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WDM,CWDM and DWDM in Wide, Metro, and Local Area Networks



WDM, CWDM & DWDM in Wide, Metro and Local Area Networks

Written by Jeff Dominique



No matter who you are or what industry you are in, if you are responsible for a network, the odds are that you are looking for ways to expand your bandwidth capabilities not reduce them.


There are many options to consider in your quest and of course as a savvy professional, you have already learned that there is definitely a right way and a wrong way to go about it.  One thing is for certain; if you have already installed fiber optic cable in your network, bandwidth expansion is much easier to accomplish.


For years now, Wide Area Network (WAN) owners, also referred to as carriers or service providers, have enjoyed the benefits of a highly advanced technology called DWDM or Dense Wave Division Multiplexing.  The basic concept of DWDM relies on the fact that a single fiber has nearly infinite capacity and this can be exploited by using different wavelengths for each fiber channel being transmitted.


The problem with DWDM technology is that it is cost prohibited for anyone except carriers who typically have several hundred-thousand to millions of customers paying the bill for the equipment.


Today however, with the advancements in passive restricted channel multiplexers, as well as the advent of multiple SFP (Small Form-factor Pluggable) wavelengths available at relatively low costs, just about any network owner with a quality built fiber optic network can play the game. The emphasis is on the words "quality built” fiber optic network, because as you stack multiple wavelengths on the same fiber, the combined signal becomes increasingly sensitive proportionately.  The more wavelengths you add, the more sensitive the transmission becomes to network imperfections.  This is why we rarely try to multiplex on multimode fiber, as there are just too many inherent imperfections and restrictions in the fiber itself.


However, if you had originally selected singlemode fiber for your network, all you may need is a bit of a connection or splice upgrade in order to run the simplest form of multiplexing WDM (or) the next level CWDM (Course Wave Division Multiplexing) in your network.


Some forms of WDM are already experienced through active dual wavelength SFPs, but it is important to note that this is a more expensive form of multiplexing than the passive multiplexer.  With passive multiplexing, it is relatively easy to stack 4 to 8 independently produced SFP wavelengths onto a single fiber or fiber pair, creating a more cost-effective CDWM circuit.


When you are ready to increase the capacity of your network by 4 to 8 fold, you will have a few up front tasks to accomplish.  First, you will need to select the CWDM modules for deployment (let’s say 4 channel) and in doing so your base wavelengths will be identified.  This is because the CWDM module will be optimized for a specific range of wavelengths in order to ensure proper channel separation is maintained in accordance with ITU (International Telecom Union) mandates. Your next task will be to re-certify your network to the more specific Standards requirement for accommodating multiple wavelength applications.  This is often done by a network consultant or Independent Verification Consultant that has the appropriate instruments with programmable acceptance criteria and other test parameters such as PMD (Polarization Mode Dispersion) and CD (Chromatic Dispersion) measurements.


Once the front end work has been accomplished, the actual process of converting to an acceptable multiplexed circuit is relatively easy.  Where normally, four independent circuits would launch from independent SFPs and connect to four separate fiber pairs, the new scheme would be that the four independent circuits would plug into the individual channel inputs of the CWDM module with a single fiber or fiber pair output taking up the slot of the four fibers previously used.


And there you have it, increasing the capacity of your existing bandwidth pipeline without pulling any new fiber.


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