Research work of transmission systems-CWDM VS DWDM

CWDM-DWDM

Related CWDM-DWDM Products - cwdm mux/demux, cwdm oadm, dwdm mux/demux, dwdm oadm

 

Brief history

The first WDM system combining two carrier signals first appeared around 1985. In the early twenty-first technology to combine up to 160 signals with an effective bandwidth of about 10 Gbits per second. And carriers are testing the 40 Gbits / s. Despite the theoretical capacity of a single optical fiber in 1600 is estimated Gbit / s. So it is possible to achieve higher capacities in the future as technology advances.

 

Introduction

 

WDM (wavelength division multiplexing wavelength) is a technology that multiplexes several signals over a single optical fiber by optical carriers of different wavelength, using light from a laser or a LED. This can take greater advantage of the enormous bandwidth of optical fiber has.

 

Metropolitan networks or MAN (Metropolitan Area Network) are networks that cover areas of a city or nearby cities that interface between access networks and backbone transport over long distances. The needs of these networks are typically scalability, low cost, flexibility, robustness, transparency and bandwidth relatively high and tailored to the client. The demand for transport capacity in the metropolitan area is growing due to the introduction of services and applications with high consumption of bandwidth. This need for bandwidth in the metropolitan network a few years ago raised a great interest in WDM (Wavelength Division Multiplexing), as well as the transparency inherent in this technology is well suited to this environment, characterized by the need to integrate a wide range of clients, services and protocols. However, these systems did not meet expectations at any time, mainly because they had a very high cost and did not allow a rapid return on investment in their acquisition and deployment. However, the maturity of the technology have been realized WDM systems tailored specifically to the metropolitan area, offering high bandwidth at relatively low cost. Within the family of WDM technology, the most economically competitive over short distances is the CWDM (Coarse WDM). CWDM technology benefits from lower cost of optical components associated with a lower-tech, although limited in capacity and distance, is well suited to the needs of enterprise networks and metropolitan short distance.
Transmission system with optical fiber WDM

Within the family WDM two systems:

• DWDM which in turn can be ultra long distance, long distance or metropolitan.
• CWDM.

CWDM (Coarse Wavelength Division Multiplexing), which means division multiplexing light wavelengths. CWDM is a technique for transmitting signals via optical fiber which belongs to the family division multiplexing wavelength (WDM) is used in the early 80's to carry video signal (CATV) in fiber- multimode, was standardized by the ITU-T (International Telecommunication Union - Telecommunication sector), on the recommendation of the G.694.2 standard in 2002,based on a grid or wavelength separation of 20 nm (or 2500 GHz) in the range of 1270-1610 nm, thus being able to carry up to 18 wavelengths in a single monomode optical fiber.

Accordingly, they have two important characteristics inherent in systems employing CWDM optical components which allow easier and therefore also cheaper than in DWDM systems:
 

Higher wavelength spacing. Thus, in CWDM lasers can be used with a greater spectral bandwidth and stabilized, ie the central wavelength can be shifted due to manufacturing imperfections or changes in temperature which it is subjected to the laser and even so, being in band. This allows the manufacture of lasers following manufacturing processes less critical than those used in DWDM, and that the lasers do not have sophisticated cooling circuits for correcting deviations of the wavelength due to temperature changes to which the chip is subjected, it which significantly reduces the space occupied by the chip and the power consumption, in addition to the cost of manufacture. Usually used in CWDM distributed feedback lasers or DFB (Distributed Feed-Back) directly modulated channel supporting speeds up to 2.5 Gbps over distances up to 80 km in the case of using G.652 fiber. On the other hand, uses CWDM optical filters and multiplexers and demultiplexers based on thin film technology or TFF (Thin-Film-Filter), where the number of layers of the filter increases as the channel spacing is less. This again implies a greater capacity for integration and reduced cost. These filters CWDM broadband, allowed variations in the nominal wavelength 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 radically decreased despite increasing the separation between them is not possible because in CWDM optical amplifiers are used Erbium doped fiber or EDFA (Erbium Doped Filter Amplifier) as in DWDM for distances above 80 km The EDFA are components used before transmitting or receiving optical fiber to amplify the power of all optical channels simultaneously, without any power level feedback. CWDM systems are used, if necessary by the distances covered or number of nodes in cascade to cross, regeneration, ie each channel undergoes a conversion optical-electrical-optical completely independently of the rest to be amplified. The cost of optoelectronics in CWDM is such that it is simpler and less expensive to regenerate be amplified. Furthermore, since the regenerators made completely amplification functions, reconstruction of the signal shape, and timing signal, compensating the dispersion accumulated all, this does not occur in the optical amplification, unless used with dispersion compensation fiber or DCF (Dispersion Compensation 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 of power per channel, which is further complicated when channels are added and removed or when it is used in DWDM networks ring, especially when systems incorporate optical amplifiers.

Properties of the products

 

It has frequency spacing of 2.500 GHz (20nm), allowing for large spectral width lasers.

 

18 wavelengths defined in the interval 1270 to 1610 nm

 

The current CWDM have their li-mite at 2.5 Gbps.

 

As to the distances covered reach about 80 km.

 

DBF laser used (distributed feedback lasers) without peltier or thermistor.

 

Use of broadband optical filters, multiplexers and demultiplexers based on FFT (thin film technology)

 

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.

 

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:

 

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

>>Anillos locales CWDM que se conectan con anillos metropolitanos DWDM

>>Access rings and passive optical networks.

Advantage

Lower energy consumption.

 

Smaller than the laser CWDM.

 

Solve the problems of bottlenecks.

 

Hardware and operating costs relating to other cheaper technologies like this.

 

Higher band widths.

 

Is simpler relating to network design, implementation and operation.

 

Ease of installation, configuration and maintenance of the network.

 

High degree of flexibility and security in the metro optical networking.

 

It can carry any short-range service as SDH, CATV, ATM, FTTH - PON 10Gibagit, among others.




DWDM (Dense Wavelength Division Multiplexing), which means division multiplexing in dense wave longitude. One s DWDM transmission technique señales through optical fiber using the C band (1550 nm).
That is a multiplexing method very similar to the frequency division multiplexing is used in electromagnetic transmission means. Several carrier signals (optical) are transmitted by a single optical fiber using different wavelengths of laser beam each. Each optical carrier is an optical channel that can be treated independently of other channels that share the medium (fiber optic) and contain different types of traffic. In this way can multiply the effective bandwidth of the optical fiber, so as to provide bidirectional communications. This is a very attractive transmission technique 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 receiver side it. Unlike him CWDM, DWDM is in numbers mayor Get optical channels reducing the chromatic dispersion of each 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 he ello. Currently it pueden get 40, 80 to 160 optical channels separated 100 GHz, 50 GHz to 25 GHz respectively.

Properties of the products

• The large scale manufacture of optical fiber has allowed a reduction in costs and an improvement in transmission characteristics of the fiber.
• Flat gain optical amplifiers for a given range of wavelengths coupled in line with the fiber acting as repeaters eliminating the need for regenerators.
• Solid state integrated filters smaller and can be integrated on the same substrate along with other optical components.
• New photo detectors and laser sources that allow integration producing more compact designs.
• Multiplexers and demultiplexers based optical passive optical diffraction.
• Filters selectable wavelength, which can be employed as optical multiplexers.
• The optical add-drop multiplexers (OADM) technology have allowed implants in DWDM networks of various types.
• The optical components of connection (OXC), which can be implemented with different manufacturing technologies, and have made possible the purely optical switching.

The scope of DWDM is in long distance networks of ultra-wide band, so as metropolitan networks, intercity or very 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 low cost implementation in many types of networks.

 

DWDM technology requires specialized optical devices based on the properties of light and the optical, electrical and mechanical 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 1340 to 1440 nm. WDM systems use wavelengths in the two possible ranges (from 1,300 to 1.34o nm 's1.440 to 1,550 nm). There are special fibers that allow transmission at all wavelengths between 1,530 and 1,565 nm without absorption. 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 the range close to 1,550 nm ITU-T recommendation G.692 defines its 43 channels in the range of 1,530 to 1,565 nm with a spacing of 100 GHz, each channel will carry an OC-192 traffic at 10 Gbps. However, each day coming to market systems with more channels. A 40-channel DWDM system at 10 Gbps per channel provides an aggregate speed 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 spacing of 50 GHz frequency spacing This indicates the proximity of the channels between them. A channel not only uses a single wavelength, each channel has a certain bandwidth around the central wavelength, each band is separated from the next by a guard band area several GH, thus seeks to avoid possible overlap or interference between adjacent channels.

 

These problems are due to drifts in the laser emitters by temperature or time, to have no optical amplifiers ios a gain 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 transmit data at a distance of 80 km without amplification. Placing 8 cascaded optical amplifiers, the distance may increase to 640 km.

Point to point topology.

The topology point-to-point can be implemented with or without OADMs. These networks are characterized by ultra-fast speeds of channels (10 to 40 [Gbps]), high integrity and reliability of the signal, and fast path restoration. In long-haul networks (long distance), the distance between transmitter and receiver can be several hundred kilometers, and the number of amplifiers required between the two points, is typically less than 10. MANs networks, amplifiers are often necessary.

Topologies protection in point-to-point may be provided in a couple of ways. In the first generation equipment, redundancy is a system level. 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.

A scheme of this type of topology shown in Figure 6
Figure 6 Point to point topology

 

Ring topology.

The rings are the most common architectures found in metropolitan areas and sections of a few tens of kilometers. The fiber ring may contain only four channels of wavelengths, typically less nodes and channels. The Bit Rate is in the range of 622 [Mbps] to 10 [Gb] per channel.

With the use of OADMs, which rise and fall of wavelengths in a transparent, meaning that the others are not affected, ring architectures allow nodes to access the network elements such as routers, switches and servers, with the rise and fall of wavelength channels in the optical domain. With the increase in the number of OADMs, the signal is subject to loss and amplifiers may be required.

For protection in this topology using the 1 +1 scheme. It has two lines of connection, information is sent by one of them. If the ring fails, another path switchea ring. An outline of this topology shown in Figure 7

Figure 7 Ring topology

Mesh topology.

The mesh architecture is the future of optical networks. As networks evolve, the architecture of ring and point-to-point would have a place, but the mesh would be the most robust topology. Such development would be enabled by the introduction of the OXCs (Optical Cross-Connects) and configurable switches, which in some cases replace, and other would supplement, a fixed DWDM devices.

From the design standpoint, there is a graceful evolutionary path topology point-to-point and mesh. At the beginning of point-to-point, OADM nodes equipped for flexibility initially, and later in the interconnections, the network can evolve into a mesh without a complete redesign. Additionally, ring and mesh topologies can be connected to point-to-point (see Figure 8).
 Figure 8 Mesh topology

DWDM channels

 

 

Standard DWDM channel distribution:

 

- Official spacings between channels of 100 GHz (0.8 nm, 41canales) or 50 GHz (0.4 nm, 82 channels)

- Band C, conventional wavelength shorter

- L-band, wavelength longer (up to 1610 nm)

- Start using the spacing of 50 GHz (or even 25 and 12.5 GHz ultra-dense WDM) and the band (1490 nm)

 

 

DWDM Transmission

 

DWDM mono:

 

DWDM bidirectional:

 

Tunable lasers EDFA over the entire range, with spacing of 100, 50 and up to 25 GHz

 

Possibility of tunable receivers

 

New optical amplifiers

 

Speeds 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 converter

 

Optical splitter / combiner

 

Wavelength router

 

Time division multiplex optical (OTDM)

 

Optical multiplexer "Add / Drop"

Comparative table of WDM technologies as the application.

Application / parameter	CWDM Access / MAN	DWDM MAN / WAN	DWDM wide range
Channels per fiber	4-16	32-80	80-160
spectrum used	O, E, S, C, L	C, L	C, L, S
Channel spacing	20 nm (2500 GHz)	0.8 nm (100 GHz)	0.4 nm (50 GHz)
Capacity per channel	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 feedback laser)	cooled DFB	cooled DFB
Filter technology	TFF (tecn. thin film)	TFF, AWG, FBG	TFF, AWG, FBG
distance	up to 80 km	hundreds of miles	thousands of miles
cost	low	medium	high
optical amplification	no	EDFA	EDFA, Raman

MAPA MENTAL

Conclusions

 

When necessary the use of WDM in a metropolitan network, the best choice would normally be CWDM. Although it has a number of limitations (capacity, distance ...) for DWDM, in many cases meets the necessary requirements, and is 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 of infrastructure initially, and which thus encourages their creation. Furthermore, although in principle one might think that because more than likely increase the need for bandwidth, CWDM might not cover long-term needs, it is possible to upgrade from CWDM to DWDM.