Overview

Ethernet, implemented by almost everybody in communications, adapts how data [are] sent as computing technology improves. It s simple, easy to use, and pervasive. You plug it in and it works.

BOB GROW ^[1]

In Chapter 1, we looked at Ethernet as a service made available by the Media Access Control (MAC) to a MAC client, over an abstract interface called the MAC Service interface. In this chapter, we will travel a few steps down the functional stack and examine the interactions at the interface between the MAC and the physical layer.

The MAC interacts with the physical layer through the ReceiveBit function, the procedures TransmitBit and Wait, and the variables collisionDetect, carrierSense, receiveDataValid, and transmitting, as specified in Clause 4 of IEEE Std. 802.3. This may look very comforting to programmers and software engineers, but it is too abstract to allow a MAC component and a physical layer component to interact with each other in an interoperable way at a hardware level.

To allow for interchangeable physical layer components to be built, additional interfaces have been specified just below the abstract interface between the MAC and the physical layer. These interfaces, the Attachment Unit Interface (AUI) ^[2] and the Media Independent Interface (MII) family, are defined as hardware interfaces, specifying the exact number of signal pins, the precise electrical characteristics of the signals, and the connectors to be used.

As the AUI and MII reside below the abstract...