Coarse and Dense Wavelength Division Multiplexing
Wavelength Division Multiplexing (WDM) allows different data streams to be sent simultaneously over a single optical fiber network. In other articles, we’ve used the analogy of an expressway to outline how WDM works to create a single virtual fiber network. Using it to combine multiple services on one dark fiber can maximize the fiber and help organizations meet growing demands without laying or leasing more fiber until it’s absolutely necessary. There are two main types of WDM technologies used today: Coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM).
CWDM allows up to 18 channels to be transported over a single dark fiber, while DWDM supports more than 200 channels. Both technologies are independent of protocol, meaning that any mix of data, storage, voice or video can be used on the different wavelength channels. In fiber terms, the main difference between CWDM and DWDM technologies lies in how the transmission channels are spaced along the electromagnetic spectrum.
WDM technology uses infrared light, which lies beyond the spectrum of visible light. It can use wavelengths between 1260nm and 1670nm. Most fibers are optimized for the two regions 1310nm and 1550nm, which allow for effective “windows” for optical networking.
Coarse Wavelength Division Multiplexing
CWDM is a technology that allows up to 18 channels to be connected over a dark fiber pair. Two wavelength regions are most commonly associated with CWDM, 1310nm and 1550nm. The 1550nm region is more popular because it has a lower loss in the fiber (meaning the signal can travel farther).
If we use our road analogy, it’s like painting 18 lanes on the road, with nine in the 1310 region of the fiber (1270nm to 1450nm) and nine in the 1550 region (1470nm to 1610nm). To achieve this, the wavelengths of each channel are 20nm apart.
CWDM is a convenient and low-cost solution for distances up to 70km. But between 40km and its maximum distance of 70km CWDM tends to be limited to 8 channels due to a phenomena called the water peak of the fiber (more about this further down). CWDM signals cannot be amplified, making the 70km estimate an absolute maximum.