the service lifecycle. The need for automation has driven the development of optical control planes, which attempt to automate operations associated with provisioning and recovery from failures (service restoration). This chapter introduces the core functions involved in the control of optical networks, including network discovery, wavelength routing, dynamic service provisioning and seamless fault recovery. We chart the evolution from the inception of fully distributed control plane architectures to the more recent software-defined networking architectures and outline the already visible future trends in the dynamic control of optical networks, including intent-based networking and applications of artificial intelligence (AI).This chapter is structured as follows. Section 15.1 introduces the background of optical control and management planes in legacy optical networks, and introduces control plane standards. Section 15.2 illustrates one of the first software-defined control and management plane artifacts, called path computation element (path computation element (PCE)). Section 15.3 analyses the applications of software-defined networking (software-definednetwork (SDN)) in optical networks. Section 15.4 presents some emerging trends in optical control planes, such as intent-based networking and applications of AI. Section 15.5 gives a summary and a few final remarks.

### 16. Cross-Layer Design

The chaptercross-layerdesign is organized as follows. Sections 16.1 to 16.3 constitute the first part of the chapter, namely, physical network layer design. In Sect. 16.1, the physical impairments that affect long-haul optical fiber networks are described. Models for predicting and measuring the quality of transmission (qualityof transmission (QoT)) of network connections are detailed. In Sect. 16.2 various routing and wavelength assignment (routing and wavelength assignment (RWA)) algorithms are presented, some of which only guarantee QoT requirements in the presence of PLIs, while others optimize network performance while taking into account the PLIs. The latter, called PLI-aware algorithms, can significantly improve the performance of the network, i. e., lower the blocking probability. Sample numerical results illustrating the efficacy of cross-layer methods are presented. Section 16.3 addresses the design of protection and restoration techniques for physically impaired optical networks. The survivability of these networks to link failures is greatly enhanced by including information about the PLIs directly within the protection or restoration algorithms. The second part of the chapter, focusing on application-network-layer design, consists of Sects. 16.4 to 16.6. An application-aware metro-access programmable architecture is presented in Sect. 16.4. Resource allocation and path protection based on software-defined networking (software-definednetwork (SDN)) is presented in Sect. 16.5, and Sect. 16.6 presents application-aware converged wireless-access resource scheduling. The chapter is concluded in Sect. 16.7.

Suresh Subramaniam, Koteswararao Kondepu, Andrea Marotta

### 17. Optical Network Virtualization

Optical network virtualization presents the necessary technologies to provide the specified set of network requirements, while providing the necessary isolation between network slices. In this chapter, technologies from both data and control plane perspectives are presented for virtual optical networks (virtual opticalnetwork (VON)). Firstly, the benefits provided by adopting software-defined networks (software-definednetwork (SDN)) and network function virtualization (networkfunction virtualization (NFV)) technologies are introduced. Secondly, the need for a network operating system is discussed, and a reference architecture for optical networks is presented. Thirdly, a methodology for describing virtual and physical network resources is provided. Fourthly, a review of different VON resource allocation algorithms is presented. Finally, an NFV architecture supporting VON is presented, among several use cases.

Jie Zhang, Ricard Vilalta, Xiaosong Yu, Victor Lopez, Alejandro Aguado Martín

### 18. Metropolitan Networks

Metropolitan area networks, or metropolitan area network (MAN)s are at the confluence of business and home users—connecting enterprises to core networks and residential users to the rest of the Internet. This important segment of the network spans cities, regions, districts and municipalities and is a prime driver segment of broadband networking as well as being pivotal in providing connectivity to enterprises. In this chapter we begin by describing the premise of technology in metro networks (shortened form of MANs). After a detailed overview of how metro networks plug into the larger service provider scheme of things, we delve into individual technologies that are intrinsic to metro network architecture. These technologies include synchronousoptical network (SONET)/synchronousdigital hierarchy (SDH), optical transportnetwork (OTN), optical networks, WDM, Internet protocol (IP)/multiprotocol label switching (MPLS) and carrier Ethernet. Each technology is described from the metro standpoint and how it is used as a service offering medium. We then focus on futuristic technologies such as software-definednetwork (SDN) and networkfunction virtualization (NFV). A detailed guide towards best practices for provider networks summarizes the chapter.

Ashwin Gumaste

### 19. Energy Efficiency in Optical Networks

Energy efficiency is important for optical networks in terms of scalability, low-cost operation, and sustainability. At the same time, optical networks play an important role in enabling energy efficiency for the Internet as a whole and for information and communication technologies more generally. Understanding energy in the context of optical networks begins with an understanding of the constituent components and equipment that make up an optical network and how their energy use is evolving over time. The network architecture, describing how these parts are put together and operated, will also have a considerable impact on the energy efficiency of optical networks. This impact can be modeled and understood in different ways. Perhaps the largest energy impact, however, comes from including optical networks in cross-layer design and survivability strategies. These aspects of energy-efficient optical network design are examined, along with issues related to mobile and optical network convergence, nonlinear optics and optical processing, and computer and optical network cross-optimization. An introduction to resources for recommendations and guidance from standards bodies and other organizations on the energy efficiency of optical networks is also provided.

Daniel C. Kilper

### 20. Optical Packet Switching and Optical Burst Switching

Optical transmission has long been the established choice for nonwireless data transmission spanning distances longer than a few tens of meters, due to its high bandwidth and electromagnetic noise immunity. Most current high-bandwidth networks are essentially a group of fiber-optic links connected by nodes whose function is to forward incoming data to the appropriate output. The input to these nodes is data in optical form, and their output is also data in optical form. Thus, it makes sense to also process the data in the optical domain in order to have simple node architectures that improve reliability, performance, and cost. However, optical node technology cannot yet match the flexibility of electronic technology: The main roadblocks are the lack of random-access optical memories and of optical processors. Industry has addressed this lack by creating two kinds of nodes that achieve opposite extremes in the trade-off between efficiency and complexity: electronic packet switching (electronicpacket switching (EPS)) nodes and optical circuit switching (opticalcircuit switching (OCS)) nodes.

Pablo Jesus Argibay-Losada, Dominique Chiaroni, Chunming Qiao

### 21. Evolving Requirements and Trends of Datacenters Networks

In this manuscript, we present an overview of Google’s datacenter network, which has led and defined the industry over the past few decades. Starting inside the datacenter, we cover all aspects, from networking/topology to key hardware components of interconnect and switching, traffic/throughput, and energy usage/efficiency for intra-datacenter networks. Likewise, we discuss topology and interconnect for inter-datacenter networks. With particular focus on optical interconnect, we also discuss future technology directions for scaling bandwidth through a combination of higher baud rates, wavelength-division multiplexing (wavelength-division multiplexing (WDM)), coherent communication (polarization multiplexing, I/Q modulation), and space-division multiplexing (space-division multiplexing (SDM)), along with the corresponding trade-offs between these various dimensions and how these trade-offs are adjusted at different length scales. Although questions remain on the exact implementation to be adopted in the future, one thing is clear: the evolution of datacenter networks and the underlying technologies have been and will remain a critical driver for enabling new compute capabilities in the cloud.

Hong Liu, Ryohei Urata, Xiang Zhou, Amin Vahdat

### 22. Evolving Requirements and Trends of HPC

High-performance computing (HPC)high-performancecomputing (HPC) denotes the design, build or use of computing systems substantially larger than typical desktop or laptop computers, in order to solve problems that are unsolvable on these traditional machines. Today's largest high-performance computers, a.k.a. supercomputerssupercomputer, are all organized around several thousands of compute nodescompute node, which are collectively leveraged to tackle heavy computational problems. This orchestrated operation is only possible if compute nodes are able to communicate among themselves with low latency and high bandwidth.In 2004 the ASCI Purple supercomputer was the first to implement optical technologies in the interconnects that support these internode communications. However, research on optical interconnects for HPC applications dates back to the early 1990s. Historically, HPC has been a large driver for the development of short-distance optical links, such as the ones found in today's datacenters (as described elsewhere in this volume). As the number of research areas and industries that exploit HPC is growing, the need for improved HPC interconnection networks is expected to persist.In this chapter we review the requirements of current HPC systems for optical communication networks and we forecast future requirements on the basis of discernible HPC trends.

Sébastien Rumley, Keren Bergman, M. Ashkan Seyedi, Marco Fiorentino

### 23. Intra-Datacenter Network Architectures

This Handbook chapter provides an overview of existing interconnection topologies and architectures for large-scale cloud computing systems. We discuss (a) tree-based solutions (also referred to as indirect topologiesindirecttopology in this chapter), largely adopted in most of today's data center systems, as well as (b) directly connected topologiesdirectly connected topology, such as full mesh (all-to-allall-to-all), flattened butterflyflattened butterfly (FB), and HyperXHyperX. This chapter also touches upon some emerging interconnect solutions enabled by recent advances in photonic integrated technologies and switches, while more details about these aspects can be found in the Handbook Chaps. 21 , 24 , and 25 .

Roberto Proietti, Pouya Fotouhi, Sebastian Werner, S.J. Ben Yoo

### 24. System Aspects for Optical Interconnect Transceivers

Data centers have shown and will continue to show tremendous growth. The data center requirements and the resulting architectures determine the optimal optical interconnect approach. This work outlines hyperscale data center architectures and implications for the optical transceiver technology used therein. The Microsoft hyperscale data center architecture leads to the choice of the four-lane parallel single-mode fiber (PSM4) transceiver as optimal. We review the various optical transmitter technologies, including directly modulated lasers (directly modulated laser (DML)s), integrated distributed feedback laser/electroabsorption modulators (distributed feedback (DFB)/electroabsorptionmodulator (EAM)), and silicon photonics (SiPho). The increase in optical signaling rates from 25 to 50–100 Gb $$/$$ / s per wavelength is described, along with their inclusion in defined optical specifications from the IEEE Ethernet standardization organization, as well as industry multisource agreements (multisource agreement (MSA)s). Finally, we show how data center bandwidth requirements will lead to an evolution of optical packaging paradigms from pluggable modules to on-board-optics and copackaged optics.

### 25. Optical Switching for Data Center Networks

Cloud computing, the Internet of Things, and Big Data applications are imposing stringent requirements on communications within warehouse-scale data centers (DC) in terms of high bandwidth, low latency, and massive interconnectivity. Traditional DC networks based on electronic switching use hierarchical tree-structured topologies that introduce communication bottlenecks and require high energy consumption. Thus, to enable scalable growth both in the number of connected endpoints and in the exchanged traffic volume, novel architectural and technological innovations have to be investigated.Optical switching technologies are attractive due to their transparency to data rate and data format, and enable energy-efficient network architectures that eliminate layers of power-consuming optoelectronic transceivers. In particular, new architectures that exploit optical circuit switching (opticalcircuit switching (OCS)), optical packet switching (optical packetswitching (OPS)), and optical burst switching (optical burstswitching (OBS)) technologies have been widely investigated recently for intra-DC networks.This chapter reports on the technologies used to implement OCS, OPS, and OBS nodes, together with recently investigated and demonstrated optical data center network (data center network (DCN)) architectures.

Nick Parsons, Nicola Calabretta

### 26. Introduction to Optical Access Networks

Fixed-access networks have had a tremendous impact on society over the last few decades enabling residential broadband services and being a driver for the digitalization of society. With increasing broadband speeds, optical access technologies are playing an increasingly important role for fixed access. Growing capacity demand is driving deeper fiber penetration and fiber-to-the-home (fiber-to-the-home (FTTH)) deployments. An important category of optical access systems is passive optical networks (passive opticalnetwork (PON)s). PON systems are designed to meet the requirements of access networks, supporting cost effective deployment and high-end user peak rates. Several generations of PONs have been specified both in ITU-T and IEEE. Deployed systems have predominantly been based on time division multiplexingtime-division multiplexing (TDM)passive optical network (TDM-PON) (time-division multiplexing (TDM))-PON. Recent standardization in ITU-T have specified next-generationnext-generation(NG)-PON2 (NG)-PON2, which is the first multi-wavelength access standard. Beyond higher capacity residential access, optical access is also expected to play an increasingly important role in providing transport services for mobile networks, supporting growing data rates and denser radio access networks. This introductory chapter on optical-access reviews the evolution of fixed-access network architecture and presents a technology overview of optical access systems.

Björn Skubic, Lena Wosinska

### 27. Current TDM-PON Technologies

This chapter describes many aspects of time-division multiplexing-passive optical network (time-division multiplexing (TDM)-passive opticalnetwork (PON)) technology. TDM-PON is the architecture for optical access systems preferred by many network operators, due to its low operational and capital expenses. After the overview, topics related to the physical layer, such as burst-mode transmission and analog video distribution, are addressed. Then, topics in the higher layers of the network, such as the access control technologies, dynamic bandwidth allocation, security and privacy issues, protection switching and methods to improve energy efficiency like sleep modes, are described in detail for both ITU-T- and IEEE-based PONs. Finally, we conclude with a section on technologies beyond 10G PON.

Jun-ichi Kani, Doutje van Veen

### 28. Emerging PON Technologies

This chapter focuses on recent advances in optical access networks, which are also commonly termed fiber-to-the-home (FTTH) (fiber-to-the-home) or passive opticalnetwork (PON) (passive optical networks). The last decade has seen dramatic growth in these networks; they have gone from being almost nonexistent to being a worldwide presence, providing true broadband Internet connections to end users. In recent years, research and development efforts have been directed into enhancing fiber resources and increasing the number of users and the aggregate capacities of PONs. This chapter is divided into two parts. The first (Sects. 28.1–28.3) discusses the second generation of PONs (NG-PON2), which improve upon the first generation by enabling increased bit rates and including a WDM overlay. The second part (Sects. 28.4–28.6) explores proposed longer-term approaches (a future third generation of PONs) with higher scalability and flexibility that have undergone proof-of-concept testing.

Josep Prat, Luca Valcarenghi

### 29. PON Architecture Enhancements

In this chapter, possible architecture enhancements will be discussed, that allow for applying passive optical networks (passive opticalnetwork (PON)s) to a wider range of network scenarios than previously considered. The ongoing specification and early deployment of 5G wireless networks and services turns out to be an important driver for the evolution of future optical access technologies and network architectures. The interworking of wireless networks with PONs, which provide for fixed transport, will hence be the starting point for introducing advanced architectures, especially those that can support low-latency requirements as imposed by 5G radio technologies and services (Sect. 29.2). These services are provided over a flexible and versatile (long-reach) PON infrastructure on a metro scale, together with other service types over the same common network. Most hardware functions of such networks will be virtualized on data center platforms, supported by centralized resource orchestration across multiple network segments and technologies (Sect. 29.3). In some scenarios, direct optical links between the end nodes of a PON segment enable lowest transmission latency or offloading high traffic volumes from the main PON link. Sample use cases, as found in wireless and intradata center networks, are discussed in Sect. 29.4. Finally, in Sect. 29.5, optical solutions are introduced that can help in remotely supervising and managing the passive fiber infrastructure, as well as in reconfiguring the connectivity map of complex PON-based metro-access networks, while respecting access operational models and cost targets.The architectures presented are based on current PON technologies and deployment practices. Most of the modifications described that are required for accommodating advanced functionalities, such as those mentioned before, are either under investigation in research or even under development already. A few are still considered only on the conceptual level.

Thomas Pfeiffer

### 30. Long-Reach Passive Optical Networks and Access/Metro Integration

The physical layout of cables and nodes in many of today’s passive optical networks (passive opticalnetwork (PON)s) still dates back to the early days of copper loop installations, with customer to exchange node distances limited to a few kilometers largely by the transmission distance of analogue telephony over the copper pairs. However, modern optical communication technologies can enable much longer transmission distances, and through the use of optical amplification can effectively integrate the access and metro portions of the network into a single all-optical communication system, which is commonly referred to as “Long-reach PON” (long-reachpassive optical network (LR-PON)) or “Amplified PON”. LR-PONs offer several advantages in terms of infrastructure sharing, network node consolidation and core network delayering.This chapter describes the history of the development of LR-PONs, the technical design and the enhancements that can be added, such as flexible or dynamic wavelength assignment and the benefits for the end to end network architecture when the LR-PON is used to its full capability. We use results from recent research projects to illustrate the advantages of changing the overall network architecture to enable much higher sustained user bandwidths while reducing power consumption per user and improving economic viability. We also review recent experimental demonstrations of the end-to-end operation of such systems which validate the viability of these concepts using currently available components and technologies.

David Payne, Giuseppe Talli, Marco Ruffini, Alan Hill, Paul Townsend

### 31. Digital Optical Front-Haul Technologies and Architectures

This chapter analyzes the evolution of fronthaul interfaces and networks, based on current standardization activities. Then, since fronthaul networks require, as any transport network, multiplexing and switching technologies, different options are discussed, highlighting their pros and cons and application space: multiplexing at physical layer, both in space (relying on dark fibers) and wavelength domain; circuit multiplexing, with focus on the widespread ITU-T Optical Transport Network standard; and packet switching, where satisfying strict timing constraints and maintaining the benefits of statistical multiplexing at the same time poses new design trade-offs, which are also discussed. The final part of the chapter addresses the evolution of the fronthaul network as a multilayer network that can efficiently exploit all the above multiplexing technologies, compatibly with the split options of the radio protocol stack defined at 3GPP for 5G and the introduction of new packet fronthaul interfaces.

Fabio Cavaliere, David Larrabeiti

### 32. Analog Optical Front-Haul Technologies and Architectures

New traffic-generating technologies such as the Internet of Things and continued growth in mobile traffic are requiring mobile networks to rapidly expand their capacities. However, these increased capacity demands are not coupled to increased revenue, so the efficiencies and new architectures of the access network are required to keep pace. Optical fiber is now the medium of choice for radio access network backhaul due to the range of advantages it offers and the new architectural configurations it enables. These advantages are becoming particularly attractive as radio access networks shift, as predicted, to an increased number of small cells (micro- and picocells) to support 5G coverage. With increased bandwidths and higher-frequency systems, the case for radio-over-fiber systems grows. Although digital techniques currently dominate for fronthaul applications, the increased bandwidth requirements of digital interfaces mean that analog techniques may become favorable as capacity grows. In this chapter, we review the requirements of analog radiofrequency (radiofrequency (RF))–optical transport systems and provide an overview of the different optical configurations that are used or have been proposed for radio-over-fiber networks.

John E. Mitchell

### 33. Optical Networking for 5G and Fiber-Wireless Convergence

Due to the drastically increased capacity and exploitation of higher RF bands by the upcoming 5G New Radio (5GNew Radio (5G NR)) standards, revolutionary changes are set to occur in next-generation mobile data networks. In the 5G NR specification being developed by 3GPP NR, three different application scenarios are defined: enhanced mobile broadband (enhancedmobile broadband (eMBB)), massive machine-type communication (massive machine-type communication (mMTC)), and ultra-reliable low-latency communication (ultra-reliable low-latency communication (URLLC)). Furthermore, following recent debate, the major standards bodies have agreed that both low-density parity check (low-density parity check (LDPC)) and polar codes should be employed as data and control channel coding options, respectively. Thus, it is clear that, in addition to higher throughput, the flexibility to meet different needs in various application scenarios and to utilize the advantages of different technologies will be key to the construction of 5G fiber-wireless converged heterogeneous networks. In this chapter, to show how optics can support 5G heterogeneous wireless communications, technologies used in fiber-wireless integrated radio access networks (radio accessnetwork (RAN)s) are reviewed. Aspects such as mobile fronthaul evolution, all-spectrum fiber wireless access technologies, and optical signal processing techniques in 5G converged networks are also discussed.

Gee-Kung Chang, Mu Xu, Feng Lu

### 34. Space Optical Links for Communication Networks

Future spacecraft will require a paradigm shift in the way information is transmitted, in light of the continuous increase in the amount of data requiring space links. Current radio-frequency-based communication systems impose a bottleneck on the volume of data that can be transmitted back to Earth due to technological as well as regulatory reasons. Free-space optical communication has finally emerged as a key technology for solving the increasing bandwidth limitations for space communication while reducing the size, weight and power of satellite communication systems, and taking advantage of a license-free spectrum.This chapter focuses on communication links where one of the terminals is in space, and it is organized as follows: Section 34.1 gives a quick overview of key concepts of free-spaceoptical communication (FSOC) that aid in understanding how this technology is used to design lasercom links. Section 34.2 describes how the laser communication signals are affected as they propagate through the atmosphere. Sections 34.3 and 34.4 explain the two applications with greater potential, each having unique characteristics and advantages. Finally, Sect. 34.5 provides an insight of what optical satellite networks may progress toward in the future.

### 35. Visible Light Communications

Current wireless communications need to fulfill two important requirements according to different applications. The first is to achieve high-speed and long-distance data transmission, and the second is to realize ubiquitous and short-range information services. As a carrier for wireless communications, radio, infrared radiation, and visible light (VL) are complementary transmission media, and different applications call for the use of one medium or the others. Radio and infrared radiation are favored for the first requirements. For the second requirement, however, visible light communication (visible light communication (VLC)) is an advancing field, which has received much attention. VLC transmits data by intensity modulation of light sources, such as light emitting diodes (LEDs) and laser diodes (LDs), which are faster than the response of the human eye. VLC merges lighting and data communications in applications such as indoor lighting, signboards, wireless local area networks (wirelesslocal area network (W-LAN)), streetlights, vehicles, traffic signals, underwater signals, and so on. This chapter describes the methods and application of VLC and covers topics such as light sources, receivers, VLC technologies, and current applications including ubiquitous indoor information services, visible-light wireless LAN (visible-light wireless LAN (VLW-LAN)), and underwater visible-light wireless communication (underwater visible-light wireless communication (UVLWC)).

Xin Lin, Tomokuni Matsumura

### 36. Optical Communications and Sensing for Avionics

This chapter is a review of avionicsavionics optical fiber communication and sensing systems. Optical fibers are the most common means of optical communication. One important extension of fiber-optic communication technology for aerospace applications is the fly-by-light (fly-by-light (FBL)) approach for aircraft control systems. Inherent characteristics of the FBL approach such as its light weight, compact size, large bandwidth, and immunity to electromagnetic interference (electromagnetic interference (EMI)) make it an ideal technology for future flight controlflight control systems. FBL control systems offer some advantages for the new aircraft generation within more hostile military environments. In addition to their value in communication, optical fibers have the capability to sense and provide information about the environment they are exposed to. There are various types of fiber-optic sensorfiber-optic sensors that can provide detailed data on parameters such as temperature, strain, pressure, vibration, and acoustic emissions. Accurate knowledge of these parameters is vital to the structural design and safe operation of various components that must often operate in high temperature or mechanically hostile environments; well-designed optical fiber sensors can provide measurement possibilities in these environments. This chapter will provide insight into a number of these opticalsensingoptical sensing techniques, which have demonstrated significant potential for enabling accurate measurements in harsh environments.

Alireza Behbahani, Mehrdad Pakmehr, William A. Stange

### Backmatter

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