Ethernet: The Definitive Guide, 2nd Edition

Chapter 4. The Ethernet Frame and Full-Duplex Mode

The tutorial in Chapter 3 introduced the Ethernet system and provided a brief look at how it works. In this chapter, we take a more detailed look at the Ethernet frame and the full-duplex mode of operation. You don’t need to know all the details of the frame and Ethernet system operation in order to build and use Ethernets. However, an understanding of these elements can certainly help when designing networks or troubleshooting problems.

The original half-duplex mode Media Access Control (MAC) protocol was designed to allow a set of stations to compete for access to a shared Ethernet channel, based on coaxial cable segments linked with signal repeaters. The half-duplex media access control protocol is based on carrier sense with multiple access and collision detection, which gives rise to the CSMA/CD acronym.

The development of full-duplex media systems made it possible for Ethernet links to operate in full-duplex mode, providing a higher-performance mode of operation than the one supported over shared channels using CSMA/CD. The Auto-Negotiation protocol described in Chapter 5 automatically selects the highest-performance mode of operation over a link, typically resulting in full-duplex mode for Ethernet connections. Today, the vast majority of Ethernet links operate in full-duplex mode, which we will describe in this chapter.

However, half-duplex mode is still supported for Ethernet interfaces operating at 10 or 100 Mb/s over twisted-pair ...