DWDM vs CWDM

Definition of wavelength division multiplexingEarly fiber optic transmission systems put information onto strands of glassthrough simple pulses of light. A light was flashed on and off to represent the“ones” and “zeros” of digital information. The actual light could be of almostany wavelength (also known as color or frequency) from roughly 670nm to1550nm.

During the 1980s, fiber optic data communications modems used low-costLEDs to put near-infrared pulses onto low-cost fiber. As the need forinformation increased, the need for bandwidth also increased. Early SONETsystems used 1310nm lasers to deliver 155 Mb/s data streams over very longdistances. But this capacity was quickly exhausted. Advances in optoelectroniccomponents allowed design of systems that simultaneously transmittedmultiple wavelengths of light over a single fiber. Multiple high-bit rate datastreams of 2.5 Gb/s, 10 Gb/s and, more recently, 40 Gb/s and 100Gb/s couldbe multiplexed through divisions of several wavelengths. And so was bornWavelength Division Multiplexing (WDM).

CWDM - Coarse Wavelength Division Multiplexing. WDM systems with fewerthan eight active wavelengths per fiber.DWDM - Dense Wavelength Division Multiplexing. WDM systems with morethan eight active wavelengths per fiber.

Types of WDM

Currently, there are two types of WDM in existence today: Coarse WDM(CWDM) and Dense WDM (DWDM).Backwards as it may seem, DWDM came well before CWDM, which appearedonly after a booming telecommunications market drove prices to affordablelows. Whereas CWDM breaks the spectrum into big chunks, DWDM dices itfinely. DWDM fits 40-plus channels into the same frequency range used fortwo CWDM channels.

CWDM is defined by wavelengths. DWDM is defined in terms of frequencies.DWDM’s tighter wavelength spacing fit more channels onto a single fiber, butcost more to implement and operate.

Distinctive CWDM differences

CWDM can—in principle—match the basic capabilities of DWDM but at lowercapacity and lower cost. CWDM enables carriers to respond flexibly to diversecustomer needs in metropolitan regions where fiber may be at a premium.However, it’s not really in competition with DWDM as both fulfill distinct rolesthat largely depend upon carrier-specific circumstances and requirementsanyway. The point and purpose of CWDM is short-range communications. Ituses wide-range frequencies and spreads wavelengths far apart from eachother. Standardized channel spacing permits room for wavelength drift aslasers heat up and cool down during operation. By design, CWDM equipmentis compact and cost-effective as compared to DWDM designs.

Distinctive DWDM differences

DWDM is designed for long-haul transmission where wavelengths are packedtightly together. Vendors have found various techniques for cramming 32, 64,or 128 wavelengths into a fiber. When boosted by Erbium Doped-FiberAmplifiers (EDFAs)—a sort of performance enhancer for high-speedcommunications—these systems can work over thousands of kilometers.Densely packed channels aren’t without their limitations. First, high-precisionfilters are required to peel away one specific wavelength without interferingwith neighboring wavelengths. Those don’t come cheap. Second, precisionlasers must keep channels exactly on target. That nearly always means suchlasers must operate at a constant temperature. High-precision, high-stabilitylasers are expensive, as are related cooling systems.

CWDM and DWDM scenarios

CWDM doesn’t span long distances because its light signal isn’t amplified,which keeps costs down but also limits maximum propagation distances.Vendors may cite working ranges of 50 to 80 kilometers, with 160 kilometerdistances achievable through signal amplifiers. CWDM supports fewerchannels and that may be adequate for metro carriers who prefer to start smalland expand later as demand increases.

Non-amplified signaling systems keep entry costs down and can still retainhigh loss tolerance. Whenever a non-amplified signal is used, there is atrade-off between capacity and distance. Either you make long networks withfewer nodes or smaller networks with lots of nodes.

Comparison between CWDM and DWDM will be illustrated from the followingaspects:

CWDM DWDMDefined by wavelengths Defined by frequenciesShort-range communications Long-haul transmissionsUses wide-range frequencies Narrow frequenciesWavelengths spread far apart Tightly packed wavelengths

Wavelength drift is possible Precision lasers required to keepchannels on target

Breaks the spectrum into big chunks Dices the spectrum into small piecesLight signal isn’t amplified Signal amplification maybe used

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