INTRODUCTION

Wavelength Division Multiplexing (WDM) is a technology that multiplexes several signals over a single optical fiber byoptical carriers of different wavelengths, using light from a laser or a LED. This can take greater advantage of theenormous bandwidth that optical fiber has.

The first WDM system combining two carrier signals first appeared around 1985. In the early twenty-first, the WDMtechnology can combine up to 160 signals with an effective bandwidth of about 10 Gbps. And carriers are testing the40 Gbps or even 100 Gbps, despite the theoretical capacity of a single optical fiber in 1600 is estimated Gbps. So itis possible to achieve higher capacities in the future as technology advances.

Metropolitan networks, or Metropolitan Area Networks (MANs), are networks that cover areas of a city or nearbycities that interface between access networks and backbone transport over long distances. The needs of thesenetworks are typically scalability, low cost, flexibility, robustness, transparency and bandwidth relatively high andtailored to the client. The demand for transport capacity in the metropolitan area is growing due to the introductionof services and applications with high consumption of bandwidth. This need for bandwidth in the MAN a few yearsago raised a great interest in the WDM technology, as well as the transparency inherent in this technology is wellsuited to this environment, characterized by the need to integrate a wide range of clients, services and protocols.These systems did not meet expectations at any time, mainly because they had a very high cost and did not allow arapid Return On Investment (ROI) in their acquisition and deployment.

However, the maturity of the technology have been realized. WDM systems tailored specifically to the metropolitanarea, offering high bandwidth at relatively low cost. Within the family of WDM technology, the most economicallycompetitive over short distances is the Coarse WDM (CWDM). CWDM technology benefits from lower cost of opticalcomponents associated with a lower-tech. Although limited in capacity and distance, CWDM is well suited to theneeds of enterprise networks and metropolitan short distance. Within the family of WDM technology, Dense WDM(DWDM) which in turn can be ultra long distance, long distance or metropolitan.

CWDM TECHNOLOGY

CWDM means division multiplexing light wavelengths. It is a technique for transmitting signals via optical fiber whichbelongs to the WDM family. The CWDM technology was standardized by the ITU-T based on a grid or wavelengthseparation of 20 nm in the range of 1270-1610 nm, thus being able to carry up to 18 wavelengths in a single modefiber. Accordingly, there are two important characteristics inherent in systems employing CWDM optical componentswhich allow easier and therefore also cheaper than in DWDM systems.

Higher Wavelength Spacing: CWDM lasers can be used with a greater spectral bandwidth and stabilized, i.e. thecentral wavelength can be shifted due to manufacturing imperfections or changes in temperature which it issubjected to the laser and even so, being in band. This allows the manufacture of lasers following manufacturingprocesses less critical than those used in DWDM, and that the lasers do not have sophisticated cooling circuits forcorrecting deviations of the wavelength due to temperature changes to which the chip is subjected, it whichsignificantly reduces the space occupied by the chip and the power consumption, in addition to the cost ofmanufacture. Usually used in CWDM distributed feedback lasers or DFB (Distributed Feed-Back) directly modulatedchannel supporting speeds up to 2.5 Gbps over distances up to 80 km in the case of using G.652 fiber. On the otherhand, uses CWDM optical filters and multiplexers and demultiplexers based on thin film technology or Thin Film Filter(TFF), where the number of layers of the filter increases as the channel spacing is less. This again implies a greatercapacity for integration and reduced cost. These filters CWDM broadband, allowed variations in the nominalwavelength of the source of up to about ± 7.6 nm and are generally available as a filter or two channels.

High Optical Spectrum: This, which allows the number of channels capable of being used not see radicallydecreased despite increasing the separation between them is not possible because in CWDM optical amplifiers areused Erbium-Doped Fiber or Erbium-Doped Fiber Amplifier (EDFA) as in DWDM for distances above 80 km. TheEDFA are components used before transmitting or receiving optical fiber to amplify the power of all optical channelssimultaneously, without any power level feedback. CWDM systems are used, if necessary by the distances covered ornumber of nodes in cascade to cross, regeneration, ie each channel undergoes a conversion optical-electrical-opticalcompletely independently of the rest to be amplified. The cost of optoelectronics in CWDM is such that it is simplerand less expensive to regenerate be amplified. Furthermore, since the regenerators made completely amplificationfunctions, reconstruction of the signal shape, and timing signal, compensating the dispersion accumulated all, thisdoes not occur in the optical amplification, unless used with dispersion compensation fiber or DCF (DispersionCompensation Fiber), high cost and also usually require a preamp stage prior given the high attenuation introduced.

In addition, CWDM is very simple in terms of network design, implementation, and operation. CWDM works with few

parameters that need optimization by the user, while DWDM systems require complex calculations of balance ofpower per channel, which is further complicated when channels are added and removed or when it is used in DWDMnetworks ring, especially when systems incorporate optical amplifiers.

PROPERTIES OF THE CWDM PRODUCTS

1. It has frequency spacing of 2.500 GHz (20nm), allowing for large spectral width lasers.2. 18 wavelengths defined in the interval 1270 to 1610 nm3. The current CWDM have their li-mite at 2.5 Gbps.4. As to the distances covered reach about 80 km.5. DBF laser used (distributed feedback lasers) without peltier or thermistor.6. Use of broadband optical filters, multiplexers and demultiplexers based on FFT (thin film technology).7. Spacing greater wavelengths, indicating that if there is a variation in the central wavelength due to

imperfections of the laser produced by manufacturing processes less critical in this wave band are maintained.8. High optical spectrum, this allows us to have a number of channels to use without these be diminished

because of the separation between them.

As Topological

CWDM can support the following topologies.

1. Rings point to point and passive optical network (PON eliminates all active components in the network, tointroduce passive components such as splitter or splitter, and reduce maintenance costs and the network).

2. Anillos locales CWDM que se conectan con anillos metropolitanos DWDM.

3. Access rings and passive optical networks.

Advantages

1. Lower energy consumption2. Smaller than the laser CWDM3. Solve the problems of bottlenecks.4. Hardware and operating costs relating to other cheaper technologies like this.5. Higher band widths6. Is simpler relating to network design, implementation and operation.7. Ease of installation, configuration and maintenance of the network.8. High degree of flexibility and security in the metro optical networking.9. It can carry any short-range service as SDH, CATV, ATM, FTTH - PON 10Gibagit, among others.

DWDM (Dense Wavelength Division Multiplexing), which means divisionmultiplexing in dense wave longitude. One s DWDM transmission techniqueseñales through optical fiber using the C band (1550 nm).

That is a multiplexing method very similar to the frequency divisionmultiplexing is used in electromagnetic transmission means. Several carriersignals (optical) are transmitted by a single optical fiber using differentwavelengths of laser beam each. Each optical carrier is an optical channelthat can be treated independently of other channels that share themedium (fiber optic) and contain different types of traffic. In this way canmultiply the effective bandwidth of the optical fiber, so as to providebidirectional communications. This is a very attractive transmissiontechnique for telecom operators by allowing them to increase capacity without additional wiring or trenching.

To transmit using DWDM is needed from complementary devices: a transmitter side muxer y demuxer at a receiverside it. Unlike him CWDM, DWDM is in numbers mayor Get optical channels reducing the chromatic dispersion ofeach channel through it using a laser Mayor Quality, baja fiber dispersion through him the use of DCM modules"Dispersion Compensation Modules." In this MANERO you can combine more channels reducing space between heello. Currently it pueden get 40, 80 to 160 optical channels separated 100 GHz, 50 GHz to 25 GHz respectively.

PROPERTIES OF THE DWDM PRODUCTS

1. The large scale manufacture of optical fiber has allowed a reduction in costs and an improvement in transmissioncharacteristics of the fiber.

2. Flat gain optical amplifiers for a given range of wavelengths coupled in line with the fiber acting as repeaterseliminating the need for regenerators.

3. Solid state integrated filters smaller and can be integrated on the same substrate along with other opticalcomponents.

4. New photo detectors and laser sources that allow integration producing more compact designs.

5. Multiplexers and demultiplexers based optical passive optical diffraction.

6. Filters selectable wavelength, which can be employed as optical multiplexers.

7. The optical add-drop multiplexers (OADM) technology have allowed implants in DWDM networks of various types.

8. The optical components of connection (OXC), which can be implemented with different manufacturingtechnologies, and have made possible the purely optical switching.

9. The scope of DWDM is in long distance networks of ultra-wide band, so as metropolitan networks, intercity orvery high speed.

As the deployment of DWDM increases its cost is decreasing gradually, mainly due to the large number of optical

components that are manufactured. Consequently, are expected to become a DWDM technology that enables lowcost implementation in many types of networks.

DWDM technology requires specialized optical devices based on the properties of light and the optical, electrical andmechanical properties of semiconductors. Among these optical devices include optical transmitters, ADC and OXC.

Conventional single mode fibers can transmit in the range of 1,300 a1.550 nm. absorbing the wavelengths from 1340to 1440 nm. WDM systems use wavelengths in the two possible ranges (from 1,300 to 1.34o nm 's1.440 to 1,550nm). There are special fibers that allow transmission at all wavelengths between 1,530 and 1,565 nm withoutabsorption. However, not all optoelectronic components work with the same efficiency at all wavelengths.

DWDM systems employ the latest advances in optical technology to generate a large number of wavelengths in therange close to 1,550 nm ITU-T recommendation G.692 defines its 43 channels in the range of 1,530 to 1,565 nmwith a spacing of 100 GHz, each channel will carry an OC-192 traffic at 10 Gbps. However, each day coming tomarket systems with more channels. A 40-channel DWDM system at 10 Gbps per channel provides an aggregatespeed of 400 Gbps.

Currently, commercial DWDM systems have 16 to 40 and 80 channels and is expected to market the next 128-channel systems. Systems with 40 channels have a channel spacing of 100 GHz, having 80 channels have a spacingof 50 GHz frequency spacing This indicates the proximity of the channels between them. A channel not only uses asingle wavelength, each channel has a certain bandwidth around the central wavelength, each band is separatedfrom the next by a guard band area several GH, thus seeks to avoid possible overlap or interference betweenadjacent channels.

These problems are due to drifts in the laser emitters by temperature or time, to have no optical amplifiers ios again constant for all wavelengths and potential scattering effects, among others.

The number of channels depends also on the type of fiber used. A single strand of single mode fiber can transmitdata at a distance of 80 km without amplification. Placing 8 cascaded optical amplifiers, the distance may increase to640 km.

Point-To-Point Topology

The topology point-to-point can be implemented with or withoutOADMs. These networks are characterized by ultra-fast speeds ofchannels (10 to 40 [Gbps]), high integrity and reliability of the signal,and fast path restoration. In long-haul networks (long distance), thedistance between transmitter and receiver can be several hundredkilometers, and the number of amplifiers required between the twopoints, is typically less than 10. MANs networks, amplifiers are oftennecessary.

Topologies protection in point-to-point may be provided in a coupleof ways. In the first generation equipment, redundancy is a systemlevel. Redundant parallel lines connected at both ends.

In the second generation equipment, redundancy is the board level.Parallel lines connect one system at both ends that contain transponders, multiplexers and redundant CPUs.

Ring Topology

The rings are the most common architectures found in metropolitan areasand sections of a few tens of kilometers. The fiber ring may contain onlyfour channels of wavelengths, typically less nodes and channels. The BitRate is in the range of 622 Mbps to 10 Gb per channel.

With the use of OADMs, which rise and fall of wavelengths in atransparent, meaning that the others are not affected, ring architecturesallow nodes to access the network elements such as routers, switchesand servers, with the rise and fall of wavelength channels in the opticaldomain. With the increase in the number of OADMs, the signal is subjectto loss and amplifiers may be required.

For protection in this topology using the 1 +1 scheme. It has two lines ofconnection, information is sent by one of them. If the ring fails, anotherpath switchea ring.

Mesh Topology

The mesh architecture is the future of optical networks. Asnetworks evolve, the architecture of ring and point-to-pointwould have a place, but the mesh would be the most robusttopology. Such development would be enabled by theintroduction of the OXCs (Optical Cross-Connects) andconfigurable switches, which in some cases replace, andother would supplement, a fixed DWDM devices.

From the design standpoint, there is a graceful evolutionarypath topology point-to-point and mesh. At the beginning ofpoint-to-point, OADM nodes equipped for flexibility initially,and later in the interconnections, the network can evolveinto a mesh without a complete redesign. Additionally, ringand mesh topologies can be connected to point-to-point.

DWDM channels

Standard DWDM channel distribution:

Official spacings between channels of 100 GHz (0.8nm, 41canales) or 50 GHz (0.4 nm, 82 channels)Band C, conventional wavelength shorterL-band, wavelength longer (up to 1610 nm)Start using the spacing of 50 GHz (or even 25 and 12.5GHz ultra-dense WDM) and the band (1490 nm)

DWDM Transmission

DWDM monoDWDM bidirectionalTunable lasers EDFA over the entire range, with spacing of 100, 50 and up to 25 GHzPossibility of tunable receiversNew optical amplifiersSpeeds up to 10 and 40 Gbps (depending on the length) for each λ

DWDM components

Optical multiplexer "Add / Drop" (OADM)Crossed optical switch (OXC)Wavelength converterOptical splitter / combinerWavelength routerTime division multiplex optical (OTDM)Optical multiplexer "Add / Drop"

Comparative table of WDM technologies as the application.

Application /parameter CWDM Access / MAN DWDM MAN /

WANDWDM widerange

Channels per fiber 4-16 32-80 80-160spectrum used O, E, S, C, L C, L C, L, S

Channel spacing 20 nm (2500 GHz) 0.8 nm (100GHz)

0.4 nm (50GHz)

Capacity perchannel 2.5 Gbit / s 10 Gbit / s 10-40 Gbit / s

Capacity of fiber 20-40 Gbit / s 100-1000 Gbit /s > 1 Tbit / s

Laser Type uncooled DFB (distributed feedbacklaser) cooled DFB cooled DFB

Filter technology TFF (tecn. thin film) TFF, AWG, FBG TFF, AWG, FBG

distance up to 80 km hundreds ofmiles

thousands ofmiles

cost low medium highoptical amplification no EDFA EDFA, Raman

Conclusions

When necessary the use of WDM in a metropolitan network, the best choice would normally be CWDM. Although ithas a number of limitations (capacity, distance ...) for DWDM, in many cases meets the necessary requirements, andis also around 50% of DWDM, since the equipment needed for CWDM is cheaper.

The fact of the price reduction is very important given the significant investments that require this type ofinfrastructure initially, and which thus encourages their creation. Furthermore, although in principle one might thinkthat because more than likely increase the need for bandwidth, CWDM might not cover long-term needs, it ispossible to upgrade from CWDM to DWDM.

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