AuI

AUI stands for Attachment Unit Interface, a historical standard in Ethernet networks that defines the interface between the Media Access Control (MAC) sublayer and the physical layer transceiver. Implemented as part of the IEEE 802.3 family in the early 1980s, the AUI architecture was designed to separate the MAC from the actual transmission medium, enabling different media to be used with the same MAC logic.

In this arrangement, the MAC connects to an external Attachment Unit (AU) through the AUI. The AU, in turn, connects to the chosen transmission medium via a Media Attachment Unit (MAU). The MAU implements the medium-specific transceiver functions such as encoding, timing, and the electrical or optical interface, allowing a single MAC to work with coaxial, twisted-pair, or fiber media by swapping MAUs.

The physical interface uses a 15-pin D-sub connector (DB-15) on both ends of an AUI cable, carrying the data signals and relevant control signals between the MAC and the AU. This setup supported multiple Ethernet media, including thick Ethernet (10BASE5) and thin Ethernet (10BASE2), through appropriate MAUs and adapters, as well as fiber-based options like 10BASE-F.

With the rise of 10BASE-T and RJ-45 connectors in the late 1980s and 1990s, AUI ports became less common in new equipment. Modern Ethernet largely replaced AUI with integrated transceivers and media-specific interfaces, though some legacy devices and specialized equipment still include an AUI/MAU path for compatibility or historical reasons.

In summary, the AUI was a key component of early Ethernet architecture, enabling media independence by decoupling the MAC from the physical medium via the AU and MAU arrangement.