Ethernet: The Definitive Guide, 2nd Edition

Chapter 3. The Ethernet System

An Ethernet network is made up of hardware and software working together to deliver digital data between computers. To accomplish this task, several basic elements combine to make an Ethernet system. This chapter describes these elements, as a familiarity with the basic elements provides a good background for working with Ethernet. We will also see how the Ethernet system is used by high-level network protocols to send data between computers.

This chapter discusses the original half-duplex mode of operation, because that’s the system that Ethernet began with. Half-duplex simply means that only one computer can send data over the Ethernet channel at any given time. In half-duplex mode, multiple computers share access to a single Ethernet channel by using the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Media Access Control (MAC) protocol. Until the introduction of Ethernet switches, half-duplex was the typical mode of operation for Ethernet devices.

However, these days almost all Ethernet devices are connected directly to a port on an Ethernet switch that is operating in full-duplex mode, and do not share the Ethernet signal channel with other devices. When Ethernet devices are connected to switch ports, the Auto-Negotiation protocol will typically select full-duplex mode, in which the original CSMA/CD protocol is shut off and the two devices on the link can send data whenever they like. Half- and full-duplex mode are both forms ...