Frequency Division Multiplexing (FDM) is a circuit switching protocol by which network connectivity is allocated across a frequency band. Each network connection is allocated a specific frequency band in which data can be transferred.
FDM is used among technologies such as FM radio; a total frequency spectrum of 88Mhz-108MHz is allocated for network communications (R). The FM spectrum is divided into channels, each 200Hz (.2Mhz) wide. The “bandwidth” of FM radio can be said to be 200Hz (R).
In the image below, a 400Mhz wide spectrum has been sub-divided into 4 x 100MHz bands each of which would have 400/100 transmission capacity of the total network but could transmit freely without delay from other bands.
In FDM, each networked device is permitted to transmit freely on their assigned band but cannot use the bandwidth allocated for another network system. As such, the entirety of a network’s transmission capacity may be underutilized during silent periods when end systems are not transmitting data.
In such cases, bandwidth is not dynamically expanded to allow higher throughput for active devices and, effectively, sits idle until used. In the context of FM, this is called dead air. FDM is similar to time-division multiplexing in this regard.