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Transmission system technologies typically refer to physical layer protocol duties such as modulation, demodulation, line coding, equalization, error control, bit synchronization and multiplexing, but it may also involve higher-layer protocol duties, for example, digitizing an analog signal, and data compression.
Transmission of a digital message, or of a digitized analog signal, is known as data transmission.
Examples of transmission are the sending of signals with limited duration, for example, a block or packet of data, a phone call, or an email.