Ethernet: The Definitive Guide

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.

Since there is no central controller in an Ethernet system, each Ethernet interface operates independently while using the same MAC protocol. By equipping all interfaces with the same set of rules, all stations connected to a shared Ethernet channel operate the same way. Therefore, the MAC protocol functions as a kind of “Robert’s Rules for Robots.”

You don’t need to know all the details of the MAC protocol in order to build and use Ethernet LANs. However, an understanding of the MAC protocol can certainly help when designing networks or troubleshooting problems. The media guidelines and MAC protocol are not some arbitrary laws dreamed up by a standards committee, ...