The field of telecommunication consists of the theory and the practice of communication at a distance, principally electronic communication. Many systems for telecommunication now take the form of large, complex, interconnected data networks with both wired and wireless segments, and the design of such systems is based on a rich theory. Communication theory studies methods for the design of signaling waveforms to transmit information from point to point, as within a telecommunication system. Communication theory is that part of information theory that is concerned with the explicit design of suitable waveforms to convey messages and with the performance of those waveforms when received in the presence of noise and other channel impairments. Digital telecommunication theory, or modem theory, is that part of communication theory in which digital modulation and demodulation techniques play a prominent role in the communication process, either because the information to be transmitted is digital or because the information is temporarily represented in digital form for the purpose of transmission.

Digital communication systems are in widespread use and are now in the process of sweeping away even the time-honored analog communication systems, such as those used in radio, television, and telephony. The main task of communication theory is the design of efficient waveforms for the transmission of information over band-limited or power-limited channels. The most sweeping conclusion of information theory is that all communication is essentially digital.