Chapter 3. The Media Access Control Protocol

The tutorial in Chapter 2, introduced the Ethernet system and provided a brief look at how it works. In this chapter we take a much more detailed look at the original mode of operation used for Ethernet, which is based on the CSMA/CD media access control (MAC) protocol. This is also called half-duplex mode, to distinguish it from the full-duplex mode of operation. Full-duplex mode, which has no need for a MAC protocol, is described in Chapter 4.

In the original half-duplex mode, the MAC protocol allows a set of stations to compete for access to a shared Ethernet channel in a fair and equitable manner. The protocol's rules determine the behavior of Ethernet stations, including when they are allowed to transmit a frame onto a shared Ethernet channel and what to do when a collision occurs.