CONTENTS · Seminar Report ‘03 0 CONTENTS • Definition and Overview • Fundamentals of DWDM...

Seminar Report ‘03 0

CONTENTS

• 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


Definition 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.

Overview 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.


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.


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.


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


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.

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

demand.

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.


Optical Transport to Optical Networking: Evolution of the Phototonics

Layer

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.


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

networks.


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


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

exhaust.

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


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.


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.


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.


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


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.


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


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

photodetector.

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.


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.


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

components


Transponders

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


Operation of a Transponder Based DWDM System 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

wavelength.

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

node.

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

(optional).

6. A pre-amplifier boosts the signal before it enters the end system

(optional).


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.


References

1. Computer Networks-S.Tenanbaum 2. www.iec.org

3. www.itpapers.com


ACKNOWLEDGEMENT

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


ABSTRACT

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.

CONTENTS · Seminar Report ‘03 0 CONTENTS • Definition and Overview • Fundamentals of DWDM...

Documents

Transcript of CONTENTS · Seminar Report ‘03 0 CONTENTS • Definition and Overview • Fundamentals of DWDM...