How are computers physically connected in a network?

Computers are physically connected by a variety of mediums, including "Wi-Fi," "Ethernet," and "fiber optics." The only requirement for these connections to physically exist is that each end of the connection must have a way to interface with the medium being used over a common physical protocol. For Wi-Fi, for example, both sides must be equipped with antennas and broadcasters capable of receiving and sending signals at the proper frequencies. The OSI network model refers to these connections as the "Physical Layer", the lowest level in the model. In all cases, both ends communicating at the Physical Layer level must be using the same medium and physical signalling characteristics.