Table of Contents

1. Introduction. 2

2. The challenges of Today’s Telecommunications Network. 3

3. Achieving Bandwidth Capacity Goals. 4

4. Dense Wavelength Division Multiplexing. 7

5. DWDM Architecture. 13

6. Measurements of Performance. 15

7. Optical SNR and Transmitted power requirements of DWDM systems. 15

8. Key DWDM System Characteristics. 16

9. Fiber non-linearities. 17

10. Applications for DWDM. 18

11. Conclusion. 19

12. References. 20

Sri Venkateshwara College Of Engineering. ECE Department.

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1.Introduction

Over the last decade, fiber optic cables have been installed by carriers as the backbone of their interoffice networks, becoming the mainstay of the telecommunications infrastructure. Using time division multiplexing (TDM) technology, carriers now routinely transmit information at 2.4Gb/s on a single fiber, with some deploying equipment that quadruples that rate to 10Gb/s. The revolution in high bandwidth applications and the explosive growth of the Internet, however, have created capacity demands that exceed traditional TDM limits. As a result, the once seemingly inexhaustible bandwidth promised by the deployment of optical fiber in the 1980s is being exhausted. To meet growing demands for bandwidth, a technology called Dense Wavelength Division Multiplexing (DWDM) has been developed that multiplies the capacity of a single fiber.

DWDM systems being deployed today can increase a single fiber’s capacity sixteen fold, to a throughput of 40 Gb/s! This cutting edge technology when combined with network management systems and add-drop multiplexers enables carriers to adopt optically-based transmission networks that will meet the next generation of bandwidth demand at a significantly lower cost than installing new fiber.

Definition

Dense Wavelength Division Multiplexing (DWDM) is a fiber-optic transmission technique. It involves the process of multiplexing many different wavelength signals onto a single fiber. So each fiber have a set of parallel optical channels each using slightly different light wavelengths. It employs light wavelengths to transmit data parallel-by-bit or serial-by-character. DWDM is a very crucial component of optical networks that will allow the transmission of data: voice, video-IP, ATM

Hence with the development of WDM technology, optical layer provides the only means for carriers to integrate the diverse technologies of their existing networks into one physical infrastructure. For example, though a carrier might be operating both ATM and SONET networks, with the use of DWDM it is not necessary for the ATM signal to be multiplexed up to the SONET rate to be carried on the DWDM network. Hence carriers can quickly introduce ATM or IP without having to deploy an overlay network for multiplexing.

Sri Venkateshwara College Of Engineering. ECE Department.

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2. The Challenges of Today’s Telecommunications Network

To understand the importance of DWDM and optical networking, these capabilities must be discussed in the context of the challenges faced by the telecommunications industry and in particular, service providers. Most U.S. networks were built using estimates that calculated bandwidth use by employing concentration ratios derived from classical engineering formulas such as poisson and Reeling. Consequently, forecasts of the amount of bandwidth capacity needed for networks were calculated on the presumption that a given individual would only use network bandwidth six minutes of each hour. These formulas did not factor in the amount of traffic generated by Internet access(300 percent growth per year), faxes, multiple phone lines, modems, teleconferencing, and data and video transmission. Had these factors been included, a far different estimate would have emerged. In fact, today many people use the bandwidth equivalent of 180 minutes or more each hour.

Therefore, an enormous amount of bandwidth capacity is required to provide the services demanded by consumers. For perspective, in 1997, a long-distance carrier made major strides when it increased its bandwidth capacity to 1.2 Gbps(billion of bits per second) over one fiber pair. At the transmission speed of one Gbps, one thousand books can be transmitted per second. However today, if one million families decide they want to see video on Web sites and sample the new emerging video applications, then network transmission rates of terabits (trillions of bits per second[Tbps]) are required. With a transmission rate of one Tbps, it is possible to transmit 20 million simultaneous 2-way phone calls or transmit the text from 300 years-worth of daily newspapers per second.

No one could have predicted the network growth necessary to meet the demand. For example, one study estimated that from 1994 to 1998 the demand on the U.S.interexchange carriers’(IXCs’) network would increase sevenfold, and for the U.S.local exchange carriers’(LECs’) network, the demand would increase fourfold. In actuality, one company indicated that its growth was 32 times that of the previous year, while another company’s rate of growth in 1997 alone was the same size as its entire network in 1991. Yet another has said that the size of its network doubled every six months in that four-year period.

In addition to this explosion in consumer demand for bandwidth, many service providers are coping with fiber exhaust in their networks. An industry survey indicated that in 1995, the amount of embedded fiber already in use in the average network was between 70 percent and 80 percent. Today, many carriers are nearing one hundred-percent capacity utilization across significant portion of their networks. Another problem for carriers is the challenge of

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deploying and integrating diverse technologies in one physical infrastructure. Customer demands and competitive pressures mandate that carriers offer diverse services economically and deploy them over the embedded network. DWDM provides service providers an answer to that demand.

Use of DWDM allows providers to offer services such as e-mail, video, and multimedia carried as Internet protocol(IP) data over asynchronous transfer mode(ATM) and voice carried over SONET/SDH. Despite the fact the these formats-IP,ATM, and SONET/SDH-provide unique bandwidth management capabilities, all three can be transported over the optical layer using DWDM. This unifying capability allows the service provider the flexibility to respond to customer demands over one network.

A platform that is able to unify and interface with these technologies and position the carrier with the ability to integrate current and next-generation technologies is critical for a carrier’s success.

3. Achieving BandwidthCapacity GoalsConfronted by the need for more capacity, carriers have three possible solutions:

• Install new fiber.

• Invest in new TDM technology to achieve faster bit rates.

• Deploy Dense Wavelength Division Multiplexing.

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3.1 Installing New Fiber to Meet Capacity Needs

For years, carriers have expanded their networks by deploying new fiber and transmission equipment. For each new fiber deployed, the carrier could add capacity up to 2.4 Gb/s. Unfortunately, such deployment is frequently difficult and always costly. The average cost to deploy the additional fiber cable, excluding costs of associated support systems and electronics, has been estimated to be about $70,000 per mile, with costs escalating in densely populated areas. While this projection varies from place to place, installing new fiber can be a daunting prospect, particularly for carriers with tens of thousands of route miles. In many cases, the right-of way of the cable route or the premises needed to house transmission equipment is owned by a third party, such as a railroad or even a competitor. Moreover, singlemode fiber is currently in short supply owing to production limitations, potentially adding to costs and delays. For these reasons, the comprehensive deployment of additional fiber is an impractical, if not impossible, solution for many carriers.

3.2Higher Speed TDM — DeployingSTM-64/OC-192 (10 Gb/s)

As indicated earlier, STM–64/OC–192 is becoming an option for carriers seeking higher capacity, but there are significant issues surrounding this solution that may restrict its applicability. The vast majority of the existing fiber plant is single-mode fiber (SMF) that has high dispersion in the 1550 nm window, making STM–64/OC–192 transmission difficult. In fact, dispersion has a 16 times greater effect with STM–64/OC–192 equipment than with STM–16/OC–48. As a result, effective STM–64/OC–192 transmission requires either some form of dispersion compensating fiber or entire new fiber builds using non-zero dispersion shifted fiber (NZDSF) which costs some 50 percent more than SMF. The greater carrier transmission power associated with the higher bit rates also introduces nonlinear optical effects that cause degraded waveform quality. The effects of Polarization Mode Dispersion (PMD) which, like other forms of dispersion affects the distance a light pulse can travel without signal degradation is of particular concern for STM-64/OC–192.

This problem, barely noticed until recently, has become significant because as transmission speeds increase, dispersion problems grow exponentially thereby dramatically reducing the distance a signal can travel. PMD appears to limit the reliable reach of STM–64/OC–192 to about 70 kms on most embedded fiber. Although there is a vigorous and ongoing debate within the industry over the extent of PMD problems, some key issues are already known.

• PMD is particularly acute in the conventional singlemode fiber that comprises the vast majority of the existing fiber plant, as well as in aerial fiber.

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•Unlike other forms of dispersion that are fairly predictable and easy to measure, PMD varies significantly from cable to cable. Moreover, PMD is affected by environmental conditions, making it difficult to determine ways to offset its effect on high bit rate systems.

• As a result, carriers must test nearly every span of fiber for its compatibility with STM–64/OC–192; in many cases, PMD will rule out its deployment altogethers.

3.3. A Third Approach – DWDM

Dense Wavelength Division Multiplexing (DWDM) is a technology that allows multiple information streams to be transmitted simultaneously over a single fiber at data rates as high as the fiber plant will allow (e.g. 2.4 Gb/s).

The DWDM approach multiplies the simple 2.4 Gb/s system by up to 16 times, giving an immense and immediate increase in capacity using embedded fiber! A sixteen channel system (which is available today) supports 40 Gb/s in each direction over a fiber pair, while a 40 channel system under development will support 100 Gb/s, the equivalent of ten STM–64/OC–192 transmitters!The benefits of DWDM over the first two options adding fiber plant or deploying STM–64/OC–192 for increasing capacity are clear.

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4. Dense WavelengthDivision Multiplexing

DWDM technology utilizes a composite optical signal carrying multiple information streams, each transmitted on a distinct optical wavelength. Although wavelength division multiplexing has been a known technology for several years, its early application was restricted to providing two widely separated “wideband” wavelengths, or to manufacturing components that separated up to four channels. Only recently has the technology evolved to the point that parallel wavelengths can be densely packed and integrated into a transmission system, with multiple, simultaneous, extremely high frequency signals in the 192 to 200 terahertz (THz) range. By conforming to the ITU channel plan, such a system ensures interoperability with other equipment and allows service providers to be well positioned to deploy optical solutions throughout their networks. The 16 channel system in essence provides a virtual 16–fiber cable, with each frequency channel serving as a unique STM–16/OC–48 carrier.  

To transmit 40 Gb/s over 600 kms using a traditional system would require 16 separate fiber pairs with regenerators placed every 35 kms for a total of 272 regenerators. A 16 channel DWDM system, on the other hand, uses a single fiber pair and 4 amplifiers positioned every 120 kms for a total of 600 kms.

The most common form of DWDM uses a fiber pair one for transmission and one for reception. Systems do exist in which a single fiber is used for bidirectional traffic, but these configurations must sacrifice some fiber capacity by setting aside a guard band to prevent channel mixing; they also degrade amplifier performance. In addition, there is a greater risk that reflections occurring during maintenance or repair could damage the amplifiers. In any event, the availability of mature supporting technologies, like precise demultiplexers and Erbium Doped Fiber Amplifiers (EDFA), has enabled DWDM with eight, sixteen, or even higher channel counts to be commercially delivered.

4.1. DWDM SYSTEM

As mentioned earlier, optical networks use Dense Wavelength Multiplexing as the underlying carrier. The most important components of any DWDM system are transmitters, receivers, Erbium-doped fiber Amplifiers, DWDM multiplexors and DWDM demultiplexors. Fig 1 gives the structure of a typical DWDM system.

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  Fig.1 Block Diagram of a DWDM System

4.2. Optical Transmission Principles

The DWDM system has an important photonic layer, which is responsible for transmission of the optical data through the network. Some basic principles, concerning the optical transmission, are explained in this section. These are necessary for the proper operation of the system.

Channel Spacing

The minimum frequency separation between two different signals multiplexed in known as the Channel spacing. Since the wavelength of operation is inversely proportional to the frequency, a corresponding difference is introduced in the wavelength of each signal. The factors controlling channel spacing are the optical amplifiers bandwidth and the capability of the receiver in identifying two close wavelengths sets the lower bound on the channel spacing. Both factors ultimately restrict the number of unique wavelengths passing through the amplifier.

Signal Direction

An optical fiber helps transmit signal in both directions. Based on this feature, a DWDM system can be implemented in two ways:

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Unidirectional: All wavelengths travel in the same direction within the fiber. It is similar to a simplex case. This calls in for laying one another parallel fiber for supporting transmission on the other side.

Bi-directional: The channels in the DWDM fiber are split into two separate bands, one for each direction. This removes the need for the second fiber, but in turn reduces the capacity or transmission bandwidth.

 

Signal Trace

The procedure of detecting if a signal reaches the correct destination at the other end. This helps follow the light signal through the whole network. It can be achieved by plugging in extra information on a wavelength, using an electrical receiver to extract if from the network and inspecting for errors. The receiver the reports the signal trace to the transmitter.

Taking into consideration the above two factors, the international bodies have established a spacing of 100GHz to be the worldwide standard for DWDM. This means that the frequency of each signal is less than the rest by atleast 0.1THz.

4.3 Demultiplexers

With signals as precise and as dense as those used in DWDM, there needed to be a way to provide accurate signal separation, or filtration, on the optical receiver. Such a solution also needed to be easy to implement and essentially maintenance free. Earlyfiltering technology was either too imprecise for DWDM, too sensitive to temperature variations and polarization, too vulnerable to crosstalk from neighboring channels, or too costly. This restricted the evolution of DWDM. To meet the requirements for higher performance, a more robust filtering technology was developed that makes DWDM possible on a cost effective basis: the in–fiber Bragg grating.

The new filter component, called a fiber grating, consists of a length of optical fiber wherein the refractive index of the core has been permanently modified in a periodic fashion, generally by exposure to an ultraviolet interference pattern. The result is a component which acts as a wavelength dependent reflector and is useful for precise wavelength separation. In other words, the fiber grating creates a highly selective, narrow bandwidth filter that functions somewhat like a mirror and provides significantly greater wavelength selectivity than any other optical technology. The filter wavelength can be controlled during fabricationthrough simple geometric considerations which enable reproducible accuracy. Because this is a passive device, fabricated into glass fiber, it is robust and durable.

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4.4. Optical Amplifer.

The advent of the Erbium Doped Fiber Amplifier (EDFA) enabled commercial development of DWDM systems by providing a way to amplify all the wavelengths at the same time. This optical amplification is done by incorporating Erbium ions into the core of aspecial fiber in a process known as doping. Optical pump lasers are then used to transfer high levels of energy to the special fiber, energizing the Eribum ions which then boost the optical signals that are passing through. Significantly, the atomic structure of Erbium providesamplification to the broad spectral range required for densely packed wavelengths operating in the 1550–nm region, optically boosting the DWDM signals. Instead of multiple electronic regenerators, which required that the optical signals be converted to electrical signals then back again to optical ones, the EDFA directly amplifies the optical signals. Hence the composite optical signals can travel up to 600 kms without regeneration and up to 120kms between amplifiers in a commercially available, terrestrial, DWDM system.

It is the optical amplifier that has made WDM economically feasible. The usable bandwidth by using EDFAs is about 30nm (1530nm-1560nm). However, attenuation is minimum in the range of 1500nm to 1600nm. Hence that implies very less utilizations. Also typically what happens is that with the need to place as many wavelengths (channels) as possible in a single fiber, the distance between two channels is very small (0.8-1.6nm). This results in the Interchannel crosstalk becoming a very important issue at this point.

It became imperative that the amplifier's bandwidth had to be increased while eliminating crosstalk. So this led to the development of Silica Erbium fiber-based Dual-band fiber amplifier (DBFA). These fibers are similar to the EDFAs and have been able to generate

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terabit transmission successfully. However, the most important feature of the DBFA is its bandwidth =>1528nm-1610nm. The DBFA has two sub-band amplifiers. The first is in the range of the EDFA and the second one is what is known asExtended band fiber amplifier (EBFA). It has been shown that this EBFA has several attractive features compared to the traditional EDFA.:

Flat Gain: EBFAs achieve a flat gain over a range of wide range (35nm) as compared to the EDFAs

Slow Saturation: EBFAs reach saturation slower than the EDFAs. Saturation is the state where output remains constant even though input level keeps increasing.

Low Noise: EBFAs exhibit lower noise than EDFAs

Therefore, the 1590-nm EBFA represents a huge leap in meeting the ever-increasing demands of high-capacity fiber-optic transmission systems. A similar product is Lucents Bell Labs of an "Ultra-Wideband Optical Amplifier (UWOA) that can amplify upto 100 wavelengh channels as they travel along a single optical fiber and has a usable bandwidth of 80nm. This bandwidth spans the 1530-1565nm channel (C-band) and also the long wavelength channels beyond 1565-1620nm(L-band).

4.5 DWDM COMPONENTS

Important components of a DWDM system are the Add/Drop Multiplexer (ADM), the Optical CrossConnect (OXC), Optical Splitter. The Add/Drop Multiplexer as the name suggests, selectively adds/drops wavelengths without having to use any SONET/SDH terminal equipment. We require the ADM to add new wavelengths to the network or to drop some wavelengths at their terminating points. There are two types of implementations of the ADM, the Fixed WADM and the Reconfigurable WDM.

Fig.3 Block Diagram of the WADM

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The Optical CrossConnect acts a crossconnect between n-input ports and n-output ports. It allows the efficient network management of wavelengths at the optical layer. The variety of functions that it provides are signal monitoring, restoration, provisioning and grooming.

Fig.4 Block Diagram of the OXC

Optical Splitters are being suggested for use in multicast-capable wavelength-routing switches to provide optical multicasting. It is a passive device that will help in replicating optical signals.

Optical Gateways are devices that will allow the smooth transition of traffic to the optical layer. We can have high-speed ATM networks or a mix of SONET and ATM services with such a gateway. They provide the maximum benefits of optical networks.

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5. DWDM ARCHITECTURE

Using some of the basic concepts of DWDM systems, it is possible to form an All-Optical layer. Transport of Gigabit Ethernet , ATM, SONET, IP on different channels is feasible. By achieving this, the system becomes more flexible and any signal format can be connected to, without the addition of any extra equipment that acts as a translator between the formats. In this section we will talk about the various types of technologies that can be used over DWDM systems. In particular, we will discuss ATM over DWDM and IP over DWDM.

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5.1 ATM over DWDM

As bandwidth requirements increase, Telcos are faced with huge investments in order to fulfil the capacity demands. Along with this the demand for QoS has increased. There seems to be a general move towards providing QoS while still maintaining the same capacity. ATM over DWDM solves the bandwidth and Quality of Service issues in a cost-effective way. In DWDM networks, if there is a carrier that operates both ATM and SONET networks there is no need for the ATM signal to be multiplexed upto the SONET rate. This is because the optical layer can carry any type of signal without any additional multiplexing. This results in the reduction of a lot of overlay network.

While there are a lot of advantages of running ATM over DWDM, there are certain issues that are of importance that need to be considered. They are channel spacing (four Wave Mixing) and optical attenuation. Hence, we need good wavelength conditioning techniques to solve this problem. The techniques used are Forward Error Correction Technique and the pilot light technique. By using the latter technique network management systems are able to ensure connectivity, signal on each channel and also identify faults. This network management is similar to the way test cells are used on specific Virtual Channels in ATM.

Testing ATM over DWDM

Testing of ATM over DWDM consists of similar concepts to those provided in ATM over SONET. However, with DWDM it is more complex because we now have multiple parallel links on a single fiber. So besides the need of taking into account the connectivity and the conformance to QoS agreements, we need to make sure that these parallel links are all mutually exclusive. Hence, the following parameters need to be measured:

Signal-to-noise ratio Channel power Channel center wavelength and spacing Crosstalk Total Optical Power Chromatic dispersion Polarization Mode Dispersion

5.2 IP over DWDM (or IP over lambda)

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The ultimate solution would be to take IP directly over DWDM. This will bring about scalability and cost-effectiveness. Now we have industry products that actually implement IP over DWDM for example Monterey Networks( bought by Cisco in August '99) have their Monterey 20000 Series Wavelength Router& trade. They claim that by using their product, "service providers can traffic-engineer and rapidly scale up survivable mesh optical cores without introducing intermediate ATM switches or proliferating legacy SONET multiplexers and cross-connects".

In effect we are totally eliminating ATM and SONET layers from the networks. The proponents of IP over DWDM say that SONETs reliability is due to a lot of redundancy. This overkill prevents the network from using a large portion of its resources. The real test is whether it would be possible to create an end-to-end optical Internet operating from OC-3 to OC-48 and build systems around an optical Internet backbone. Compare that with the news that SONET handles OC-192 smoothly and can touch OC-768. As of March99, all the IP over DWDM systems that were operational were all SONET frame based.

With the development of erbium-doped fiber amplifiers most systems that use IP over DWDM using SONET frames have removed the SONET multiplexors. GTS Carrier Service in March, launched the first high capacity transport platform in Europe that uses IP over DWDM technology. Further more, major carriers such as AT&T, Sprint, Enron, Frontier, Canarie, have all begun to realize the huge economic potential of IP over DWDM and there is no longer any skepticism about this technology.

6. Measurements of Performance

There are several aspects that make the design of DWDM systems unique. A spectrum of DWDM channels may begin to accumulate tilt and ripple effects as the signals propagate along a chain of amplifiers. Furthermore, each amplifier introduces amplified spontaneous emissions (ASE) into the system, which cause a decrease in the signal to noise ratio, leading to signal degradation. Upon photodetection, some other features of optically amplified systems come into play. The Bit Error Rate (BER) is determined differently in an optically amplified system than in a conventional regenerated one. The probability of error in the latter is dominated by the amount of receiver noise. In a properly designed optically amplified system, the probability of error in the reception of a binary value of one is determined by the signal mixing with the ASE, while the probability of error in the reception of a binary value of zero is determined by the ASE noise value alone.

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7. Optical SNR and Transmitted PowerRequirements of DWDM Systems

Ultimately, the BER performance of a DWDM channel is determined by the optical SNR that is delivered to the photodetector. In a typical commercial system, an optical SNR of approximately 20 dB, measured in a 0.1 nm bandwidth, is required for an acceptably low BER of 10–15. This acceptable SNR is delivered through a relatively sophisticated analysis of signal strength per channel, amplifier distances, and the frequency spacing between channels. For a specific SNR at the receiver, the amount of transmit power required in each channel is linearly proportional to the number of amplifiers as well as the noise and SNR of each amplifier, and is exponentially proportional to the loss between amplifiers. Because total transmit power is constrained by present laser technology and fiber nonlinearities, the workable key factor is amplifier spacing. This is illustrated in the accompanying graph by showing the relationship for a fiber plant with a loss of .3 dB/km, a receiver with a .1nm optical bandwidth, and optical amplifiers with a 5 dB noise figure. The system illustrated is expected to cover 600 kms and the optical SNR required at the receiver is 20 dB measured in the 0.1 nm bandwidth.

8. Key DWDM System Characteristics

There are certain key characteristics of acceptable and optimal DWDM systems.

A sixteen channel system(which is available today) supports 40 Gb/s in each direction over a fiber pair, while a 40 channel system under development will support 100 Gb/s.

The spacing between the channels is less than 1nm.

Transport of Gigabit Ethernet, ATM, SONET, IP on different channel is feasible.

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Well-engineered DWDM systems offer component reliability, system availability, and system margin.

Automatic adjustment of the optical amplifiers when channels are added or removed achieves optimal system performance. This is important because if there is just one channel on the system with high power, degradation in performance through self-phase modulation can occur. On the other hand, too little power results in not enough gain from the amplifier.

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9. Fiber non linearities.

In addition to ASE accumulation and dispersion, there are several types of fiber nonlinearities that can further limit the performance of any fiber optic transmission system—including those that use DWDM. These nonlinearities fall into two broad groups: scattering and refractive index phenomena.

Scattering Phenomena

One subtype of this phenomena is known as Stimulated Brillouin Scattering (SBS), which is caused by the interaction between the optical signal and acoustic waves in the fiber. The result is that power from the optical signal can be scattered back towards the transmitter. SBS is a narrowband process that affects each channel in a DWDM system individually, but

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which is even more pronounced in STM–64/OC–192 systems, due to the greater power levels required for their transmission. A second form of scattering is known as Stimulated RamanScattering (SRS), which is prompted by the interaction of the optical signal with silica molecules in the fiber. This interaction can lead to the transfer of power from shorter wavelength, higher photon energy channels, to longer wavelength, lower photon energy channels. Unlike SBS, SRS is a wideband phenomena that affects the entire optical spectrum that is being transmitted. SRS can actually cause a spectrum of equal amplitude channels to tilt as it moves through the fiber. Moreover, its impact worsens as power is increased and as the total width of the DWDM spectrum widens. One way to combat this phenomena is to usemoderate channel powers as well as a densely packed channel plan that minimizes the overall width of the spectrum.

Refractive Index Phenomena

This group of nonlinearities includes self-phase modulation (SPM), cross-phase modulation (CPM), and four-wave mixing (FWM). These are caused because the index of refraction, and hence the speed of propagation in a fiber, is dependent on the intensity of light—a dependency that can have particularly significant effects in long–haul applications. SPM, which refers to the modulation that a light pulse has on its own phase, acts on each DWDM channel independently. The phenomena causes the signal’s spectrum to widen and can lead to crosstalk or an unexpected dispersion penalty. By contrast, CPM is due to intensity fluctuations in another channel and is an effect that is unique to DWDM systems. Finally, four-wave mixing refers to the nonlinear combination of two or more optical signals in such a way that they produce new optical frequencies. Although four-wave mixing is generally not a concern in conventional single-mode fiber, it can be particularly troublesome in the dispersion shifted fiber that is used to propagate STM64/OC192. As a result, carriers that opt for STM–64/OC–192 equipment to relieve today’s congestion may unintentionally be limiting their ability to grow their capacity through future deployment of DWDM.

All three types of refractive index phenomena can be controlled either through careful choice of channel power or increases in channel spacing.

10. Applications for DWDM

As occurs with many new technologies, the potential ways in which DWDM can be used are only beginning to be explored. Already, however, the technology has proven to be particularly well suited for several vital applications.

• DWDM is ready made for long-distance telecommunications operators that use either point–to–point or ring topologies. The sudden availability of 16 new transmission

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channels where there used to be one dramatically improves an operator’s ability to expand capacity and simultaneously set aside backup bandwidth without installing new fiber.

• This large amount of capacity is critical to the development of self-healing rings, which characterize today’s most sophisticated telecom networks. By deploying DWDM terminals, an operator can construct a 100% protected, 40 Gb/s ring, with 16 separate communication signals using only two fibers.

• Operators that are building or expanding their networks will also find DWDM to be an economical way to incrementally increase capacity, rapidly provision new equipment for needed expansion, and future–proof their infrastructure against unforeseen bandwidth demands.

• Network wholesalers can take advantage of DWDM to lease capacity, rather than entire fibers, either to existing operators or to new market entrants. DWDM will be especially attractive to companies that have low fiber count cables that were installed primarily for internal operations but that could now be used to generate telecommunications revenue.

• The transparency of DWDM systems to various bit rates and protocols will also allow carriers to tailor and segregate services to various customers along the same transmission routes. DWDM allows a carrier to provide STM–4/OC–12 service to one customer and STM–16/OC–48 service to another all on a shared ring!

• In regions with a fast growing industrial base DWDM is also one way to utilize the existing thin fiber plant to quickly meet burgeoning demand.

11.Conclusion

Optical networking provides the backbone to support existing and emerging technologies with almost limitless amounts of bandwidth capacity. All-optical networking(not just point-

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to-point transport) enabled by optical cross-connects, optical programmable add/drop multilexers, and optical switches provides a unified infrastructure capable of meeting the telecommunications demands of today and tomorrow. Transparently moving trillions of bits of information efficiently and cost-effectively will enable service providers to maximize their embedded infrastructure and position themselves for the capacity demand of the next millennium.

12. REFERENCES

Articles:

[ GERWIG98] Optical Networks: A Ray of Light, Kate Gerwig,editor: CMP Media Inc�s InternetWeek Ma, 4 pages,

[EDN98] Optical networking lightens carrier-backbone burden, EDN Access Magazine, October 8 1998

[INTTEL99] DWDM Rising,Telephony Magazine, April 19 1999, http://www.internettelephony.com/archive/4.19.99/Cover/cover.htm

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[SAMIR99] Enlightening the effects and implications of nearly infinite bandwidth: Samir Chatterjee and Suzanne Pawlowski Comm. Of ACM June 1999

[CHATT97] Requirements for success in gigabit Networking: S. Chatterjee, ACM July 1997

[GREEN96] Optical Networking Update: P.E.Green (IEEE Journal on Selected Areas, June 1996

[WILLN97] Mining the Optical Bandwidth for a terabit per second: A.E.Willner, IEEE Spectrum, April 1997

[ALCATEL99] Optical Networks: Alcatel ,August 1999, 29 pages,(http://www.webproforum.com/wpf_all.html)

[YANG98] Amplifiers at 1590nm double the DWDM bandwidth: Dan Yang, AFC Technologies, Sept 1998 (http://www.broadband-guide.com/lw/reports/report09985.html)

12.1 . Books:

[MUKHERJEE97] Biswanath Mukherjee, "Optical Communication Networks", McGraw Hill, July 1997, 575 pages, http://networks.cs.ucdavis.edu/users/mukherje/book/toc.html.

[GREEN93] P.E.Green, "Fiber-Optic Networks", Prentice-Hall 1993

[GERARD98] Gerard Lachs, "Fiber-Optic Communications, McGraw-Hill Telecommunications 1998

[KEISER93] Gerd Keiser,Optic!l fiber Communications, McGraw-Hill 1983

12.2. LIST OF ACRONYMS

ADM            -Add/Drop Multiplexor ANSI           -American National Standards Institute ATM            -Asynchronous Transfer Mode DBFA           -Erbium Fiber-based Dual-band Fiber Amplifier DWDM        -Dense Wavelength Division Multiplexing EBFA           -Extended Band Fiber Amplifier ERION        -Ericsson Intelligent Optical Network IP                 -Internet Protocol ITU              -International Telecommunications Union

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MMR          -Modified MultiMeta Ring MPLS          -Multiprotocol Label Switching MWRS         -Multicast-capable Wavelength-Routing Switch OA              -Optical Amplifier OADM        -Optical Add/Drop Multiplexor OASn         -Optical Amplifier Section layer OCh           -Optical Channel layer O-E-O        -Optical to Electronic to Optical OMSn         -Optical Multiplex Section layer OXC           -Optical CrossConnects QKD          -Quantum Key Distribution UWOA       -Ultra-Wideband Optical Amplifier RWA          -Routing and Wavelength Assignment SDH           -Synchronous Digital Hierarchy SONET      -Synchronous Optical Network SR 3             -Synchronous Round Robin with Reservation SRR           -Synchronous Round Robin TDM          -Time Division Multiplexing WAM         -Wide Area Metro WDM         -Wavelength Division Multiplexing

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