The document discusses several models of communication that were developed over time:
1) The earliest model was the "Continuous Loop Model" which depicted communication as a machine with feedback loops. It had flaws in assuming communication is always continuous.
2) Shannon and Weaver's 1949 model was more practical, studying phone conversations over 2 years. It depicted information sources, transmitters, receivers, noise sources, and feedback.
3) Berlo's late 1970s/early 1980s model tried to be all-inclusive but was never tested, so it is considered a "theory" rather than a true model. It depicted the roles of senders and receivers.