we need to emphasize that although FDMA and FDM conceptually seem similar,
there are differences between them. FDM, as we saw in chapter 6, is a physical layer
technique that combines the loads from low-bandwidth channels and transmits them by
using a high-bandwidth channel. The channels that are combined are low-pass. The
multiplexer modulates the signals, combines them, and creates a bandpass signal. The
bandwidth of each channel is shifted by the multiplexer.