Coherent Optics for Access Networks

Zhensheng Jia, Luis Alberto Campos, Zhensheng Jia, Luis Alberto Campos

• Publisher: CRC Press
• Year: 2019
• Print ISBN:
  9780367245764
• eBook ISBN:
  9781000736960
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

This book will highlight the motivation for coherent optics in access and introduce digital coherent optical system in detail, including advanced modulation formats, architecture of modulation and detection, and DSP flow for both transmitter and receiver. This book will also demonstrate potential approaches to re-design and re-engineer the digital coherent concept from long-haul and metro solutions to the access network, leveraging reduction in complexity and cost as well as the benefits of capacity increases and operational improvements. This book will illustrate the details on optimization of the digital, optical, and electrical complexity and standardization and interoperability.

1 Access Networks Evolution

Luis Alberto Campos and Zhensheng Jia

CONTENTS

• 1.1 Introduction
• 1.2 Traffic Growth
• 1.3 Telco, Mobile and Cable Access Networks
• 1.4 Data Center Interconnectivity Networks
• 1.5 Access Networks Characteristics
• 1.6 Fiber Deployment in the Access
• References

1.1 Introduction

There has been an amazing growth in data networks since the early days of the Internet. In the beginning, the Internet consisted of a network of connected servers at educational and industrial locations that users would access by being directly connected to the server on site or indirectly through remote terminals with modems connected to the main computers through telephone lines. These remote telephony access links represent the very first access networks. These networks have grown at exponential rates to the point that today they cover more than 3 billion users worldwide connected to the Internet across fixed and wireless networks [1]. The predominant way subscribers access the Internet is through a service provider’s access network.

Access networks can either be connected through wires or be wireless. Wired access networks connect subscribers via twisted pair of wires, coaxial transmission lines, through fiber and even through power lines. Wireless access networks provide Internet access through the cellular mobile infrastructure, wireless access points using WiFi, fixed networks that rely on static point-to-point wireless connections, and satellite connectivity.

The capability of these networks has also been growing to keep up with the exponential growth of demand for capacity. By extrapolating Nielsen’s law of Internet bandwidth, broadband service rates are expected to reach 10 Gbps by 2023 and 100 Gbps by 2029 [2].

Access networks have been originally designed for the delivery of specific services but have been adapted for Internet connectivity.

In all cases, providers have optimized cost as they have leveraged much of their existing infrastructure to provide Internet connectivity. Evolution of networks through fiber optic links has played a greater role in meeting the increased demand for capacity. Optical transport has been a tool used by telephony, cable TV and mobile service providers. These networks have introduced more and more fiber as the demand is keep increasing. Service providers have not limited themselves to provide connectivity to the masses, but they have also provided connectivity to enterprises, building metropolitan and regional aggregation networks.

This chapter analyzes different types of optical transport and access network architectures. The optical transport technologies that have been dominant in the access environment such as analog optics and baseband digital optics leveraging intensity-modulation and direct-detection (IM-DD) are also discussed. The characteristics and limitations of these technologies are important to understand the drivers for introducing coherent optics in the access network as a long-term technology strategy. Current use of point-to-point (P2P) and point-to-multipoint (P2MP) technologies in the access is also discussed.

1.2 Traffic Growth

Video-intensive technologies require the most bandwidth, and immersive applications leveraging Virtual Reality/Augmented Reality (VR/AR) are the most demanding out of all video applications. Current VR applications are little more than 360° video/panoramas. A low-quality 360° video requires at least a 30 Mbps connection, HD quality streams easily surpass 100 Mbps, and retina quality (4k+) streams approach Gbps territory. Looking one step ahead in the evolution of immersive videos, holographic displays or light-field displays are being prototyped. As the name indicates, light-field displays reproduce the original light fields through a display, generating the original electromagnetic waves that would have been generated from actual objects from a display window. This allows the viewer to move and change viewing angles and perceive the natural changes of the image, such as seeing objects behind the foreground appear with viewing angle changes. Light-field displays provide true 3D and immersive visual representation without headsets. It is estimated that a commercial size display would require a capacity around 1.5 Gbps [3].

However, there are still many things holding back its use beyond showrooms and proof of concepts, the most glaring problem being the network’s capacity.

These days, it seems that just about everything is getting smarter, from thermostats to refrigerators, and becoming ever more connected. Each of these devices – physical objects with data sensing, analyzing and recording functions plus the ability to communicate remotely – collectively forms the “Internet of Things” (IoT). Clearly, expansion in the use of smart devices is an unstoppable force, but one thing could hamper this growth – inadequate bandwidth. While most of the devices that comprise the IoT communicate wirelessly all the world over, all the data they send must be transmitted over a physical wireline network between wireless access points. High-throughput, low-latency and high-reliability networks will be needed for applications such as video analytics in public safety and to support self-driving cars.

In the upcoming 5G era, the impacts of massive MIMO (Multiple-Input Multiple-Output), Carrier Aggregation, Multi-band support and radio cell densification necessitate bandwidth requirements, while the impact of coexistence between macro-, micro-, pico- and small cells in a centralized/virtualized processing environment calls for flexibility requirements. Fiber and optical access technologies are expected to play more and more important roles in the fronthaul and backhaul services to meet the aggressive performance goals of 5G.

Access networks have been evolving to meet the ever-increasing demand for capacity. Three dominant approaches have been used to address the increasing demand for capacity: first, increasing optical transport efficiency with greater capacity per bandwidth; second, segmenting network into smaller serving areas and smaller wireless cells to dedicate the same target capacity to fewer end devices and to be closer to subscribers for higher transport performance; and third, adding more spectrum. The fourth, least desired and more costly approach entails deploying more fiber.

1.3 Telco, Mobile and Cable Access Networks

An access network is the one that a network operator uses to provide connectivity to a large number of subscribers. Subscriber connectivity is provided through an aggregation and distribution center. Telephone and cellular aggregation and distribution centers are called central offices, while in the case of cable, they are often referred to as hubs.

While telephone, cellular and cable networks were, in the past, differentiated from each other in topology, architecture and the services they carry, in recent years, an evolution from different starting points, converging toward the delivery of IP services, has resulted in these networks resembling each other as they continue to evolve.

Earlier, cable networks focused on delivery of video services through end-to-end coaxial transport, but later on, they evolved toward a bidirectional hybrid fiber coaxial (HFC) network to carry data, voice and video services.

Telephone networks initially focused on telephony services through point-to-point twisted pair connectivity from central office to subscribers. Analog voice transport evolved to 56/64 kbps digital voice, and this evolved again into digital subscriber line (DSL) services, leveraging twisted pair infrastructure with digital signal processing to achieve high data-carrying capacity.

Cellular networks consisted of macro-cells to deliver wireless telephony services, but by augmenting the spectrum they use and increasing the number of cells, cellular networks evolved toward the transport of high-bandwidth data in addition to voice services.

All of the above networks, in their quest to meet ever-increasing capacity requirements of subscribers, have deployed more fiber in their networks.

Many of these service providers have introduced fiber-to-the-home (FTTH) networks to address capacity issues in high-demand areas. These fiber networks are also known as passive optical networks or PONs. The dominant digital 10G PONs standards are XG-PON/XGS-PON [4,5] and 10G- Ethernet passive optical network (10G-EPON) [6]. XG-PON supports 10 Gbps downlink (DL) and 2.5 Gbps uplink (UL), XGS-PON provides symmetrical 10 Gbps speed, while 10G-EPON supports 10 Gbps DL and 1.25 Gbps UL. In these PON systems, the network transitions from one optical line termination (OLT) unit to 32 or 64 optical end-points or optical network units (ONUs) typically traverse a distance of no more than 20 km. Today, PON technology uses optical IM-DD schemes. The sensitivity of IM-DD limits the reach of PON to 20 km.

1.4 Data Center Interconnectivity Networks

Data center networks are very different. Even though many data center networks may be deployed in metropolitan area settings and may share similar distances as those found in access networks, they should not be considered access networks. Data centers and their networks have different growth characteristics, both in the number of links that interconnect data centers and in the capacities of those links. Data center topologies are also quite different from access network topologies. While access networks for the most part are implemented to establish connectivity from one to a very large number of end-points, data center networks are implemented establishing full mesh interconnectivity between fewer data center end-points. The capacity between any two data center end-points is many times massive, requiring a large number of parallel links to establish connectivity between the different data center end-points.

1.5 Access Networks Characteristics

Service selection, transmission medium and transport technologies unique to each type of access network have implications on the network size and capabilities, on how these access networks have been deployed and how they have evolved. Some details of how the different access technologies correspond to and differ from each other in these areas, including the fiber infrastructure associated with each type of access technology, are discussed in the following.

Telcos have fiber links that connect remote terminal digital subscriber line access multiplexers (DSLAMs) and remote G.fast units, which are capable of much greater speed than central office (CO)–based DSLAMs when in closer proximity to subscribers. Cellular companies have evolved from macro-cells to small cells to increase capacity/cell, while cable companies have evolved to distributed access architectures to increase capacity per node and transport efficiency. The next-generation transport architectures of both cellular and cable networks incorporate function splits that impact not only optical network topology but also optical transport resources.

Today, optical backhaul capacity to these smaller serving areas is provided through PON technology or point-to-point gigabit ethernet. These fiber links typically carry no more than 10 Gbps to the aggregation center such as a central office or a hub location. Newer PON standards are aimed to extend the capacity up to 50 Gbps [7].

Remote DSLAMs should be able to handle more than 500 users, while fiber-to-the-distribution-point (FTTdp) and G.Fast nodes would handle fewer users consistent with the shorter loop reach.

In cable, subscribers within a serving group are, in general, served from a coaxial network that extends from a fiber node. In cable, the serving ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication Page
• Table of Contents
• Foreword
• Preface
• Acknowledgments
• Editors
• Contributors
• Chapter 1 Access Networks Evolution
• Chapter 2 Direct-Detection Systems for Fiber-Access Networks
• Chapter 3 Digital Coherent Optical Technologies
• Chapter 4 Coherent Technology Transition to Access Networks
• Chapter 5 Coherent Optics Use Cases in Access Networks
• Chapter 6 Simplified Coherent Optics for Passive Optical Networks
• Chapter 7 Access Environment Considerations for Coherent Optics Systems
• Chapter 8 Photonics Integrated Circuits for Coherent Optics
• Chapter 9 Standard Development
• Chapter 10 Concluding Remarks
• Abbreviations
• Index