MAC Transceiver

The Medium Access Control (MAC) transceiver (which is sometimes known by the older term "Medium Access Unit" (MAU)) contains the electronics to interface a network interface card to an Ethernet cable. The unit contains a line driver (the transmitter), a line receiver (the receiver), a carrier detect circuit (used to sense whether the cable is in use) and control electronics to ensure correct operation of the device.

A 10 Mbps Ethernet Transceiver is connected to the Attachment Unit Interface (AUI) port of a network interface card, which is a 15-pin D-type connector. The port is connected with the transceiver using an AUI drop cable. This cable consists of 5 pairs of twisted copper wires each covered by an individual screen. The drop-cable has a AUI connector at each end and may be up to 50 meters in length (25m in some specific cases). (The Belden part number for this type of cable is 9901.)

The IEEE added a number of additional control signals to the AUI cable, which although are often present, are now seldom used.

MAC Transceiver and Attachment Unit Interface

The transceiver control contains the necessary electronic circuits to :

Similar interfaces also exist when using higher speed Ethernet, allowing external transceivers (e.g. fibre optic transceivers) to be added to NICs and Ethernet switches. Each interface is however designed to operate at one speed only and can not support transceivers designed for a different speed.


A transmitter may attempt to send more than the permitted frame size. In practice this can only arise through a hardware or software fault. Even so, since such errors have been observed on real systems, they must be guarded against. This is particularly significant since excessive jabber can very significantly reduce the performance of an Ethernet LAN.

The way chosen to prevent Jabber, is for the transceiver to count the number of bytes sent. If this number rises above the maximum allowed, a jabber control circuit forces the transceiver to terminate the frame, stopping the node from sending more data.

See also:


10BT Interface

Giants & Runts

Bridge / Switch

Examples of LAN Equipment

Gorry Fairhurst - Date: 01/02/2006 EG3557