20 INTRODUCTION TO OPTICAL NETWORKS Figure 1.8 An IP over SONET network. (a) The network has IP switches with SONET adaptors that are connected to a SONET network. (b) The layered view of this network. Figure 1.9 The layered view of an IP over ATM over SONET network. Another example of this sort of layering arises in the context of an IP over ATM over SONET network. Some service providers are deploying an ATM network operating over a SONET infrastructure to provide services for IP users. In such a network, IP packets are converted to ATM cells at the periphery of the network. The ATM switches are connected through a SONET infrastructure. The layered view of such a network is shown in Figure 1.9. Again, the IP network treats the ATM network as its link layer, and the ATM network uses SONET as its link layer. The introduction of second-generation optical networks adds yet another layer to the protocol hierarchy the so-called optical layer. The optical layer is a server layer 1.4 The Optical Layer 21 Figure 1.10 A layered view of a network consisting of a second-generation optical network layer that supports a variety of client layers above it. that provides services to other client layers. This optical layer provides lightpaths to a variety of client layers, as shown in Figure 1.10. Examples of client layers residing above a second-generation optical network layer include IP, ATM, and SONET/SDH, as well as other possible protocols such as Gigabit Ethernet, ESCON (enterprise serial connection~a protocol used to interconnect computers to storage devices and other computers), or Fibre Channel (which performs the same function as ESCON, at higher speeds). As second-generation optical networks evolve, they may provide other services besides lightpaths, such as packet-switched virtual circuit or datagram services. These services may directly interface with user applications, as shown in Figure 1.10. Several other layer combinations are possible and not shown in the figure, such as IP over SONET over optical, and ATM over optical. The client layers make use of the lightpaths provided by the optical layer. To a SONET network operating over the optical layer, the lightpaths are simply replace- ments for hardwired fiber connections between SONET terminals. As described earlier, a lightpath is a connection between two nodes in the network, and it is set up by assigning a dedicated wavelength to it on each link in its path. Note that individual wavelengths are likely to carry data at fairly high bit rates (a few gigabits per second), and this entire bandwidth is provided to the higher layer by a lightpath. Depending on the capabilities of the network, this lightpath could be set up or taken down in response to a request from the higher layer. This can be thought of as a circuit-switched service, akin to the service provided by today's telephone network: the network sets up or takes down calls in response to a request from the user. Al- ternatively, the network may provide only permanent lightpaths, which are set up 22 INTRODUCTION TO OPTICAL NETWORKS at the time the network is deployed. This lightpath service can be used to support high-speed connections for a variety of overlying networks. Optical networks today provide functions that might be thought of as falling primarily within the physical layer from the perspective of its users. However, the optical network itself incorporates several sublayers, which in turn correspond to the link and network layer functions in the classical layered view. Before the emergence of the optical layer, SONET/SDH was the predominant transmission layer in the telecommunications network, and it is still the dominant layer in many parts of the network. We will study SONET/SDH in detail in Chap- ter 6. For convenience, we will use SONET terminology in the rest of this section. The SONET layer provides several key functions. It provides end-to-end, managed, circuit-switched connections. It provides an efficient mechanism for multiplexing lower-speed connections into higher-speed connections. For example, low-speed voice connections at 64 kb/s or private line 1.5 Mb/s connections can be multiplexed all the way up into 2.5 Gb/s or 10 Gb/s line rates for transport over the network. Moreover, at intermediate nodes, SONET provides an efficient way to extract indi- vidual low-speed streams from a high-speed stream, using an elegant multiplexing mechanism based on the use of pointers. SONET also provides a high degree of network reliability and availability. Car- riers expect their networks to provide 99.99% to 99.999% of availability. These numbers translate into an allowable network downtime of less than one hour per year and five minutes per year, respectively. SONET achieves this by incorporating sophisticated mechanisms for rapid service restoration in the event of failures in the network. This is a subject we will look at in Chapter 10. Finally, SONET includes extensive overheads that allow operators to monitor and manage the network. Examples of these overheads include parity check bytes to determine whether frames are received in error or not, and connection identifiers that allow connections to be traced and verified across a complex network. SONET network elements include line terminals, add/drop multiplexers (ADMs), regenerators, and digital crossconnects (DCSs). Line terminals multiplex and demul- tiplex traffic streams. ADMs are deployed in linear and ring network configurations. They provide an efficient way to drop part of the traffic at a node while allowing the remaining traffic to pass through. The ring topology allows traffic to be rerouted around failures in the network. Regenerators regenerate the SONET signal wherever needed. DCSs are deployed in larger nodes to switch a large number of traffic streams. Today's DCSs are capable of switching thousands of 45 Mb/s traffic streams. The functions performed by the optical layer are in many ways analogous to those performed by the SONET layer. The optical layer multiplexes multiple lightpaths into a single fiber and allows individual lightpaths to be extracted efficiently from the composite multiplex signal at network nodes. It incorporates sophisticated service 1.4 The Optical Layer 23 Figure 1.11 Example of a typical multiplexing layered hierarchy. restoration techniques and management techniques as well. We will look at these techniques in Chapters 9 and 10. Figure 1.11 shows a typical layered network hierarchy, highlighting the optical layer. The optical layer provides lightpaths that are used by SONET and IP network elements. The SONET layer multiplexes low-speed circuit-switched streams into higher-speed streams, which are then carried over lightpaths. The IP layer performs statistical multiplexing of packet-switched streams into higher-speed streams, which are also carried over lightpaths. Inside the optical layer itself is a multiplexing hi- erarchy. Multiple wavelengths or lightpaths are combined together into wavelength bands. Bands are combined together to produce a composite WDM signal on a fiber. The network itself may include multiple fibers and multiple fiber bundles, each of which carries a number of fibers. So why have multiple layers in the network that perform similar functions? The answer is that this form of layering significantly reduces network equipment costs. Different layers are more efficient at performing functions at different bit rates. For example, the SONET layer can efficiently (that is, cost-effectively) switch and process traffic streams up to, say, 2.5 Gb/s today. However, it is very expensive to have this layer process 100 10 Gb/s streams coming in on a WDM link. The optical layer, on the other hand, is particularly efficient at processing traffic on a wavelength-by-wavelength basis, but not particularly good at processing traffic streams at lower granularities, for example, 155 Mb/s. Therefore, it makes sense to use the optical layer to process large amounts of bandwidth at a relatively coarse level and the SONET layer to process smaller amounts of bandwidth at a relatively finer 24 INTRODUCTION TO OPTICAL NETWORKS level. This fundamental observation is the key driver to providing such functions in multiple layers, and we will study this in detail in Chapter 7. A similar observation also holds for the service restoration function of these networks. Certain failures are better handled by the optical layer and certain others by the SONET layer or the IP layer. We will study this aspect in Chapter 10. 1.5 Transparency and All-Optical Networks A major feature of the lightpath service provided by second-generation networks is that this type of service can be transparent to the actual data being sent over the lightpath once it is set up. For instance, a certain maximum and minimum bit rate might be specified, and the service may accept data at any bit rate and any protocol format within these limits. It may also be able to carry analog data. Transparency in the network provides several advantages. An operator can pro- vide a variety of different services using a single infrastructure. We can think of this as service transparency. Second, the infrastructure is future-proof in that if protocols or bit rates change, the equipment deployed in the network is still likely to be able to support the new protocols and/or bit rates without requiring a complete overhaul of the entire network. This allows new services to be deployed efficiently and rapidly, while allowing legacy services to be carried as well. An example of a transparent network of this sort is the telephone network. Once a call is established in the telephone network, it provides 4 kHz of bandwidth over which a user can send a variety of different types of traffic such as voice, data, or fax. There is no question that transparency in the telephone network today has had a far-reaching impact on our lifestyles. Transparency has become a useful feature of second-generation optical networks as well. Another term associated with transparent networks is the notion of an all-optical network. In an all-optical network, data is carried from its source to its destination in optical form, without undergoing any optical-to-electrical conversions along the way. In an ideal world, such a network would be fully transparent. However, all-optical networks are limited in their scope by several parameters of the physical layer, such as bandwidth and signal-to-noise ratios. For example, analog signals require much higher signal-to-noise ratios than digital signals. The actual requirements depend on the modulation format used as well as the bit rate. We will study these aspects in Chapter 5, where we will see that engineering the physical layer is a complex task with a variety of parameters to be taken into consideration. For this reason, it is very difficult to build and operate a network that can support analog as well as digital signals at arbitrary bit rates. 1.5 Transparency and All-Optical Networks 25 The other extreme is to build a network that handles essentially a single bit rate and protocol (say, 2.5 Gb/s SONET only). This would be a nontransparent network. In between is a practical network that handles digital signals at a range of bit rates up to a specified maximum. Most optical networks being deployed today fall into this category. Although we talk about optical networks, they almost always include a fair amount of electronics. First, electronics plays a crucial role in performing the intelli- gent control and management functions within a network. However, even in the data path, in most cases, electronics is needed at the periphery of the network to adapt the signals entering the optical network. In many cases, the signal may not be able to remain in optical form all the Way to its destination due to limitations imposed by the physical layer design and may have to be regenerated in between. In other cases, the signal may have to be converted from one wavelength to another wavelength. In all these situations, the signal is usually converted from optical form to electronic form and back again to optical form' Having these electronic regenerators in the path of the signal reduces the trans- parency of that path. There are three types of electronic regeneration techniques for digital data. The standard one is called regeneration with retiming and reshaping, also known as 3R. Here the bit clock is extracted from the signal, and the signal is reclocked. This technique essentially produces a "fresh" copy of the signal at each regeneration step, allowing the signal to go through a very large number of regen- erators. However, it eliminates transparency to bit rates and the framing protocols, since acquiring theclock usually requires knowledge of both of these. Some limited form of bit rate transparency is possible by making use of programmable clock re- covery chips that can work at a set of bit rates that are multiples of one another. For example, chipsets that perform clock recovery at either 2.5 Gb/s or 622 Mb/s are commercially available today. An implementation using regeneration of the optical signal without retiming, also called 2R, offers transparency to bit rates, without supporting analog data or different modulation formats [GJR96]. However, this approach limits the number of regeneration steps allowed, particularly at higher bit rates, over a few hundred megabits per second. The limitation is due to the jitter, which accumulates at each regeneration step. The final form of electronic regeneration is 1R, where the signal is simply received and retransmitted without retiming or reshaping. This form of regeneration can handle analog data as well, but its performance is significantly poorer than the other two forms of regeneration. For this reason, the networks being deployed today use 2R or 3R electronic regeneration. Note, however, that optical amplifiers are widely used to amplify the signal in the optical domain, without converting the signal to the electrical domain. These can be thought of as 1R optical regenerators. 26 INTRODUCTION TO OPTICAL NETWORKS Table 1.1 Different types of transparency in an optical network. Transparency type Parameter Fully transparent Practical Nontransparent Analog/digital Both Digital Digital Bit rate Arbitrary Predetermined maximum Fixed Framing protocol Arbitrary Selected few Single Table 1.1 provides an overview of the different dimensions of transparency. At one end of the spectrum is a network that operates at a fixed bit rate and framing protocol, for example, SONET at 2.5 Gb/s. This would be truly an opaque network. In contrast, a fully transparent network would support analog and digital signals with arbitrary bit rates and framing protocols. As we argued earlier, however, such a network is not practical to engineer and build. Today, a practical alternative is to engineer the network to support a variety of digital signals up to a predeter- mined maximum bit rate and a specific set of framing protocols, such as SONET and Gigabit Ethernet. The network supports a variety of framing protocols either by making use of 2R regeneration inside the network or by providing specific 3R adaptation devices for each of the framing protocols. Such a network is shown in Figure 1.12. It can be viewed as consisting of islands of all-optical subnetworks with optical-to-electrical-to-optical conversion at their boundaries for the purposes of adaptation, regeneration, or wavelength conversion. 1.6 Optical Packet Switching So far we have talked about optical networks that provide lightpaths. These networks are essentially circuit-switched. Researchers are also working on optical networks that can perform packet switching in the optical domain. Such a network would be able to offer virtual circuit services or datagram services, much like what is provided by ATM and IP networks. With a virtual circuit connection, the network offers what looks like a circuit-switched connection between two nodes. However, the band- width offered on the connection can be smaller than the full bandwidth available on a link or wavelength. For instance, individual connections in a future high-speed network may operate at 10 Gb/s, while transmission bit rates on a wavelength could be 100 Gb/s. Thus the network must incorporate some form of time division mul- tiplexing to combine multiple connections onto the transmission bit rate. At these rates, it may be easier to do the multiplexing in the optical domain rather than in the electronic domain. This form of optical time domain multiplexing (OTDM) may 1.6 Optical Packet Switching 27 Figure 1.12 An optical network consisting of all-optical subnetworks interconnected by optical-to-electrical-to-optical (OEO) converters. OEO converters are used in the network for adapting external signals to the optical network, for regeneration, and for wavelength conversion. be fixed or statistical. Those that perform statistical multiplexing are called optical packet-switched networks. For simplicity we will talk mostly about optical packet switching. Fixed OTDM can be thought of as a subset of optical packet switching where the multiplexing is fixed instead of statistical. An optical packet-switching node is shown in Figure 1.13. The idea is to create packet-switching nodes with much higher capacities than can be envisioned with electronic packet switching. Such a node takes a packet coming in, reads its header, and switches it to the appropriate output port. The node may also impose a new header on the packet. It must also handle contention for output ports. If two packets coming in on different ports need to go out on the same output port, one of the packets must be buffered, or sent out on another port. Ideally, all the functions inside the node would be performed in the optical do- main, but in practice, certain functions, such as processing the header and controlling the switch, get relegated to the electronic domain. This is because of the very limited processing capabilities in the optical domain. The header itself could be sent at a lower bit rate than the data so that it can be processed electronically. The mission of optical packet switching is to enable packet-switching capabilities at rates that cannot be contemplated using electronic packet switching. However, designers are handicapped by several limitations with respect to processing signals in the optical domain. One important factor is the lack of optical random access 28 INTRODUCTION TO OPTICAL NETWORKS Figure 1.13 An optical packet-switching node. The node buffers the incoming packets, looks at the packet header, and routes the packets to an appropriate output port based on the information contained in the header. memory for buffering. Optical buffers are realized by using a length of fiber and are just simple delay lines, not fully functional memories. Packet switches include a high amount of intelligent real-time software and dedicated hardware to control the network and provide quality-of-service guarantees, and these functions are ,difficult to perform in the optical domain. Another factor is the relatively primitive state of fast optical-switching technology, compared to electronics. For these reasons, optical packet switching is still in its infancy today in research laboratories. Chapter 12 covers all these aspects in detail. 1.7 Transmission Basics In this section, we introduce and define the units for common parameters associated with optical communication systems. 1.7.1 Wavelengths, Frequencies, and Channel Spacing When we talk about WDM signals, we will be talking about the wavelength, or frequency, of these signals. The wavelength )~ and frequency f are related by the equation c=f~., where c denotes the speed of light in free space, which is 3 x 108 m/s. We will reference all parameters to free space. The speed of light in fiber is actually somewhat lower (closer to 2 x 108 m/s), and the wavelengths are also correspondingly different. 1.7 Transmission Basics 29 To characterize a WDM signal, we can use either its frequency or wavelength in- terchangeably. Wavelength is measured in units of nanometers (nm) or micrometers (#m or microns). (1 nm = 10 -9 m, 1 #m = 10 -6 m.) The wavelengths of interest to optical fiber communication are centered around 0.8, 1.3, and 1.55 ~m. These wavelengths lie in the infrared band, which is not visible to the human eye. Frequen- cies are measured in units of hertz (or cycles per second), more typically in megahertz (1 MHz = 106 Hz), gigahertz (1 GHz = 109 Hz), or terahertz (1 THz = 1012 Hz). Using c - 3 • 108 m/s, a wavelength of 1.55 #m would correspond to a frequency of approximately 193 THz, which is 193 • 1012 Hz. Another parameter of interest is the channel spacing, which is the spacing between two wavelengths or frequencies in a WDM system. Again the channel spacing can be measured in units of wavelengths or frequencies. The relationship between the two can be obtained starting from the equation C f ~ o Differentiating this equation around a center wavelength )~0, we obtain the relation- ship between the frequency spacing Af and the wavelength spacing A)~ as r Af ) ~ A)~. This relationship is accurate as long as the wavelength (or frequency) spacing is small compared to the actual channel wavelength (or frequency), which is usually the case in optical communication systems. At a wavelength )~0 = 1550 nm, a wavelength spacing of 0.8 nm corresponds to a frequency spacing of 100 GHz, a typical spacing in WDM systems. Digital information signals in the time domain can be viewed as a periodic se- quence of pulses, which are on or off, depending on whether the data is a 1 or a 0. The bit rate is simply the inverse of this period. These signals have an equivalent representation in the frequency domain, where the energy of the signal is spread across a set of frequencies. This representation is called the power spectrum, or sim- ply spectrum. The signal bandwidth is a measure of the width of the spectrum of the signal. The bandwidth can also be measured either in the frequency domain or in the wavelength domain, but is mostly measured in units of frequency. Note that we have been using the term bandwidth rather loosely. The bandwidth and bit rate of a digital signal are related but not exactly the same. Bandwidth is usually specified in kilohertz or megahertz or gigahertz, whereas bit rate is specified in kilobits/second (kb/s), megabits/second (Mb/s), or gigabits/second (Gb/s). The relationship between the two depends on the type of modulation used. For instance, a phone line offers 4 kHz of bandwidth, but sophisticated modulation technology allows us to realize . specified, and the service may accept data at any bit rate and any protocol format within these limits. It may also be able to carry analog data. Transparency in the network provides several advantages types of transparency in an optical network. Transparency type Parameter Fully transparent Practical Nontransparent Analog/digital Both Digital Digital Bit rate Arbitrary Predetermined maximum. islands of all -optical subnetworks with optical- to-electrical-to -optical conversion at their boundaries for the purposes of adaptation, regeneration, or wavelength conversion. 1 .6 Optical Packet