Optical WDM Networks: Concepts and Design Principles / Edition 1 available in Hardcover
Optical WDM Networks: Concepts and Design Principles / Edition 1
- ISBN-10:
- 0471671703
- ISBN-13:
- 9780471671701
- Pub. Date:
- 08/04/2004
- Publisher:
- Wiley
Optical WDM Networks: Concepts and Design Principles / Edition 1
Hardcover
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Product Details
| ISBN-13: | 9780471671701 |
|---|---|
| Publisher: | Wiley |
| Publication date: | 08/04/2004 |
| Pages: | 312 |
| Product dimensions: | 6.40(w) x 9.37(h) x 0.74(d) |
About the Author
HUSSEIN T. MOUFTAH, PhD, is Canada Research Chair Professor in the School of Information Technology and Engineering at the University of Ottawa, Canada. Formerly a professor and associate head of the Department of Electrical and Computer Engineering at Queen’s University, he is an IEEE Fellow and served as editor-in-chief of IEEE Communications magazine and later as IEEE Communications Society Director of Magazines. Dr. Mouftah is author or coauthor of three books and more than 700 technical papers and eight patents.
Read an Excerpt
Optical WDM Networks
Concepts and Design PrinciplesBy Jun Zheng Hussein T. Mouftah
John Wiley & Sons
ISBN: 0-471-67170-3Chapter One
Introduction
1.1 Optical Networks: A Brief Picture
The rapid evolution of telecommunications networks is always driven by ever-increasing user demands for new applications as well as continuous advances in enabling technologies. In the past ten years, we have witnessed the huge success and explosive growth of the Internet, which has attracted a large number of users surging into the Internet. Individual users are using the Internet insatiately for information, communication, and entertainment, while enterprise users are increasingly relying on the Internet for their daily business operations. As a result, Internet traffic has experienced an exponential growth in the past ten years, which is consuming more and more network bandwidth. On the other hand, the emergence of time-critical multimedia applications, such as Internet telephony, video conferencing, video on demand, and interactive gaming, is also swallowing up a large amount of network bandwidth. All these facts are imposing a tremendous demand for bandwidth capacity on the underlying telecommunications infrastructure.
To meet the unprecedented demand for bandwidth capacity, a bandwidth revolution has taken place in telecommunications networks with the introduction of fiber optics. As the core of this revolution, optical fibers have proved to be an excellent physical transmission medium for providing huge bandwidth capacity. Theoretically, a single single-mode fiber has a potential bandwidth of nearly 50 terabits per second (Tbps), which is about four orders of magnitude higher than the currently achievable electronic processing speed of a few gigabits per second (Gbps) [1]. Apart from the huge bandwidth capacity, optical fibers also have a number of other significant characteristics, such as low signal attenuation (about 0.2 dB/km), low error bit rate (typically, [10.sup.-12]), low signal distortion, low power requirement, low space requirement, and low cost [1-3]. However, because of the limit of the electronic processing speed, it is unlikely that all the bandwidth of an optical fiber will be exploited by using a single high-capacity optical channel or wavelength. For this reason, it is desirable to find an effective technology that can efficiently exploit the huge potential bandwidth capacity of optical fibers. The emergence of wavelength division multiplexing (WDM) technology has provided a practical solution to meeting this challenge. With WDM technology, multiple optical signals can be transmitted simultaneously and independently in different optical channels over a single fiber, each at a rate of a few gigabits per second, which significantly increases the usable bandwidth of an optical fiber. Recently, commercial WDM systems with up to 160 OC-192 (10 Gbps) channels have been announced [4]. In addition to the increased usable bandwidth of an optical fiber, WDM also has a number of other advantages, such as reduced electronic processing cost, data transparency, and efficient failure handling [1]. As a result, WDM has become a technology of choice for meeting the tremendous bandwidth demand in the telecommunications infrastructure. Optical networks employing WDM technology have been widely considered a promising network infrastructure for next-generation telecommunications networks and optical Internet [4-10].
Today, WDM technology is being deployed in various types of telecommunications networks. However, the deployment is mainly for point-to-point transmission [1]. All the routing and switching functions are still performed electronically at each network node. Optical signals must go through opto-electronic (O/E) and electro-optical (E/O) conversion at each intermediate node as they propagate along an end-to-end path from one node to another node. Consequently, a network node may not be capable of processing all the traffic carried by all its input signals, including the traffic intended for the node as well as the traffic that is just passing through the node to other network nodes, causing an electronic bottleneck. For example, a node with four input fiber links and 16 wavelengths at 2.5 Gbps on each fiber link must handle a maximum data rate of 160 Gbps, which is normally beyond the electronic processing capability of the node. To overcome the electronic bottleneck, it is desirable to incorporate optical routing and switching functions at each network node in order to bypass those signals that carry traffic not intended for the node directly in the optical domain. With the advent of reconfigurable optical devices, such as optical add-drop multiplexers (OADMs) and optical cross-connects (OXCs) [1-2], this has become possible. At the time of this writing, OADM and OXC products are commercially available from several product vendors and more advanced products are expected to become available in the near future [4]. A number of WDM networks have been tested and are being tested in the United States, Europe, and many other countries. It is expected that WDM technology will be widely deployed in all types of telecommunications networks, not only in wide area networks or backbone networks, but also in metropolitan and local area networks [11-15].
1.2 WDM Technology
Wavelength division multiplexing (WDM) is an optical multiplexing technology for exploiting the huge bandwidth capacity inherent in optical fibers. Conceptually, it is similar to frequency division multiplexing (FDM) that has already been used in radio communication systems for over a century. The basic principle is to divide the huge bandwidth of an optical fiber into a number of nonoverlapping subbands or optical channels and transmit multiple optical signals simultaneously and independently in different optical channels over a single fiber, each signal being carried by a single wavelength.
Figure 1.1 shows the transmission spectrum of an optical fiber. In this spectrum, there are two low-attenuation areas. One is centered at 1300 nanometers (nm) and the other at 1500 nm. Both areas have a range of about 200 nm with an attenuation loss less than 0.5 dB/km. Theoretically, these two areas can provide a total amount of 50 Tbps low-attenuation transmission bandwidth. However, because the maximum rate at which an end device can access an optical channel is limited by its electronic processing speed, it is technically impossible to take advantage of all the bandwidth of an optical fiber by using a single optical channel or wavelength. With WDM, the huge bandwidth is carved up into a number of optical channels with each channel operating at any feasible rate, say, a few gigabits per second to be compatible with current electronic processing speeds. Accordingly, a single fiber is theoretically capable of supporting over 1000 optical channels or wavelengths at a few gigabits per second.
Figure 1.2 shows a block diagram of a basic WDM transmission system. The network medium can be a simple fiber link, a passive star coupler, or any type of optical network. The transmitter consists of a laser and a modulator. The laser is the light source, which generates an optical carrier signal at either a fixed wavelength or a tunable wavelength. In the modulator, the carrier signal is modulated by an electronic signal and is then sent to the multiplexer (MUX). The multiplexer combines multiple optical signals on different wavelengths at its input ports into a single optical signal, which is transmitted to a common output port or optical fiber. The demultiplexer (DMUX) uses optical filters to separate the optical signal received on the input port into multiple optical signals on different wavelengths, which are then sent into the receivers. The receiver consists of a detector (e.g., photodiode) that can convert an optical signal to an electronic signal. The optical amplifiers are used to maintain the power strength of an optical signal at appropriate locations in the transmission system.
Figure 1.3 illustrates the WDM of a transmission bandwidth of 250 Gbps. The transmission bandwidth is divided into a number of optical channels, each having a smaller bandwidth at a particular wavelength. Note that it is important to have sufficient spacing between the wavelengths of adjacent channels to avoid interchannel cross talk or interference caused by spectrum overlapping and transmission imperfections. Accordingly, the size of the frequency spacing should be no less than the bandwidth of each channel. The density of channels depends on both the channel bandwidth and the frequency spacing. For example, if the bandwidth of each channel is 2.5 GHz and the frequency spacing is also 2.5 GHz, there are a total of 100 channels available over a single fiber, resulting in a total transmission capacity of 250 Gbps, as shown in Figure 1.3(a). This is an extreme case in which there is no spacing between the wavelengths of two adjacent channels. In this case, it is practically very difficult to prevent adjacent signals from interfering with each other because of the impairments occurring in transmission, such as dispersion and nonlinearity. If the channel bandwidth becomes 10 GHz and the frequency spacing becomes 20 GHz, only 13 channels are available, resulting in a transmission capacity of 130 Gbps, as shown in Figure 1.3(b). Therefore, given the bandwidth of each channel, the frequency spacing determines the density of optical channels.
On the other hand, an optical channel provides a bit rate of a few gigabits per second. To make efficient use of this bandwidth, it is often necessary to share a single optical channel among multiple low-bit-rate channels. The most common way to implement this is to use time division multiplexing (TDM) technology [1]. Although a single fiber can theoretically support a number of optical channels or wavelengths, the development of more wavelengths over a single fiber depends on advances in enabling technologies as well as the commercial availability of WDM devices. Fortunately, the last few years have seen a rapid growth of WDM transmission systems. Five years ago, commercially available WDM systems could only offer up to 32 optical channels at 2.5 Gbps over a single fiber. At the time of this writing, however, 160 channels at 10 Gbps (OC-192) have been announced as commercially available. It is expected that WDM systems supporting more channels or wavelengths will come into the marketplace in the next few years.
1.3 WDM Network Architectures
WDM networks can be classified into two broad categories: broadcast-and-select WDM networks and wavelength-routed WDM networks.
1.3.1 Broadcast-and-Select WDM Networks
A WDM network that shares a common transmission medium and employs a simple broadcasting mechanism for transmitting and receiving optical signals between network nodes is referred to as a broadcast-and-select WDM network. The most popular topologies for a broadcast-and-select WDM network are the star topology and the bus topology, as shown in Figure 1.4 and Figure 1.5, respectively. In the star topology, a number of nodes are connected to a passive star coupler by WDM fiber links. Each node has one or more optical transmitters and receivers, which can be either fixed-tuned or tunable. A node transmits its signal on an available wavelength. Different nodes can transmit their signals on different wavelengths simultaneously and independently. The star coupler receives and combines all the signals and broadcasts them to all the nodes in the network. To receive a signal, a node tunes one of its receivers to the wavelength on which the signal is transmitted.
In the bus topology, a number of nodes are connected to a bus through 2x2 couplers (see Section 2.3) by WDM fiber links. Each node transmits its signal to the bus on an available wavelength through a coupler and receives a signal from the bus through another coupler. In the transmitting coupler only one of the output ports is used, and in the receiving coupler only one of the input ports is used. Both the star and bus topologies use optical couplers. A star coupler can be made out of 2x2 couplers. The two topologies differ in the number of couplers used and in the manner in which the couplers are connected. In most cases, the star topology has proven to be a better choice for many types of networks [1].
The advantage of a broadcast-and-select WDM network lies in its simplicity and broadcasting capability. However, this type of network needs a large number of wavelengths because the wavelengths cannot be reused in the network. A wavelength can only be used by one node at a given time. As a result, the network is not scalable to the number of nodes in the network. On the other hand, because the transmitted power from a node is split among all the nodes in the network, each node can only receive a very small fraction of the transmitted power. Accordingly, this type of network cannot span a long distance and is most suitable for deployment in local area networks (LANs) or metropolitan area networks (MANs). Because broadcast-and-select WDM networks are not the focus of this book, the readers are referred to [1-2] for more discussion.
1.3.2 Wavelength-Routed WDM Networks
A WDM network that employs wavelength routing to transfer data traffic is referred to as a wavelength-routed WDM network. A wavelength-routed WDM network typically consists of routing nodes interconnected by point-to-point WDM fiber links in an arbitrary mesh topology. Each routing node employs a set of transmitters and receivers for transmitting signals to and receiving signals from fiber links and an optical cross-connect (OXC) or wavelength cross-connect (WXC) to route and switch different wavelengths from an input port to an output port. Each fiber link operates in WDM and supports a certain number of optical channels or wavelengths. An access node can be connected to a routing node, which is used as an interface between the optical network and the electronic client networks. At the source side, an access node performs traffic aggregation and E/O conversion functions. At the destination side, traffic deaggregation and O/E conversion are performed. In the context of this book, an access node and its associated routing node are collectively referred to as a network node or simply a node unless otherwise stated. The architecture of a wavelength-routed WDM network is shown in Figure 1.6.
A wave length-routed WDM network is a circuit-switched network in which a pair of network nodes communicates through an end-to-end optical connection that may consist of one or more all-optical connections called lightpaths. A lightpath is a unidirectional all-optical connection between a pair of network nodes, which may span multiple fiber links and use one or multiple wavelengths without undergoing any O/E and E/O conversion at each intermediate node. Two lightpaths cannot share the same wavelength on a common fiber link, which is referred to as the wavelength-distinct constraint. However, two lightpaths can use the same wavelength on different fiber links, which is referred to as the wavelength-reuse property. In the absence of any wavelength conversion capability, a lightpath must use the same wavelength on all the links it spans, which is known as the wavelength-continuity constraint. This constraint is unique to wavelength-routed WDM networks and makes such networks different from conventional circuit-switched networks. Because of this constraint, network performance in terms of wavelength utilization and blocking probability would be largely degraded. For this reason, it is desirable to eliminate the wavelength-continuity constraint in order to improve network performance. This can be achieved by deploying wavelength converters at network nodes to provide wavelength conversion capability in the network.
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Table of Contents
1. Introduction.2. Fundamentals of WDM network devices.
3. Routing and wavelength assignment.
4. Virtual topology design.
5. Distributed lightpath establishment.
6. Optical layer survivability.
7. IP over WDM.
8. Future trends in optical networks.
Appendix A: Basics of graph theory.
Appendix B: Dijkstra's algorithm.