Introduction of CWDM and DWDM Transmission systems, More Information: www.fiberstore.com

1. INTRODUCTION
Wavelength Division Multiplexing (WDM) is a technology that multiplexes several signals over a single optical fiber by
optical carriers of different wavelengths, using light from a laser or a LED. This can take greater advantage of the
enormous bandwidth that optical fiber has.
The first WDM system combining two carrier signals first appeared around 1985. In the early twenty-first, the WDM
technology can combine up to 160 signals with an effective bandwidth of about 10 Gbps. And carriers are testing the
40 Gbps or even 100 Gbps, despite the theoretical capacity of a single optical fiber in 1600 is estimated Gbps. So it
is 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 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 MAN a few years
ago raised a great interest in the WDM technology, 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.
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 (ROI) 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 Coarse WDM (CWDM). CWDM technology benefits from lower cost of optical
components associated with a lower-tech. Although limited in capacity and distance, CWDM is well suited to the
needs 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 which
belongs to the WDM family. The CWDM technology was standardized by the ITU-T based on a grid or wavelength
separation of 20 nm in the range of 1270-1610 nm, thus being able to carry up to 18 wavelengths in a single mode
fiber. Accordingly, there are two important characteristics inherent in systems employing CWDM optical components
which 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. 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 Thin Film Filter
(TFF), 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 Erbium-Doped Fiber Amplifier (EDFA) 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

2. 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 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 nm
3. 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, to
introduce 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 consumption
2. Smaller than the laser CWDM
3. Solve the problems of bottlenecks.
4. Hardware and operating costs relating to other cheaper technologies like this.
5. Higher band widths
6. 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 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 DWDM PRODUCTS
1. The large scale manufacture of optical fiber has allowed a reduction in costs and an improvement in transmission
characteristics of the fiber.
2. Flat gain optical amplifiers for a given range of wavelengths coupled in line with the fiber acting as repeaters
eliminating the need for regenerators.
3. Solid state integrated filters smaller and can be integrated on the same substrate along with other optical
components.
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 manufacturing
technologies, 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 or
very high speed.
As the deployment of DWDM increases its cost is decreasing gradually, mainly due to the large number of optical

3. 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.
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.
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.
DWDM channels

4. 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
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.
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