Physical layer protocol
In computer and telecommunications networking, a physical layer protocol defines either the method by which individual devices (e.g., computers) connect to the transmission medium, characteristics of the medium itself, or both. The usage here is broader than that of the Open Systems Interconnection Reference Model, which dealt purely with the first aspect. There are a number of situations where there is an electrical or optical conversion between the signals seen on the computer interface, and those that are used in the larger transmission system.
Some differences between the computer connections specification deal with installation convenience and interface cost, while others can be safety-related. Yet others deal with the transmission system containing multiple user paths, with the aggregated information capacity far greater than available for any one user.
Discussed in the Ethernet article is an early, if not extremely effective, simplification. With the original specification, the physical medium, called 10BASE5, was a relatively stiff cable, which had to run in a fairly straight line, making turns with wide and gentle bends. At first, this was not a huge problem, in that personal computers with local area network interfaces were not common, and Ethernet primarily connected computers in a computer room. Still, there were some desktop devices that did need access to the LAN, so a technique was developed in which a transceiver physically attached to the main cable, and to a less electrically critical computer interface, which ran on a more flexible cable, called the attachment unit interface (AUI). The AUI interface also was more electrically complex than the Ethernet, but in a manner more compatible with typical computer electronics.
Without enumerating them, the transceiver and AUI technique had significant disadvantages, although it remained operationally useful, in special cases, for some time. The next variation of what was then the IEEE 802.3 LAN specification reduced the length of the main cable, in exchange for using a much more flexible cable that could be brought to desktops, called 10BASE2.