Wavelength Division Multiplexing: An Introduction to WDM Technology (Part 1)

Fiber scarcity is a common problem for many network operators, including metro carriers, health care campuses, university campuses, school districts, and more. Wavelength division multiplexing (WDM) is a reliable, cost-effective solution for adding many new services or increasing bandwidth over limited fiber. In this article, we’ll explain how the technology works, how to evaluate the options and a couple of common misconceptions about WDM technology.

What Is WDM?

Wavelength division multiplexing is a technology that joins multiple optical signals with different wavelengths onto a common strand of fiber. It can be performed by either a complex active system or relatively simple passive filters. At each end of the link, the passive or active device contains two components: multiplexer (mux) that combines the multiple signals to be sent over the common fiber, and a demultiplexer (demux) that separates the signals on the other end of the link. The WDM solutions described throughout this article support data rates of 1G through 10G, and multiple protocols including Ethernet and Fibre Channel.

How Does It Work?

Let’s say you have a large building on one side of your campus or district that requires eight 10G fiber connections, and you need to connect all of them to a switch in the central office several kilometers away:



You could run 8 pairs of fiber all the way across your campus or district…



…but this would be an extremely expensive proposition, and multiplies your risk of fiber breakage.

A more cost-effective, reliable alternative would be to install a WDM mux/demux at each location, as shown below:



The mux/demux installed at each location multiplexes (muxes) each of the 8 signals together, sends it across one common pair of fibers and demultiplexes (demuxes) the signal at the other end.

WDM Wavelength Options

There are two sets of wavelengths available:

  • Coarse Wave Division Multiplexing (CWDM). The CWDM wavelength set consists of a series of 18 wavelengths spaced 20nm apart, from 1270nm to 1610nm.



  • Dense Wave Division Multiplexing (DWDM). As its name would suggest, DWDM technology packs many more wavelengths in a much narrower band of the spectrum than CWDM, as shown below:



Because the wavelengths are so close together (fractions of a nanometer apart), their spacing is more commonly given in frequencies (usually 50GHz or 100GHz). Each frequency is assigned a “channel” number, as shown in the table below for 100GHz spacing:



Want to learn more about WDM? Check out WDM Part 2, in which we discuss how to begin deciding between CWDM and DWDM, as well as addressing some common misconceptions about WDM technology in general.

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