Appendix C. External Transceivers
In this appendix, we describe two external transceivers and transceiver interfaces that were once widely used but are no longer sold as new equipment. These are the AUI cable and external MAU for 10 Mb/s systems, and the MII cable and external PHY for 100 Mb/s systems. The equipment described here is obsolete and no longer used in new installations. We will use the present tense to describe the operation of these components, but keep in mind that this information is included solely for the sake of completeness.
The attachment unit interface (AUI) cable, also called a transceiver cable, was developed as part of the original 10 Mb/s Ethernet system. New media systems based on twisted-pair and fiber optic link segments were later invented for the 10 Mb/s system, and in the early 1990s the 10BASE-T twisted-pair system became the most widely implemented networking system.
The AUI makes it possible to connect an Ethernet interface to any one of the several 10 Mb/s media systems, while isolating the interface from any details of the specific media system in use. The development of the AUI as a medium-independent attachment was actually a side effect of the design of the original thick coaxial cabling system, which requires the use of external transceivers connected directly to the coax cable.
The need to provide a connection between the Ethernet interface electronics in the station and the external transceiver located on the coaxial cable was what led to ...