FDM stnds for frequency division multiplexing and it is used only in case of analog signals because analog signals are continuous in nature and the signal have frequency. TDM-stands for time division multiplexing and it is used only in case of digital signals because digital signals are discrete in nature and are in the form of 0 and 1s. and are time dependent.
Analog Carrier System is a transmission system that uses repeaters that compensate for analog medium impairments, and produce output signals that are linear-scaled versions of input signals. Analog carrier systems can carry speech, data, video and supervisory signals although they are best suited for speech signals.
As a rule a hub does not contain a repeater. Amplifiers would be used in analog communication, not digital communication. Attenuation is a process that occurs all the time in networks; you would place a repeater in an area where the attenuation would cause the signal to degrade to the point where it wouldn't be usable. You would want the repeater to be placed before that point.
repeater
ADC analog digital converter