Wave division multiplexing is a way for service providers to increase the capacity of fiber optic networks without adding new fiber. This is done by combining optical signals of different wavelengths on different channels, transmitting them together on one strand of fiber, and separating them again when they arrive at their destination.
The International Telecommunications Union defines two different variations of WDM: coarse wave division multiplexing (CWDM) and dense wave division multiplexing (DWDM). DWDM can support more channels and more bandwidth. According to Wes Oxlee, director of wireless connectivity solutions at CommScope, with a passive WDM device one strand of fiber can transmit up to 18 channels using CWDM and 40 channels using DWDM. The single channel bit rate can be as high as 160 Gbps with DWDM, but with CDWM it will not get much higher than 3 Gbps, according to the RP Phototonics Encyclopedia.
Active or passive
Coarse wave division multiplexing devices and dense wave division multiplexing devices can be either active or passive. Active components require power; passive components do not.
“With active WDM you can continue to use gray small form factor pluggables in your radios,” Oxlee said. “You have all the photonics management in an active system and it does also allow you to have that monitoring of the circuit as well, but active WDM does impart some latency into the CPRI signal, so you can look at doing it by a passive WDM system as well. They don’t require any power – they are passive units. If you look at the small cell densification sites, perhaps using the active WDM is an overkill when you can do it via passive. But there are also some downsides to using passive. You do need to use colored SFPs to be able to break out the individual waveforms.”
Oxlee also sees potential for passive WDM equipment to expand the capacity of existing fiber networks.
“Why don’t we re-use the existing passive optical networks that are out there?” he said. “A large number of carriers have existing fiber-to-the-home assets that are running across many, many cities around the world. Typically they are only using two or three channels at the moment to provide that fiber-to-the-home service. By utilizing WDMs and converging the wireless network onto the wireline network, it frees up that existing infrastructure to be able to service these small cells’ requirements, and that gives them access to that very valuable fiber asset in high-density areas and it opens up potentially additional revenue streams for the carriers as well — they can sell that spare capacity on their network to other mobile network operators.”
