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					Seminar Report ‘03                                          0


   • Definition and Overview
   • Fundamentals of DWDM Technology
   • Development of DWDM Technology
        •    WDM with Two Channels
        •    Evolution of DWDM
   • The Challenges of Today's Telecommunications Network
   • Resolving the Capacity Crisis
   • Capacity Expansion and Flexibility
   • Capacity Expansion Potential
   • DWDM Incremental Growth
   • The Optical Layer as the Unifying Layer
   • Optical Amplifiers
   • Multiplexers and Demultiplexers
   • DWDM System Functions
   • Enabling Technologies
   • Components and Operation
   • Transponders
   • Operation of a Transponder Based DWDM System
   • Key DWDM System Characteristics
   • Conclusion
Seminar Report ‘03                                                          1

   Dense wavelength division multiplexing (DWDM) is a fiber-optic
   transmission technique that employs light wavelengths to transmit data
   parallel-by-bit or serial-by-character.

   The role of scalable DWDM systems in enabling service providers to
   accommodate consumer demand for ever-increasing amounts of
   bandwidth is important. DWDM is discussed as a crucial component of
   optical networks that allows the transmission of e-mail, video,
   multimedia, data, and voice—carried in Internet protocol (IP),
   asynchronous transfer mode (ATM), and synchronous optical
   network/synchronous digital hierarchy (SONET/SDH), respectively,
   over the optical layer.
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   Fundamentals of DWDM Technology

   The emergence of DWDM is one of the most recent and important
   phenomena in the development of fiber optic transmission technology.
   The functions and components of a DWDM system, including the
   enabling technologies, and a description of the operation of a DWDM
   system are discussed below.
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   Development of DWDM Technology

   Early WDM began in the late 1980s using the two widely spaced
   wavelengths in the 1310 nm and 1550 nm (or 850 nm and 1310 nm)
   regions, sometimes called wideband WDM. Figure below shows an
   example of this simple form of WDM. One of the fiber pair is used to
   transmit and the other is used to receive. This is the most efficient
   arrangement and the one most found in DWDM systems.

   WDM with Two Channels

   The early 1990s saw a second generation of WDM, sometimes called
   narrowband WDM, in which two to eight channels were used. These
   channels were now spaced at an interval of about 400 GHz in the 1550-
   nm window. By the mid-1990s, dense WDM (DWDM) systems were
   emerging with 16 to 40 channels and spacing from 100 to 200 GHz. By
   the late 1990s DWDM systems had evolved to the point where they
   were capable of 64 to 160 parallel channels, densely packed at 50 or
   even 25 GHz intervals.
Seminar Report ‘03                                                         4

   The progression of the technology can be seen as an increase in the
   number of wavelengths accompanied by a decrease in the spacing of
   the wavelengths. Along with increased density of wavelengths, systems
   also advanced in their flexibility of configuration, through add-drop
   functions, and management capabilities.

   Evolution of DWDM
Seminar Report ‘03                                                            5

   The Challenges           of    Today's       Telecommunications

   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.
   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. No one
   could have predicted the network growth necessary to meet the
   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 portions of their networks.
   Another problem for carriers is the challenge of 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.
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     Optical Transport to Optical Networking: Evolution of the Phototonics

   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 that 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.
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   Resolving the Capacity Crisis

   Faced with the multifaceted challenges of increased service needs, fiber
   exhaust, and layered bandwidth management, service providers need
   options to provide an economical solution. One way to alleviate fiber
   exhaust is to lay more fiber, and, for those networks where the cost of
   laying new fiber is minimal, this will prove the most economical
   solution. However, laying new fiber will not necessarily enable the
   service provider to provide new services or utilize the bandwidth
   management capability of a unifying optical layer.

   A second choice is to increase the bit rate using time division
   multiplexing (TDM), where TDM increases the capacity of a fiber by
   slicing time into smaller intervals so that more bits (data) can be
   transmitted per second. Traditionally, this has been the industry method
   of choice (DS–1, DS–2, DS–3, etc.). However, when service providers
   use this approach exclusively, they must make the leap to the higher bit
   rate in one jump, having purchased more capacity than they initially
   need. Based on the SONET hierarchy, the next incremental step from
   10 Gbps TDM is 40 Gbps—a quantum leap that many believe will not
   be possible for TDM technology in the near future. This method has
   also been used with transport networks that are based on either the
   synchronous optical network (SONET) standard for North America or
   the synchronous digital network (SDH) standard for international
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   Capacity Expansion and Flexibility: DWDM
   The third choice for service providers is dense wavelength division
   multiplexing (DWDM), which increases the capacity of embedded
   fiber by first assigning incoming optical signals to specific frequencies
   (wavelength, lambda) within a designated frequency band and then
   multiplexing the resulting signals out onto one fiber. Because incoming
   signals are never terminated in the optical layer, the interface can be
   bit-rate and format independent, allowing the service provider to
   integrate DWDM technology easily with existing equipment in the
   network while gaining access to the untapped capacity in the embedded
   fiber.DWDM combines multiple optical signals so that they can be
   amplified as a group and transported over a single fiber to increase
   capacity. Each signal carried can be at a different rate (OC–3/12/24,
   etc.) and in a different format (SONET, ATM, data, etc.) A system with
   DWDM can achieve all this gracefully while maintaining the same
   degree of system performance, reliability, and robustness as current
   transport systems—or even surpassing it. Future DWDM terminals will
   carry up to 80 wavelengths of OC–48, a total of 200 Gbps, or up to 40
   wavelengths of OC–192, a total of 400 Gbps—which is enough
   capacity to transmit 90,000 volumes of an encyclopedia in one second.
                     Increased Network Capacity—WDM
Seminar Report ‘03                                                          9

   Capacity Expansion Potential

   By beginning with DWDM, service providers can establish a grow-as-
   you-go infrastructure, which allows them to add current and next-
   generation TDM systems for virtually endless capacity expansion.
   DWDM also gives service providers the flexibility to expand capacity
   in any portion of their networks—an advantage no other technology
   can offer. Carriers can address specific problem areas that are
   congested because of high capacity demands. This is especially helpful
   where multiple rings intersect between two nodes, resulting in fiber
       Capacity Expansion Evolution: A Strategy for the Long Term

   Service providers searching for new and creative ways to generate
   revenue while fully meeting the varying needs of their customers can
   benefit from a DWDM infrastructure as well. By partitioning and
   maintaining different dedicated wavelengths for different customers,
   for example, service providers can lease individual wavelengths—as
   opposed to an entire fiber—to their high-use business customers.
   Compared with repeater-based applications, a DWDM infrastructure
   also increases the distances between network elements—a huge benefit
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   for long-distance service providers looking to reduce their initial
   network investments significantly. The fiber-optic amplifier component
   of the DWDM system enables a service provider to save costs by
   taking in and amplifying optical signals without converting them to
   electrical signals. Furthermore, DWDM allows service providers to do
   it on a broad range of wavelengths in the 1.55µm region. For example,
   with a DWDM system multiplexing up to 16 wavelengths on a single
   fiber, carriers can decrease the number of amplifiers by a factor of 16 at
   each regenerator site. Using fewer regenerators in long-distance
   networks results in fewer interruptions and improved efficiency.
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   DWDM Incremental Growth

   A DWDM infrastructure is designed to provide a graceful network
   evolution for service providers who seek to address their customers'
   ever-increasing capacity demands. Because a DWDM infrastructure
   can deliver the necessary capacity expansion, laying a foundation based
   on this technology is viewed as the best place to start. By taking
   incremental growth steps with DWDM, it is possible for service
   providers to reduce their initial costs significantly while deploying the
   network infrastructure that will serve them in the long run.

   Some industry analysts have hailed DWDM as a perfect fit for
   networks that are trying to meet demands for more bandwidth.
   However, these experts have noted the conditions for this fit: a DWDM
   system simply must be scalable. Despite the fact that a system of OC–
   48 interfacing with 8 or 16 channels per fiber might seem like overkill
   now, such measures are necessary for the system to be efficient even
   two years from now.

   Because OC–48 terminal technology and the related operations support
   systems (OSSs) match up with DWDM systems today, it is possible for
   service providers to begin evolving the capacity of the TDM systems
   already connected to their network. Mature OC–192 systems can be
   added later to the established DWDM infrastructure to expand capacity
   to 40 Gbps and beyond.
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   The Optical Layer as the Unifying Layer

   Aside from the enormous capacity gained through optical networking,
   the optical layer provides the only means for carriers to integrate the
   diverse technologies of their existing networks into one physical
   infrastructure. DWDM systems are bit-rate and format independent and
   can accept any combination of interface rates (e.g., synchronous,
   asynchronous, OC–3, –12, –48, or –192) on the same fiber at the same
   time. If a carrier operates both ATM and SONET networks, the ATM
   signal does not have to be multiplexed up to the SONET rate to be
   carried on the DWDM network. Because the optical layer carries
   signals without any additional multiplexing, carriers can quickly
   introduce ATM or IP without deploying an overlay network. An
   important benefit of optical networking is that it enables any type of
   cargo to be carried on the highway.

   But DWDM is just the first step on the road to full optical networking
   and the realization of the optical layer. The concept of an all-optical
   network implies that the service provider will have optical access to
   traffic at various nodes in the network, much like the SONET layer for
   SONET traffic. Optical wavelength add/drop (OWAD) offers that
   capability, where wavelengths are added or dropped to or from a fiber,
   without requiring a SONET terminal. But ultimate bandwidth
   management flexibility will come with a cross-connect capability on
   the optical layer. Combined with OWAD and DWDM, the optical
   cross-connect (OXC) will offer service providers the ability to create a
   flexible, high-capacity, efficient optical network with full optical
   bandwidth management.
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   Optical Amplifiers

   The technology that allows this high-speed, high-volume transmission
   is in the optical amplifier. Optical amplifiers operate in a specific band
   of the frequency spectrum and are optimized for operation with existing
   fiber, making it possible to boost lightwave signals and thereby extend
   their reach without converting them back to electrical form.
   Demonstrations have been made of ultrawideband optical-fiber
   amplifiers that can boost lightwave signals carrying over 100 channels
   (or wavelengths) of light.

   Due to attenuation, there are limits to how long a fiber segment can
   propagate a signal with integrity before it has to be regenerated. Before
   the arrival of optical amplifiers (OAs), there had to be a repeater for
   every signal transmitted. The OA has made it possible to amplify all
   the wavelengths at once and without optical-electrical-optical (OEO)
   conversion. Besides being used on optical links, optical amplifiers also
   can be used to boost signal power after multiplexing or before
   demultiplexing, both of which can introduce loss into the system.

   Multiplexers and Demultiplexers
   Because DWDM systems send signals from several sources over a
   single fiber, they must include some means to combine the incoming
   signals. This is done with a multiplexer, which takes optical
   wavelengths from multiple fibers and converges them into one beam.
   At the receiving end the system must be able to separate out the
   components of the light so that they can be discreetly detected.
   Demultiplexers perform this function by separating the received beam
   into its wavelength components and coupling them to individual fibers.
   Demultiplexing must be done before the light is detected, because
Seminar Report ‘03                                                             14

   photodetectors are inherently broadband devices that cannot selectively
   detect a single wavelength.

   In a unidirectional system, there is a multiplexer at the sending end and
   a demultiplexer at the receiving end. Two system would be required at
   each end for bidirectional communication, and two separate fibers
   would be needed.

        Multiplexing and Demultiplexing in a Unidirectional System

   In a bidirectional system, there is a multiplexer/demultiplexer at each
   end and communication is over a single fiber pair.

        Multiplexing and Demultiplexing in a Bidirectional System

   Multiplexers and demultiplexers can be either passive or active in
   design. Passive designs are based on prisms, diffraction gratings, or
   filters, while active designs combine passive devices with tunable
   filters. The primary challenges in these devices is to minimize cross-
   talk and maximize channel separation.
Seminar Report ‘03                                                          15

   DWDM System Functions

   At its core, DWDM involves a small number of physical-layer
   functions. Which shows a DWDM schematic for four channels. Each
   optical channel occupies its own wavelength.

                       DWDM Functional Schematic

   The system performs the following main functions:
   • Generating the signal—The source, a solid-state laser, must provide
      stable light within a specific, narrow bandwidth that carries the
      digital data, modulated as an analog signal.
   • Combining       the   signals—Modern     DWDM     systems   employ
      multiplexers to combine the signals. There is some inherent loss
      associated with multiplexing and demultiplexing. This loss is
      dependent upon the number of channels but can be mitigated with
      optical amplifiers, which boost all the wavelengths at once without
      electrical conversion.
   • Transmitting the signals—The effects of crosstalk and optical signal
      degradation or loss must be reckoned with in fiber optic
      transmission. These effects can be minimized by controlling
      variables such as channel spacings, wavelength tolerance, and laser
Seminar Report ‘03                                                             16

      power levels. Over a transmission link, the signal may need to be
      optically amplified.
   • Separating the received signals—At the receiving end, the
      multiplexed signals must be separated out. Although this task would
      appear to be simply the opposite of combining the signals, it is
      actually more technically difficult.
   • Receiving the signals—The demultiplexed signal is received by a

   In addition to these functions, a DWDM system must also be equipped
   with client-side interfaces to receive the input signal. This function is
   performed by transponders. On the DWDM side are interfaces to the
   optical fiber that links DWDM systems.
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   Enabling Technologies
   Optical networking, unlike SONET/SDH, does not rely on electrical
   data processing. As such, its development is more closely tied to optics
   than to electronics. In its early form, WDM was capable of carrying
   signals over two widely spaced wavelengths, and for a relatively short
   distance. To move beyond this initial state, WDM needed both
   improvements in existing technologies and invention of new
   technologies. Improvements in optical filters and narrowband lasers
   enabled DWDM to combine more than two signal wavelengths on a
   fiber. The invention of the flat-gain optical amplifier, coupled in line
   with the transmitting fiber to boost the optical signal, dramatically
   increased the viability of DWDM systems by greatly extending the
   transmission distance.

   Other technologies that have been important in the development of
   DWDM include improved optical fiber with lower loss and better
   optical transmission characteristics, EDFAs, and devices such as fiber
   Bragg gratings used in optical add/drop multiplexers.
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   Components and Operation

   DWDM is a core technology in an optical transport network. The
   essential components of DWDM can be classified by their place in the
   system as follows:
   • On the transmit side, lasers with precise, stable wavelengths
   • On the link, optical fiber that exhibits low loss and transmission
      performance in the relevant wavelength spectra, in addition to flat-
      gain optical amplifiers to boost the signal on longer spans
   • On the receive side, photodetectors and optical demultiplexers using
      thin film filters or diffractive elements
   • Optical     add/drop     multiplexers    and   optical   cross-connect
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   Transponders convert incoming optical signals into the precise ITU-
   standard wavelengths to be multiplexed,and are currently a key
   determinant of the openness of DWDM systems.

   Within the DWDM system a transponder converts the client optical
   signal from back to an electrical signal and performs the 3R functions.
   This electrical signal is then used to drive the WDM laser. Each
   transponder within the system converts its client's signal to a slightly
   different wavelength. The wavelengths from all of the transponders in
   the system are then optically multiplexed. In the receive direction of the
   DWDM system, the reverse process takes place. Individual
   wavelengths are filtered from the multiplexed fiber and fed to
   individual transponders, which convert the signal to electrical and drive
   a standard interface to the client.

                            Transponder Functions
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   Operation of a Transponder Based DWDM
   End-to-end operation of a unidirectional DWDM system.

                        Anatomy of a DWDM System

   The following steps describe the system shown in Figure above.

   1. The transponder accepts input in the form of standard single-mode
      or multimode laser. The input can come from different physical
      media and different protocols and traffic types.
   2. The wavelength of each input signal is mapped to a DWDM
   3. DWDM wavelengths from the transponder are multiplexed into a
      single optical signal and launched into the fiber. The system might
      also include the ability to accept direct optical signals to the
      multiplexer; such signals could come, for example, from a satellite
   4. A post-amplifier boosts the strength of the optical signal as it leaves
      the system (optional).
   5. Optical amplifiers are used along the fiber span as needed
   6. A pre-amplifier boosts the signal before it enters the end system
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   7. The incoming signal is demultiplexed into individual DWDM
      lambdas (or wavelengths).
   8. The individual DWDM lambdas are mapped to the required output
      type (for example, OC-48 single-mode fiber) and sent out through
      the transponder.
Seminar Report ‘03   22
Seminar Report ‘03                    23


   1. Computer Networks-S.Tenanbaum
Seminar Report ‘03                                                           24


      I thank God Almighty for the successful completion of my seminar.
Sincere feelings of gratitude for Dr.Agnisharman Namboothiri, Head of the
Department, Information Technology. I express my heartfelt gratitude to
Staff-in-charge, Miss. Sangeetha Jose and Mr. Biju, for their valuable advice
and guidance. I would also like to express my gratitude to all other members
of the faculty of Information Technology department for their cooperation.

      I would like to thank my dear friends, for their kind-hearted
cooperation and encouragement.

                                         SHAHAN BABU.P
Seminar Report ‘03                                                           25


 DWDM(Dense wavelength division multiplexing) is a fiber-optic
 transmission technique which is an optimal solution to consumer demand
 for ever-increasing amounts of bandwidth. DWDM is a crucial component
 of optical networks that resolves capacity crisis, provides flexibility to
 expand capacity ,enables efficient and cost-effective data transfer and helps
 to integrate various technologies into single infrastructure.

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