Answer:
n Digital data, digital signal
n Equipment less complex and expensive than digital-to-analog modulation equipment
n Analog data, digital signal
n Permits use of modern digital transmission and switching equipment
n Digital data, analog signal
n Some transmission media will only propagate analog signals
n E.g., optical fiber and unguided media
n Analog data, analog signal
n Analog data in electrical form can be transmitted easily and cheaply