Medium Attachment Unit
transceiver in an Ethernet network
A Medium Attachment Unit (MAU) is a transceiver which converts signals on an Ethernet cable to and from Attachment Unit Interface (AUI) signals.
On original 10BASE5 (thicknet) Ethernet equipment, the MAU was typically clamped to the Ethernet wire via a vampire tap and connected by a multi-wire cable to the computer via a DA-15 port, which was also present on the network interface controller (NIC). This AUI cable could be up to 50 metres (160 ft) long, but was typically much shorter. With later standards, thicknet vampire taps and N connectors gave way to BNC connectors (for thinnet coax cables) and 8P8C connectors (for twisted-pair cables). MAUs for these were still connected to NICs via AUI cables, but soon the MAU ceased to be a separate adapter and was generally integrated into the NIC. Eventually, the entire Ethernet controller was often integrated into a single integrated circuit (chip) to reduce cost.
In most modern switched or hubbed Ethernet-over-twisted-pair systems, neither the MAU nor the AUI interfaces exist (apart, perhaps as notional entities for the purposes of thinking about layering the interface), and the Category 5 (CAT5) (or better) cable connects directly into a twisted-pair Ethernet port of the equipment. For backward compatibility with equipment that still had external AUI interfaces only, adapter-type MAUs with 10BASE2 or 10BASE-T connectors long remained available after the obsolescence of original vampire tap MAUs, but even adapter-type MAUs have become very rare as of the 2020s.
Similar equipment
The original Ethernet's successor standard, Fast Ethernet, introduced division into medium access control (MAC) and physical (PHY) layers connected with media-independent interface (MII). Some early Fast Ethernet hardware had physical external MII connectors, functionally similar to the AUI connector, but generally separate adaptors became obsolete. However, the tradition of using a separate low-level I/O device in networking has continued in fast optical fiber network interfaces, where the GBIC, XENPAK, XFP, and SFP transceiver modules play a similar role.
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