Attachment Unit Interface

The Attachment Unit Interface (AUI) is a physical and logical interface defined in the IEEE 802.3 standard (additionally published in FIPS PUB 107) for 10BASE5 Ethernet and the earlier DIX standard. The physical interface consists of a 15-pin D-subminiature connector that links an Ethernet node's physical signaling to the Medium Attachment Unit (MAU), sometimes referred to as a transceiver. An AUI cable can extend up to 50 metres (160 feet), though often the MAU and data terminal equipment's (DTE) medium access controller (MAC) are directly connected, bypassing the need for a cable. In Ethernet implementations where the DTE and MAU are combined, the AUI is typically omitted. The IEEE 802.3 specification officially defines the AUI as an interconnect between a DTE and the MAU. However, devices like the DEC Digital Ethernet Local Network Interconnect (DELNI) provided hub-like functionality using AUI-compatible connectors. Additionally, under certain conditions, it was possible to directly connect two AUI devices without the need for transceivers using a crossover cable. AUI connectors became increasingly rare in the early 1990s as computers and hubs directly integrated the MAU, especially with the rising adoption of the 10BASE-T standard. This shift led to the decline of 10BASE5 (thicknet) and 10BASE2 (thinnet), which made use of the interface. The electrical AUI connection remained internally within equipment for some time. Transceivers with an AUI connector were also used by 10BASE-FL Multimode fiber. With the introduction of Fast Ethernet, the AUI interface became obsolete and was replaced by the Media Independent Interface (MII). Subsequent Ethernet standards, such as Gigabit Ethernet and 10 Gigabit Ethernet, introduced the GMII and XGMII interfaces, respectively. A 10 Gigabit Ethernet interface, known as XAUI, was developed to extend the operational distance of XGMII and reduce the number of interface signals. A smaller variant called the Apple Attachment Unit...