next gen. dense wavelength division...

© 2014, IJARCSSE All Rights Reserved Page | 1000

Volume 4, Issue 6, June 2014 ISSN: 2277 128X

International Journal of Advanced Research in Computer Science and Software Engineering Research Paper Available online at: www.ijarcsse.com

Next Gen. Dense Wavelength Division Multiplexing Shaikh Bilal Anees Sameer Khan

Shah Akhtar Ali

Electronics&Telecom Dept AIKTC Electronics&Telecom Dept AIKTC Electronics&Telecom Dept AIKTC

Mumbai University, India Mumbai University, India Mumbai University, India

ABSTRACT: One of the major issues in the networking industry today is tremendous demand for more and more

bandwidth. Before the introduction of optical networks, the reduced availability of fibers became a big problem for the

network providers. However, with the development of optical networks and the use of Dense Wavelength Division

Multiplexing (DWDM) technology, a new and probably, a very crucial milestone is being reached in network evolution.

The existing SONET/SDH network architecture is best suited for voice traffic rather than today’s high-speed data traffic.

To upgrade the system to handle this kind of traffic is very expensive and hence the need for the development of an

intelligent all-optical network. Such a network will bring intelligence and scalability to the optical domain by combining

the intelligence and functional capability of SONET/SDH, the tremendous bandwidth of DWDM and innovative

networking software to spawn a variety of optical transport, switching and management related products.

The objective of this paper is to summarize the basic optical-networking approaches, briefly report on the WDM

deployment strategies, and outline the current research and development trends on WDM optical networks. And to show

how the speed of up to 1Tb/s can be achieved.

Keywords: DWDM -Dense Wavelength Division Multiplexing, IP-Internet Protocol, ITU –International

Telecommunications Union, SONET -Synchronous Optical Network, ATM -Asynchronous Transfer Mode.

I. 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 2.4 Gb/s on a single fiber, with some deploying equipment that quadruples that

rate to 10 Gb/s.[1]

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! Which is still not enough to manage the current bandwidth

requirements of the corporate, which is up to 1Tb/s? So in this paper we are going to discuss how this enormously high

bandwidth and speed could be achieved.

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.The potential bandwidth of single mode fiber is 50 Tb/s, so we are going to

demonstrate how to achieve less than 1/10th

of this enormous speed which is still quite a lot taking into consideration today’s

scenario. We are going to show the techniques to achieve the high bandwidth of 1 Tb/s.

1.1 WDM Classification

Wherever WDM systems are divided into different wavelength patterns, wide (WWDM), conventional/coarse (CWDM) and

dense (DWDM). Conventional WDM systems provide up to 8 channels in the 3rd

transmission window ( C-Band) of silica

fibers around 1550 nm. Dense wavelength division multiplexing (DWDM) uses the same transmission window but with

denser channel spacing. Channel plans vary, but a typical system would use 40 channels at 100 GHz spacing or 80 channels

with 50 GHz spacing[2].

1.2 Approach to DWDM

Confronted by the need for more capacity, carriers have three possible solutions:

http://www.ijarcsse.com/

http://en.wikipedia.org/wiki/Optical_band#Optical_bands

Anees et al., International Journal of Advanced Research in Computer Science and Software Engineering 4(6),

June - 2014, pp. 1000-1008


• Install new fiber.

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

• Deploy Dense Wavelength Division Multiplexing.

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, single mode 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.

Higher Speed TDM —

Deploying STM-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 wave form 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 single mode fiber that comprises the vast majority of the existing fiber plant,

as well as in aerial fiber.

• 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 altogether.

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[3], 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.

1.3 Dense Wavelength Division Multiplexing

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 and

SONET/SDH respectively, over the optical layer. 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. The Fig 1 below shows the

DWDM system


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II. DWDM System

Fig 1 DWDM System

2.1 Light Sources And Detectors

Light emitters and light detectors are active devices at opposite ends of an optical transmission system. Light sources, or

light emitters, are transmit-side devices that convert electrical signals to light pulses. The process of this conversion, or

modulation, can be accomplished by externally modulating a continuous wave of light or by using a device that can generate

modulated light directly. Light detectors perform the opposite function of light emitters. They are receive-side opto-electronic

devices that convert light pulses into electrical signals.

Two general types of light emitting devices are used in optical transmission, light-emitting diodes (LEDs) and laser

diodes, or semiconductor lasers. LEDs are relatively slow devices, suitable for use at speeds of less than 1 Gbps, they exhibit

a relatively wide spectrum width, and they transmit light in a relatively wide cone. These inexpensive devices are often used

in multi mode fiber communications. Semiconductor lasers, on the other hand, have performance characteristics better suited

to single-mode fiber applications.

Two types of photo-detectors are widely deployed, the positive-intrinsic-negative (PIN) photodiode and the avalanche

photodiode (APD). PIN photo diodes work on principles similar to, but in the reverse of, LEDs. That is, light is absorbed

rather than emitted, and photons are converted to electrons in a 1:1relationship. APDs are similar devices to PIN photo

diodes, but provide gain through an amplification process: One photon acting on the device releases many electrons. PIN

photo diodes have many advantages, including low cost and reliability, but APDs have higher receiver sensitivity and

accuracy.

2.2 Multiplexers And De-Multiplexers

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. De-Multiplexers perform this function by separating the received beam into its wavelength components

and coupling them to individual fibers. De-Multiplexing must be done before the light is detected, because photo detectors

are inherently broadband devices that cannot selectively detect a single wavelength. Multiplexers and De-Multiplexers 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 are to minimize cross-talk and

maximize channel separation.

2.3 Optical Add/Drop Multiplexers

Between multiplexing and de-multiplexing points in a DWDM system, as shown in Figure 2, there is an area in which

multiple wavelengths exist. It is often desirable to be able to remove or insert one or more wavelengths at some point along

this span. An optical add/drop multiplexer (OADM) performs this function. Rather than combining or separating all

wavelengths, the OADM can remove some while passing others on. OADMs are a key part of moving toward the goal of all-

optical networks. OADMs are similar in many respects to SONET ADM, except that only optical wavelengths are added and

dropped, and no conversion of the signal from optical to electrical takes place.

There are two general types of OADMs. The first generation is a fixed device that is physically configured to drop specific

predetermined wavelengths while adding others. The second generation is reconfigurable.


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Fig 2 Optical Add/Drop Multiplexers

2.4 Erbium-Doped Fiber Amplifier

By making it possible to carry the large loads that DWDM is capable of transmitting over long distances, the EDFA was a

key enabling technology. At the same time, it has been a driving force in the development of other network elements and

technologies. Erbium is a rare-earth element that, when excited, emits light around 1.54 micrometers—the low-loss

wavelength for optical fibers used in DWDM. weak signal enters the erbium-doped fiber, into which light at 980 nm or 1480

nm is injected using a pump laser. This injected light stimulates the erbium atoms to release their stored energy as additional

1550-nm light. As this process continues down the fiber, the signal grows stronger. The spontaneous emissions in the EDFA

also add noise to the signal; this determines the noise figure of an EDFA. The key performance parameters of optical

amplifiers are gain, gain flatness, noise level, and output power. Gain should be flat because all signals must be amplified

uniformly.

So addition of noise and limited wavelength makes it difficult to use in the ultra high speed optical system.

2.5 Optical Parametric Amplifier

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, as

discussed earlier. 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 de-multiplexing. Current systems are limited by the erbium doped fibre amplifier (EDFA) bands, i.e.,

about 32 nm[4]. To use larger bandwidths, it is, therefore, necessary to investigate other amplifier types. Fibre optical

parametric amplifiers (OPAs) offer prospects for amplification over large bandwidths and outside the EDFA bands, which

could be useful for future communication systems.

However, the nonlinearity of the amplifying medium, coupled with the fact that good phase matching is necessary for

parametric amplification, may lead to detrimental nonlinear crosstalk in wavelength-division multiplexing (WDM) systems.

This crosstalk can be greatly reduced by using highly uniform fibres, low signal power, polarization multiplexing, or short

fibres.

III. Challenges & Relief Paths

Types of distortion in Optical Fiber Cable

Chromatic dispersion effect.

Dispersion compensating techniques.

Optimization of residual dispersion or its map.

3.1 Chromatic Dispersion

Chromatic dispersion is a type of dispersion in which the light signal is spitted into different signals due to the prism

effect[5]. Due to which signals are affected and distorted at large. Effects of chromatic dispersion spectrum broadening then

differences in group velocity. Then pulse broadening (waveform distortion). It could be better understood in the Fig 3 below.

Fig 3 Chromatic Dispersion


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3.2 Distortion Due To Fiber Non-Linearity

This distortion results only if the there are even little manufacturing defects. It also arises when the is joining of two fibers

over a common point or when there is a bend in the path of the fiber effects of non-linearity in fibers high power intensity

then refractive index changes then frequency chirp then spectrum broadening then waveform distortion due to chromatic

dispersion. It is better demonstrated in the Fig 4 below.

Fig 4 Fiber Non-Linearties

3.3 Distortion Due To Dispersion

Dispersion can be due to many factors in the Optical Fiber cable which can be like Chromatic Dispersion, Non-linearity of

Fiber and many more reasons. This dispersion if not controlled can result in complete loss of signal by complete overlapping

of noise signal over original signal[6].

Fig 5 Dispersion Losses

The meaning of the above shown Fig 5 can be explained below:

The first image is the expected or required transmission output but as the signal travel long distance through the fiber the

output achieved is image 2, which is combination of noise + original signal which makes this signal difficult to be used

without filtering. But if we use a Dispersion Compensating Fiber (DCF) the output received is almost equivalent to the

expected output.

The DCF works as follows:

The Signal travels a long distance say 100km and we get the output as in IMAGE 2 this type of o/p is produced due to

positive dispersion property of normal OFC. But if we connect a DCF the o/p is as of IMAGE 3 this tremendous change is

because of the negative dispersion property. So in order to achieve the exact signal at the receiver o/p we need to implement

DCF in our current long haul OFC systems.

If we take an example of 100km OFC line, so in this line to achieve the required and useful o/p we need to implement DCF in

it.

For example: In 100km around 20km cable should be DFC and other 80km should be normal OFC. A ratio of around 80:20

or 8:2 should be maintained for successful reproduction of data at receiver end.


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Fig 6 Dispersion Compensation Fiber Need

The above Fig 6 shows the requirement of pre and post Distortion Compensation (DC), the need of is self explanatory that

the center wavelength has the least penalty over the longer and shorter wavelengths and the pre and post compensation only

provides that so we need to adopt that compensation technique only.

For better understanding the need of DCF,

We have taken a practical example enlightened below:

Fig 7 Trial Field

This is a 750km WDM field trial between Berlin and Darmstadt (Ref.: OFC/IOOC’99, Technical Digest TuQ2, A. Ehrhardt,

et.al.)[7]. As shown in above Fig 7. In this there are two trials made:

1st (Before Optimization) in this a normal link is used of 750km.

2nd

(After Optimization) in this a normal link is used but with introduction of Pre and Post Distortion Compensation Fiber.

And depending on the trial results a graph has been projected for better understanding of the need of DCF.


June - 2014, pp. 1000-1008


Fig 8 Field Trial Results

It is clear from the above results shown in graph i.e Fig 8 the dire needs of DCF, there are two sections to be explained:

1st (Before Optimization) in this result there is the original signal but with it there is a lot of noise which makes it difficult to

reproduce, the introduction of noise is due to different distortion factors in normal OFC.

2nd

(After Optimization) in this there is original but in pure form and the concentration of noise is to the least possibility.

IV. Modulation Techniques

Fig 9 Different Modulation Techniques

There are four different modulation techniques[8] as shown in Fig 9

1. Non-return-to-zero (NRZ)

2. Return-to-zero (RZ)

3. Carrier-Suppressed Return-to-zero

4. Optical Duo binary

If we compare the above modulation techniques with respect to the above image the RZ seems to be best of all above

because:

It has the highest bandwidth of signal when compared with others in the image, which is one of the most important

factors in OFC communication. Also the noise concentration in it is the least as shown above when compared with

Optical Duo binary.

Also in the NRZ technique both the upper, lower and the middle part carry the info which results in reduction of

bandwidth, but this is not the case in the RZ technique in this the data is carried in the lower and the middle layer

also the BW is high.

In CS-RZ the carrier is suppressed due to which the data which needs to be carried in the carrier is suppressed again

resulting in reduced BW.

So in short the RZ technique is the best form of modulation technique which can be used in the Optical

Transmission over long distances.


June - 2014, pp. 1000-1008


And the main objective of our paper is to show how the speed of 1Tb/s can be achieved.

So one of the ways to achieve this speed is to use the modulation technique Return-to-Zero Differential-Phase-Shift-

Keying or RZ-DPSK over long haul systems. And the other way is the utilization of Dispersion Compensation Fiber or

DCF

In the above section we have explained the dire need of DCF in OFC systems, and now we will show how we can or have to

do it.

The current long haul OFC systems are spread over 100s of Km underground or even under the Sea, so every time the person

cannot access the systems to change the settings or configuration of the DCF in the OFC system.

So there is need of Automatic Monitoring and Correction Systems[9] which will auto correct the errors or compensate the

noise automatically so the user doesn’t need to worry about it every now or then, and can sit back and monitor the system

without much tension of loss of data due to overlapping or noise.

Fig 10 Automatic Dispersion Control

In the Automatic Dispersion Control as shown in Fig 10 one new block is used named as Variable Dispersion Controller

(VDC).

V. Variable Dispersion Controller (VDC)

As optical signals travel through optical fibres, their waveforms are broadened by wavelength dispersion. Deterioration in

signal quality caused by wavelength dispersion can be compensated for by using variable dispersion to apply a characteristic

opposite to the wavelength dispersion in fibres. The dispersion compensator generates inverse wavelength dispersion by

dispersing wavelength division multiplexed signals using an AWG fabricated on a PLC, and by controlling the phase of each

wavelength.

VI. Advantages

i. Capacity Increase:

Large aggregate transmission capacity.

ii. Upgradability:

Customer growth without requiring additional fiber to be laid.

iii. Flexibility:

Optical ADD/DROP multiplexing (OADM) Optical cross connect (OXC)

iv. Scalability:

The possibility to add new nodes to the network.

v. Network Transparency:

Independence of data rate, format and protocol.

VII. Recent Developments

Some technologies are capable of 12.5 GHz spacing (sometimes called ultra dense WDM). Such spacings are today only

achieved by free-space optics technology. New amplification options (Raman amplification) enable the extension of the

usable wavelengths to the L-band, more or less doubling these numbers.

Acknowledgements We express sincere gratitude to our guide Mr. Awaab Fakih and Mrs. Chaya S. Ravindra for their support and valuable

guidance. They has motivated us throughout the course of the paper to work harder and achieve set goals. We are also highly

grateful to Mr. Ramzan Khatik, Head of EXTC Department and the Director, Mr. Razzak Honnutagi for providing the

facilities and conductive environment.

Special thanks to our family and friends to encourage us and provide us with practical suggestions for the improvement of

our paper.

http://en.wikipedia.org/wiki/Raman_amplification

http://en.wikipedia.org/wiki/L-band


June - 2014, pp. 1000-1008


REFERENCES

[1] Muralikrishna Gandluru ―Optical Networking And Dense Wavelenght Devision Multiplexing (DWDM)‖.

[2] Biswanath Mukherjee ―WDM Optical Communication Networks: Progress and Challenges‖.

[3] Introduction to DWDM Technology by Cisco ltd .

[4] Fibre Optic Essentials by Casimer M. DeCusatis and Carolyn J. Sher DeCusatis .

[5] Optical Fibers and RF: A Natural Combination by Malcolm Romeiser .

[6] New functionalities for advanced optical interfaces (Dispersion compensation) byKazuo Yamane Photonic systems

development dept. FUJITSU.

[7] I. P. Kaminow, et al, ―A Wideband All-Optical WDM Network‖, IEEE Journal on Selected Areas in

Communications, Vol.14, No. 5, June 1996, pp. 780 - 799.)

[8] Melián, B., Laguna, M., and Moreno, J.A., "Capacity expansion of fiber optic networks with WDM systems:

Problem formulation and comparative analysis", Computers and Operations Research, 31(3) (2004) 461-472.

[9] E. Lowe, "Current European WDM Deployment Trends", IEEE Communications Magazine, Feburary 1998, pp. 46-

50.

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