Datasheet

28
Coarse Wavelength Division Multiplexing
Overview
Why Coarse Wavelength Division Multiplexing?
Coarse Wavelength Division Multiplexing (CWDM)* is a technology
that increases the data carrying capacity or bandwidth of single-
mode fiber by transporting multiple wavelengths (lambdas) in parallel
over it, each wavelength carrying a discrete user data channel. The
technique employs completely passive devices that (on one end)
optically combine and launch multiple wavelength data channels,
and (on the other end) recovers, partitions and distributes them to
their respective user destinations. Commonly, eight discrete
wavelengths, one lambda (λ) per channel, are used to access existing
single-mode links at this time.
CWDM Technology increases user access to existing installed fiber,
offers greater system redundancy, and reduces network congestion
with a minimum infrastructure investment. With CWDM
connections, multiple network users, subnets, or VPNs can access
and traverse single-mode links that were formerly limited to single
user, Server and Switch backbone-type connections.
There are eighteen unique, ITU-defined CWDM wavelengths
available for Gigabit Ethernet and other high-speed protocols.
Currently, there are eight ITU CWDM wavelengths available for
transmission of Gigabit Ethernet, Fibre Channel and Fast Ethernet
over standard SMF-28 type single-mode fiber, with an additional
four (O-Band) wavelengths also useable. The remaining wavelengths
are subject to excessive optical attenuation over standard single-
mode fiber and require the use of special low water-peak (low
hydroxyl ion) fiber.
The eighteen wavelengths of the CWDM spectrum are optically
separated by twenty nanometers (ηm) spacing. This spacing ensures
that correct channel separation is maintained between connected
devices even though the transmitter elements are un-cooled and
the ambient transmitter temperatures vary across the normal 70ºC
network range. In this environment, current technology makes it
possible for wavelengths to not drift more than 0.1 ηm/degree C,
and with 20 ηm channel spacing, maintains sufficient margin for
proper channel separation.
Advantages of CWDM vs. DWDM (Dense Wavelength
Division Multiplexing) technology
CWDM has the advantages of less complexity, lower installed and
total lifetime cost of ownership, and relatively simple installation; it
has lower power and cooling requirements, and is optimized for
mid-range (40 to 80+ kilometers) metro and campus networks.
CWDM is limited to a maximum of twelve channels over standard
SMF-28 type single-mode fiber, and up to eighteen channels if
using low attenuation (low water-peak) single-mode fiber.
Nevertheless, an eight, twelve, or eighteen fold increase in Fiber
bandwidth represents a huge increase in available transmission
capacity over traditional approaches, with minimal infrastructure
investment.
DWDM has the potential advantage of many more (120 or more
tightly-spaced channels) over single-mode fiber, has potentially much
greater transmission range, operates over a narrow band (C and L-
bands) of light frequencies with outputs ranging between 1530
and 1620 ηm, can be used in the metro space, and can be
configured for long-haul applications. There is also considerable
field experience in deploying DWDM systems. However, these
advantages come with the serious penalties of greater system
complexity and much higher initial and lifetime system and
manpower costs. These costs are due to the following requirements:
The installation on each transmitter module of Peltier Effect Thermal
Electric Coolers (TECs) and associated control circuitry needed for
precise wavelength management, more powerful laser transceivers
with sensitive (expensive) avalanche photo-diode (APD) receivers,
greater system power and cooling requirements, erbium doped
fiber amplifiers (EDFAs) to boost long range transmission power,
more complex passive optics (that have to contend with very narrow
wavelength spacing, pass-band power-leveling, four-wave mixing,
polarization mode dispersion etc.), and a much larger spare parts
inventory.
DWDM potentially offers much greater bandwidth but the initial
start-up and life-cycle costs are daunting and significantly greater
than those required for a full-featured, high-bandwidth CWDM
solution.
NOTES
*Usage: Throughout this document the acronym CWDM is used in two contexts: It
encompasses Coarse Wavelength Division Multiplexing technology as a whole. Or
more narrowly, it can refer to Coarse Wavelength Division Multiplexer/De-Multiplexer
(mux/demux) hardware that optically combines transmitted wavelengths into a
multiplexed data stream or partitions them, when received, into individual channels.