This document discusses mathematical models for communication channels. It begins by showing a block diagram of a basic digital communication system and defines the key components. It then discusses different types of communication channels and mediums that can be used to transmit signals, including wires, wireless spectra, and optical fibers. The rest of the document discusses several common mathematical models used to represent communication channels, including additive noise channels, linear filter channels, and linear time-variant filter channels. It also discusses parameters that characterize channels and limits on data transmission rates. Finally, it covers optimum receivers for signals corrupted by additive white Gaussian noise.