CWDM: A Low-Cost Alternative for Increased Capacity
In metro and enterprise access networks, coarse wavelength division multiplexing technology promises to extend the life of network equipment by adding inexpensive capacity where it is needed most.
Jenkin A. Richard
Cost savings realized in deploying coarse wavelength division multiplexing
(CWDM) systems have helped this transport architecture gain acceptance as a viable
alternative to dense WDM (DWDM) in short-haul metro-area and enterprise networks.
As CWDM evolves technically, its advantages over DWDM will further secure its place
as a flexible, low-cost alternative for increasing system capacity.
What will cement CWDM’s place? Reduced hardware
costs, low power dissipation and a small device footprint will continue to place
the cost-savings advantage squarely in the CWDM camp. The savings should become
significant enough in the near future that network designers will opt to use CWDM
in new ways, including as a means of delaying more costly upgrades to extend the
life of today’s network equipment.
In a simple CWDM metro-ring architecture, network engineers can increase capacity requirements with a bolt-on
hybrid CWDM solution.
The basic difference between DWDM,
which is built for long distances, and CWDM, which is ideal for short-haul networks,
is the spacing between the wavelengths (channels) transmitted through the same fiber.
DWDM operates over a narrow band of frequencies, known as the C- and L-bands (between
1530 and 1620 nm), requiring wavelengths to be tightly crammed together at anywhere
from 1.6 to 0.4 nm apart. Using erbium-doped fiber amplifiers to boost the signal
over long distances, DWDM can transmit from 32 to 128 channels. CWDM, in contrast,
operates at a much wider range (1270- to 1610-nm center wavelengths), separating
them at 20-nm intervals, which can transmit a maximum of 18 channels over a distance
of 60 km.
Today’s CWDM systems are about
40 percent less expensive than most DWDM solutions (based on cost per channel at
OC-48 speeds), but even greater savings will be possible as the technology evolves
from its DWDM roots. The potential for further cost savings is fueled by work already
under way, including developments in subcomponent design and packaging, and in enhanced
But the real value to carriers may
be the capability to prolong the life of network equipment currently in use by providing
scalable, plug-and-play upgrades.
A key advantage of CWDM over DWDM is that it does
not require a thermoelectric cooler to compensate for thermal drift as distributed
feedback (DFB) lasers heat up and cool down. This requires the use of less expensive
wideband optical filters. With channel spacing at 20 nm, CWDM spacing is wide enough
to permit the laser wavelength to change over an operating temperature of 70 °C.
Creating high-yield devices that have
extended temperature capabilities while maintaining good operating specifications
is key to handling broader temperature extremes. One area where significant research
is under way is laser sources. Laser suppliers are working on providing the full
spectrum of sources required with acceptable dispersion and power. Maintaining tolerable
thermal drift (0.1 nm/°C) across the spectrum is another area for improvement.
While temperature requirements vary by application, a target range of 125 °C
is not unusual, especially for outdoor plant or cable TV applications.
Another important packaging issue is backscatter
tolerance, which affects the operating distance. Many vendors have developed clever
low-isolation techniques (at about 8 dB) based on uncollimated optics that are less
expensive to implement. Examples include backscatter isolation using fiber stubs
or subwavelength structures. The ideal solution, though, would require lasers that
are more tolerant to backscatter, which, in turn, could eliminate the need for isolators
in most applications. Development efforts are ongoing.
Yet another research area involves
development of sources with better divergence symmetry to increase coupling efficiency.
The existing problem is that asymmetric divergence forms an elliptical image that
does not efficiently couple into a circular mode field. Though coupling efficiency
is not a problem unique to CWDM, it is especially important for technology that
does not rely on amplifiers (which are being used in some channels).
A problem with today’s laser sources is the lack of symmetric divergence, which results in
an elliptical image that doesn’t couple efficiently into a circular mode field.
Finally, low-power requirements highlight
key packaging and usability differences between CWDM and DWDM. The latter consumes
significantly greater power and dissipates more heat because of the Peltier coolers
designed into its optical subassemblies. The coolers can consume about 4 W per
wavelength, whereas a CWDM laser transmitter uses only about 0.5 W. A related issue
is avalanche photodiode usability. Today, PIN detectors offer a convenient, lower-cost
alternative to avalanche photodiodes, but they do not offer a sufficient link budget
for many applications.
As a result, researchers continue to
work on next-generation avalanche photodiodes, which promise lower voltages, better
sensitivity and, in some cases, better response over temperature. Once these devices
can be easily integrated into transceivers, they will further reduce CWDM costs.
Indeed, hot-pluggable transceivers
hold the greatest potential for cost savings. The CWDM industry leveraged the cost/volume
advantages of the hot-pluggable transceiver early on, which helped standardize and
reduce the platform costs much more quickly than DWDM.
Currently, DWDM transceivers can be
up to five times more expensive, largely because of the packaging and testing of
a DWDM laser (at about ±0.1 nm) relative to a CWDM laser (±2 to 3 nm).
The tighter tolerances translate into smaller yields and, thus, higher costs.
Additionally, to minimize the cost
of equipment designed for a staged deployment, network designers today want hot-pluggable
transceivers. Because of the high thermal dissipation of DWDM systems, only one
vendor has been successful at bringing a hot-pluggable DWDM transceiver to market.
Fundamental problems with such plug-and-play devices, however, include poor rack
system densities and the inventory expense of supporting high-channel counts. Furthermore,
DWDM is a low-volume/high-mix technology that effectively dismisses the fundamental
advantage of transceivers, namely volume-driven manufacturing and standardization.
Plug and play
CWDM, by comparison, can provide standardized
small-form-factor transceiver modules with lower power requirements — i.e.,
less thermal dissipation — which increases rack density and offers an important
advantage in today’s space-constrained central offices.
Standardization is important for ensuring
that multiple vendors are available for hot-pluggable CWDM transceivers. The ITU
has already approved a grid that has been proposed by the Full Spectrum CWDM Alliance.
Development work is ongoing on application-specific standards.
Hybrid CWDM components illustrate its
flexibility in architectural choices over DWDM today. Because of lower device costs,
customers can use CWDM as an easy plug-and-play solution to add incremental channel
capacity when it is needed. Likewise, customers can convert or upgrade one or more
CWDM bands to higher-capacity DWDM as traffic increases dictate.
Using a hybrid CWDM bolt-on solution, end users can carve out a wide
express band at the 1530, 1550 and 1570 bands. Using a bandpass filter, they can
inexpensively tack on five channels to increase system capacity.
In low-channel-count networks in metro-area
or enterprise networks, customers can use such hybrid technology to “bolt
on” expansion channels, which can increase network capacity fivefold. For
example, if an existing link uses a standard transceiver, the end user can expect
that the 1530-, 1550- and 1570-nm bands will be employed (essentially three of the
eight possible CWDM upgrade channels). By adding a hybrid bolt-on, it is possible
to carve out a wide express band and then, using a bandpass filter, to efficiently
tack on five channels to the system. This can be done without displacing the current
Similarly, the strategy also can help
upgrade long-wave Ethernet networks. In SONET or Ethernet 1300-nm networks, network
designers can express all traffic on legacy signals and still create six to 10 expansion
channels. Alternatively, CWDM can provide an elegant solution for Ethernet over
SONET that uses a CWDM multiplexer to deliver both services transparently.
There is an easy upgrade path to DWDM
as traffic grows. To that end, CWDM transceivers can be easily replaced by DWDM
equipment to expand only those nodes that require additional channels, while leaving
the rest of the system unchanged.
Amplified DWDM devices are ideal for addressing
the needs of long-haul 10 Gigabit Ethernet market requirements, but will always
come at a higher price. Many networks simply do not require the horsepower and capacity
that DWDM provides, making it an overkill and costly solution in these environments.
Comparatively, CWDM technology provides
a mix of low cost and low capacity in short-distance networks. As the technology
evolves, it will continue to outpace DWDM in these key application areas. This,
in turn, will boost the usefulness of hybrid solutions to extend the life of network
equipment by adding short-term capacity while providing an easy upgrade path to
DWDM when capacity demands rise.
Ultimately, CWDM will take its place
as a viable and important stand-alone transport architecture that complements DWDM
and will play a pivotal role in the communications networks of the future.
Meet the author
Jenkin A. Richard is chief technology officer for Gigabit Optics Corp. in Sunnyvale, Calif.
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