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Über dieses Buch

Lo, soul! seest thou not God's purpose from the first? The earth to be spann'd, connected by net-work From Passage to India! Walt Whitman, "Leaves of Grass", 1900. The Internet is growing at a tremendous rate today. New services, such as telephony and multimedia, are being added to the pure data-delivery framework of yesterday. Such high demands on capacity could lead to a "bandwidth-crunch" at the core wide-area network resulting in degra­ dation of service quality. Fortunately, technological innovations have emerged which can provide relief to the end-user to overcome the In­ ternet's well-known delay and bandwidth limitations. At the physical layer, a major overhaul of existing networks has been envisaged from electronic media (such as twisted-pair and cable) to optical fibers - in the wide area, in the metropolitan area, and even in the local area set­ tings. In order to exploit the immense bandwidth potential of the optical fiber, interesting multiplexing techniques have been developed over the years. Wavelength division multiplexing (WDM) is such a promising tech­ nique in which multiple channels are operated along a single fiber si­ multaneously, each on a different wavelength. These channels can be independently modulated to accommodate dissimilar bit rates and data formats, if so desired. Thus, WDM carves up the huge bandwidth of an optical fiber into channels whose bandwidths (1-10 Gbps) are compati­ ble with peak electronic processing speed.

Inhaltsverzeichnis

Frontmatter

Introduction

Chapter 1. Introduction

Abstract
The emergence of high-speed applications such as video-conferencing and the rapid growth in the number of networked users demand a network infrastructure which is capable of delivering huge amounts of data in real-time over a wide area. Optical fiber has emerged as an excellent medium for such future networks in view of its tremendous bandwidth potential (around 50 THz). It is already being used as a transmission medium of choice by several telecommunications companies in the United States and abroad (Com, 1998; Chaffee, 1987). Apart from its huge bandwidth capability, optical fiber also offers a low attenuation loss (roughly 0.2 dB/km between 1200 nm and 1600 nm wavelengths) and extremely low bit-error rates, making it ideally suited for long-haul communications.
Byrav Ramamurthy

Optical Network Devices

Chapter 2. Optical Network Devices

Abstract
Research and development on optical wavelength division multiplexed (WDM) networkshave matured considerably over the past few years, and a number of experimental prototypes have been deployed and are being tested in the U.S., Europe, and Japan. It is anticipated that the next generation of the Internet will employ WDM-based optical backbones.
Byrav Ramamurthy

LAN/MAN Architectures

Frontmatter

Chapter 3. Optimizing Amplifier Placements: The Equally-Powered Wavelengths Case

Abstract
Wavelength division multiplexing (WDM) provides the ability to utilize the enormous bandwidth offered by optical networks, using today’s electronics. WDM-based optical networks employing passive-star couplers have been proposed for deployment in local and metropolitan areas. Optical amplification is often required in such networks to compensate for the signal attenuation along the fiber links and the splitting and coupling losses in the network. However, an optical amplifier has constraints on the maximum gain and the maximum output power it can supply; thus optical amplifier placement becomes a challenging problem. A simplifying assumption for analytical tract ability requires that all wavelengths, present at a particular point in a fiber, be at the same power level, viz. the equally-powered wavelengths case. However, previous studies did not minimize the total number of amplifiers while achieving power equalization. In this work, we formulate the minimization of amplifiers with power equalization as a mixed-integer linear program (MILP) that can be solved by a linear program solver. Illustrative examples on sample networks are presented, which demonstrate the characteristics and the advantages of our optimal amplifier-placement algorithm.
Byrav Ramamurthy

Chapter 4. Optimizing Amplifier Placements: The Unequally-Powered Wavelengths Case

Abstract
Optical networks based on passive-star couplers and employing wavelength division multiplexing (WDM) have been proposed for deployment in local and metropolitan areas. These networks suffer from splitting, coupling, and attenuation losses. Since there is an upper bound on transmitter power and a lower bound on receiver sensitivity, optical amplifiers are usually required to compensate for the power losses mentioned above. Due to the high cost of amplifiers, it is desirable to minimize their total number in the network. However, an optical amplifier has constraints on the maximum gain and the maximum output power it can supply; thus, optical amplifier placement becomes a challenging problem. In fact, the general problem of minimizing the total amplifier count is a mixed-integer nonlinear problem. Previous studies have attacked the amplifier-placement problem by adding the “artificial” constraint that all wavelengths, which are present at a particular point in a fiber, be at the same power level. This constraint simplifies the problem into a less difficult mixed-integer linear program. Unfortunately, this artificial constraint can miss feasible solutions that have a lower amplifier count but do not have the equally-powered wavelengths constraint. In this chapter, we present a method to solve the minimum-amplifier-placement problem while avoiding the equally-powered wavelength constraint. We demonstrate that, by allowing signals to operate at different power levels, our method can reduce the number of amplifiers required.
Byrav Ramamurthy

WAN Architectures

Frontmatter

Chapter 5. Wavelength Conversion

Abstract
Wavelength conversion has been proposed for use in wavelength division multiplexed networks to improve efficiency. This study highlights systems challenges and performance issues which need to be addressed in order to incorporate wavelength conversion effectively. A review/survey of the enabling technologies, design methods, and analytical models used in wavelength-convertible networks is provided.
Byrav Ramamurthy

Chapter 6. Impact of Transmission Impairments on Network Performance

Abstract
In a wavelength-routed optical network, a transmitted signal remains in the optical domain over the entire route (lightpath) assigned to it between its source and destination nodes. The optical signal may have to traverse a number of crossconnect switches (XCSs), fiber segments, and optical amplifiers, e.g., erbium-doped fiber amplifiers (EDFAs). Thus, while propagating through the network, the signal may degrade in quality as it encounters crosstalk at the XCSs and also picks up amplified spontaneous emission (ASE) noise at the EDFAs. Since these impairments continue to degrade the signal quality as it progresses towards its destination, the received bit-error rate (BER) at the destination node might become unacceptably high. Previous work on the lightpath routing and wavelength assignment (RWA) problem assumed an ideal physical layer and ignored these transmission impairments. The main contribution of our work is to incorporate the role of the physical layer in setting up lightpaths by employing appropriate models of multi-wavelength optical devices (XCSs and EDFAs) such that the BER of a candidate lightpath can be computed, in advance, to determine if this lightpath should be used for the call. Features from existing RWA algorithms are integrated with our on-line BER calculation mechanism. Our simulation studies indicate that employing BER-based call-admission algorithms has a significant impact on the performance of realistic networks.
Byrav Ramamurthy

Chapter 7. Conclusions

Abstract
In this book, we investigated the impact of physical-layer issues on the design of wavelength division multiplexing (WDM)-based optical networks. Efficient algorithms were presented which significantly improved the cost-effectiveness of these networks. The non-ideal behavior of common physical devices was highlighted and incorporated into the study of network-control algorithms, leading to more realistic characterization of the performance of such networks. It is hoped that this study, and other similar studies, will lead to a more rapid deployment of a new generation of optical networks on a nation-wide, and even a global scale. Below, we summarize the contributions of this study and discuss possible directions of future research in related areas.
Byrav Ramamurthy

Backmatter

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