This type of multiplexer is used in optical communication networks when there are many input channels of light signals, and one of them needs to be selected to add to a fiber network. Because multiple signals and frequencies can be transmitted along the same fiber line, often this multiplexer is used to add a signal or drop a signal from the existing overall transmission. This is why it is referred to as an add/drop device.This device is different than the traditional multiplexer. Normally, a mux will take many signals and completely ignore any of them except the channel selected. An optical one is more advanced. Normally the entire multiplexing system consists of a demux and a mux with some intervening circuitry. The demux will separate in the incoming signal into individual components. Then these separate channels will be routed to the mux, where it will combine, add, or drop them and output a single unified signal to continue along the fiber network. So this means all or most of the signals entering an optical multiplexer have a part to play in the final output, not just one of the individual signals. Read more Read less