Digital data digital transmission communications

Digital data, digital signals: simplest form of digital encoding of digital data
Digital data, analog signal: A modem converts digital data to an analog signal so that it can be transmitted over an analog medium
Analog data, digital signals: Analog data, such as voice and video, are often digitized to be able to use digital transmission facilities
Analog data, analog signals: Analog data are modulated by a carrier frequency to produce an analog signal in a different frequency band, which can be utilized on an analog transmission system
Stallings DCC8e Fig 5.1 emphasizes the process involved in this. For digital signaling, a data source g(t), which may be either digital or analog, is encoded into a digital signal x(t). The basis for analog signaling is a continuous constant-frequency fc signal known as the carrier signal. Data may be transmitted using a carrier signal by modulation, which is the process of encoding source data onto the carrier signal. All modulation techniques involve operation on one or more of the three fundamental frequency domain parameters: amplitude, frequency, and phase. The input signal m(t) may be analog or digital and is called the modulating signal, and the result of modulating the carrier signal is called the modulated signal s(t).