The difference between a MUX and DEMUX in WDM

MUX and DEMUX at the heart of WDM

The key component in Wavelength Division Multiplexing (WDM), as the name suggests, is the multiplexer itself (often referred to as the “Mux”). The multiplexer combines data signals it receives from the transceiver into one beam of light containing multiple optical wavelengths to be transmitted simultaneously over a single fiber.

At the other end of the fiber connection, sitting at the remote site, is the demultiplexer (Demux) which takes the multiple wavelengths that it receives and separates them back into the individual data channels. It also means that one Mux & Demux is required at each end of the fiber. This combination of Mux & Demux is often itself just referred to as a Mux (or Multiplexer). Today, multiplexers are typically built into many high-capacity transceivers, which allows multiple data streams to be combined and speeds of 40G to 800G (and more) to be achieved. Pro Optix multiplexers are available for CWDM and DWDM installations.

A single strand of fiber can carry multiple wavelengths and streams of data (as we explored in 8 Facts about WDM) and the ever-evolving technology of today is enabling us to build multiplexers that allow for up to 96 channels to be carried simultaneously over a fiber pair.

In a single fiber multiplexer configuration, the data can be carried in two directions in multiple streams on the one fiber. However, the wavelengths chosen to transmit the data in both directions cannot be the same. In the diagram below, you can see that the alternative wavelengths chosen for a single fiber mux. For example, 1470nm wavelength in one direction and 1490nm on the return. Normal WDM transceivers are used for single fiber muxes since the receivers are able to detect a wide spectrum of light (1270-1610 for CWDM) even if the laser itself is only transmitting at a specific wavelength.

CWDM Mux/Demux example – 8 channels

In a dual fiber Mux, the two fiber strands are used to transmit data, the first in one direction, and the second in the opposite direction. One to send and the other to receive. This means that it is possible to use the same wavelength for both transmitting and receiving, as shown below, for example 1470nm in both directions.

The single fiber option is more complex and restricts the number of channels that can be carried, but the cost of fiber is kept down by halving the amount of fiber. Dual Fiber enables more channels to be carried using the same multiplexer, so the multiplexer is no more expensive, but using twice the amount fiber can drastically increase costs.

In conclusion, Multiplexers can be used to save on fiber, enabling multiple channels, such as voice, data, storage and video to be carried over a single fiber simultaneously to optimize and maximize the use of fiber.