Technologies for Optical Networking

The technologies used in optical networks have evolved seamlessly over the past six decades. Optical fibers with extremely low loss and enormous bandwidth are used as the transmission medium, while semiconductor lasers and LEDs serve as optical sources, and the photodetectors – pin and avalanche photodiodes – are used to receive the optical signal at the destination nodes. The transmitted optical signal has to pass through a variety of network elements, which in turn need a wide range of passive and active devices, carrying out the necessary networking functionalities. For WDM optical networks, many of these tasks need to be accomplished in the optical domain itself in a wavelength-selective manner, calling for various types of WDM-based networking elements. In this chapter, we present a comprehensive description of the optical and optoelectronic devices that are used in today’s optical networks. (137 words)

WDM Access Networks

In TDM PONs, or simply PONs, the transmission capacity of optical fibers remains under-utilized with single-wavelength transmissions for both upstream and downstream traffic. Use of WDM transmission in PONs can significantly enhance the overall network capacity and coverage area. WDM transmission in PONs can be realized in several ways, such as by using WDM exclusively or combining WDM with TDM, leading to WDM PONs and TDM-WDM PONs (TWDM PONs). This chapter gives several candidate physical configurations to realize WDM and TWDM PONs, and describes some useful dynamic wavelength-bandwidth allocation (DWBA) schemes for the latter. We also briefly discuss the needs of open access to the PON-based access networks for the various stakeholders, such as service providers and network providers. Finally, we describe briefly the roles of optical networking in the access segment of mobile networks. (135 words)

WDM Local-Area Networks

WDM-based broadcast-and-select transmission over optical passive-star topology can significantly enhance the speed of optical LANs/MANs. This type of optical LAN/MAN (or simply WDM LAN) can function using a variety of network architectures. In particular, WDM LANs can transmit packets between two nodes using direct or single-hop transmission or through intermediate nodes using multihop transmission, leading to broadly two types of network architectures: single-hop and multihop. The nodes in WDM LANs can employ two types of transmitters and receivers: tunable transmitter (TT) or fixed transmitter (FT) and tunable receiver (TR) or fixed receiver (FR), giving four types of WDM transceiver configurations: TT-TR, TT-FR, FT-TR, and FT-FR. Of the four configurations, the first three can realize single-hop communication, while the fourth generally leads to multihop networks. In this chapter, we describe various types of passive-star-based WDM LANs and examine their salient performance features. (141 words)

SONET/SDH, OTN, and RPR

With the emergence of high-speed optical transmission, the pre-existing plesiochronous digital hierarchy (PDH) appeared unsuitable for achieving network synchronization, leading to the development of the synchronous optical network (SONET) and synchronous digital hierarchy (SDH) as the two equivalent standards for circuit-switched optical networks. Several bandwidth-efficient techniques were also developed to carry packet-switched data traffic over SONET/SDH networks, offering some useful data-over-SONET/SDH architectures. Subsequently, with the increasing transmission rates for SONET/SDH and Ethernet-based LANs, a convergent networking platform called optical transport network (OTN), was developed. With the ever-increasing volume of bursty data traffic, a standard for packet-switched ring networks, called resilient packet ring (RPR), was also developed for better bandwidth realization in optical fibers. In this chapter, we first present the SONET/SDH networks and the techniques for supporting the data traffic therein, followed by a description of the basic concepts and salient features of the OTN and RPR networks. (147 words)

Elastic Optical Networks

In WDM networks using a fixed frequency grid, transmission rates can vary for different connections, leading to inefficient bandwidth utilization in optical fibers with lower-rate connections using wide frequency slots. In elastic optical networks (EONs), the frequency grid is made flexible, thereby improving the effective network capacity. A flexible frequency grid consists of smaller frequency slots, and a transmitting node can use multiple slots using suitable modulation techniques, such as optical OFDM, Nyquist-WDM and optical arbitrary waveform generation (OAWG). However, this requires bandwidth-variable transceivers (BVTs) and other devices to set up variable-rate connections. First we discuss the design challenges in EONs and describe the evolving technologies for the network elements. Then we present some offline (LP-based and heuristic) design methodologies for EONs to carry out routing and spectral allocation (RSA) for the required connections. Finally, we present some online fragmentation-aware RSA schemes for the operational EONs. (146 words)

Optical Packet and Burst-Switched Networks

In order to address poor bandwidth-utilization in circuit-switched WRONs, various techniques for optical packet-switching (OPS) have been explored, but needing complex technologies, such as real-time header extraction/insertion, packet alignment, etc. An intermediate solution between the WRONs and OPS networks – the optical burst-switched (OBS) network – has been explored, where several packets are clubbed together at ingress nodes to form optical bursts, which are transmitted with the headers sent as control packets ahead of each bursts. With this prior resource-reservation scheme at en-route nodes before burst arrivals, OBS networks overcome the challenges of OPS networks, while improving bandwidth utilization as compared to WRONs. We first present the node architectures, followed by header-processing schemes and switch designs for OPS networks. Next we present the basic concepts of OBS networking and describe the necessary network protocols, including burst assembly scheme, just enough time (JET) signaling, resource-reservation and routing schemes. (145 words)

WDM Metro Networks

With increasing traffic, legacy SONET/SDH-metro rings have mostly been upgraded using point-to-point WDM (PPWDM) transmission. For cost-effective realization, WDM metro networks also employ wavelength-routed optical networking (WRON), where a wavelength is bypassed optically at intermediate nodes. Metro networks are generally split into core and edge rings, with the metro-core ring interfacing with the long-haul backbone and the metro-edge ring interconnecting the metro-core ring with the access segment. The PPWDM/WRON transmission is employed with each wavelength using circuit-switched SONET/SDH transmission, though the metro-edge rings can also use packet-switching to enhance bandwidth utilization with bursty traffic from the access segment. In this chapter, we consider first WDM metro networks using PPWDM/WRON-based rings and present their design methodologies using LP-based and heuristic schemes. We describe some packet-switched WDM ring testbeds and examine the possible improvement in bandwidth utilization therein, as compared to the circuit-switched WRON rings. (143 words)

Optical Networks: An Overview

We present an overview of optical networks, beginning with a background of today’s telecommunication networks and the roles of optical fibers therein. Next, we describe the chronology of developments in telecommunication networks starting from the days of public-switched telephone network (PSTN) offering ‘plain old telephone service’ as the basic service, followed by the divestiture of Bell Laboratories and subsequent developments of the demonopolized regime of telecommunication networks with multiple services offered to users by the same network providers. Then we describe the salient features of the two generations of optical networks for various network segments, including single-wavelength and WDM-based LANs/MANs, accesses networks, metro and long-haul networks, datacenters, and elastic optical networks. Finally, we discuss briefly the possible network architectures with the evolving optical-networking technologies. (124 words)

Survivability of Optical Networks

Telecommunication networks with their unprecedented penetration in today’s society must deliver services that can survive unpredictable failures across the network. Recovery from failure can be made in various ways, broadly categorized in two types: protection and restoration schemes. Realization of these schemes varies for different network segments: access, metro, and long-haul networks. Protection schemes are proactive in nature and need more resources, while ensuring fast recovery from failure. However, restoration schemes are reactive in nature, as in such schemes a network starts exploring the possible alternate connections after a failure occurs, and hence they offer slower recovery while needing fewer resources. We present various protection and restoration schemes, as applicable to the respective network segments: PON, SONET/SDH and WDM-over-SONET/SDH rings, and long-haul WRONs, and we discuss the underlying issues for the implementation of these schemes. (135 words)

Optical Network Control and Management

The task of network control and management is generally realized in two logical planes – control and management – which collaboratively operate to ensure smooth, secure, and survivable traffic flow in the data plane of the network. Some of the functionalities are realized in the control plane, needing real-time execution, such as recovery from network failures, and network reconfiguration due to traffic variation. Other functionalities deal with performance monitoring, configuration management, network security, accounting and billing etc., which are less time-sensitive and are addressed by the management plane. We first discuss the philosophy of multiple-layer abstraction of telecommunication networks, including control, management, and data planes, and then describe various network control and management techniques used in optical networks: operation, administration, and management (OAM) in SONET, generalized multiprotocol label switching (GMPLS), automatically switched optical network (ASON), and software-defined optical networking (SDON) in WDM networks. (141 words)