WDM is short for wavelength division multiplexing. What it does is split the light from an optical fiber into various discrete wavelengths (colors). Each wavelength (color) is a separate channel running at a data rate of 2.5 Gbit/s, 10 Gbit/s, 40 Gbit/s or even 100 Gbit/s (still under development). So if the light in the fiber is split into 16 wavelengths (colors or channels), and each wavelength runs at a data rate of 40 Gbit/s, we get a total speed of 40 Gbit/s x 16 = 640 Gbit/s This is especially true of long and ultra long distance fiber optic communication links.
Furthermore, fibers carrying 64 and more channels (wavelengths) are already available on the market now. Which means we can run 2560 Gbit/s data rate on a single fiber. How about 48 fibers in a single fiber optic cable? That gives us an incredible link of 2560 Gbit/s x 48 = 122 880 Gbit/s. Of course, these types of high-speed, high-fiber-count links are typically only deployed for Internet backbones.
From the aforementioned samples, you can see the shocking truth about WDM. Dramatically increases the capacity of a fiber optic link while minimizing fiber optic cable equipment and cost.
What is DWDM?
DWDM stands for dense wavelength division multiplexing. Here “dense” means that the wavelength channels are very narrow and close to each other. For 100 GHz dense WDM, the gap between adjacent channels is only 100 GHz (or 0.8 nm). For example, the adjacent channels could be 1530.33nm, 1531.12nm, and 1531.90nm.
DWDM is widely used for the 1550nm band to take advantage of the capabilities of EDFAs (Erbium Doped Fiber Amplifiers). EDFAs are commonly used for 1525nm ~ 1565nm (C band) and 1570nm ~ 1610nm (L band).
Why is DWM so important?
The exploitation of DWDM has caused an explosion in transmission capacity. The amount of information that can be sent over the fiber cables that span the globe has increased so much that there is now excess capacity available.
In practice, more can be squeezed out of DWD systems by stretching the upper or lower limits of the available transmission window or by spacing wavelengths closer together, typically at 50 GHz or even 25 GHz. By doing this, providers can double or triple the number of channels. Currently, each optical channel can be routinely used for transmitting light pulses at 10 Gbit/s, or even higher data rates with 100 GHz spacing. With the help of WDM, a pair of fibers can provide a data capacity of several hundred gigabits per second.
WDM technology does not require any upgrade or replacement of the fiber infrastructure that has been installed. Therefore, we can upgrade links from one capacity level to the next simply by reconfiguring or upgrading terminal equipment and repeaters.
WDM technologies provide the raw transmission capacity. This needs to be structured in some way so that it can carry useful traffic and get routed where it needs to go. This is where the next layer of the network protocol comes into play. SDH and SONET (These are equivalent. SONET is used in the United States while SDH is used in the rest of the world.) We will talk about SDH and SONET in other tutorials.