Ethernet: The Definitive Guide

Chapter 5. Auto-Negotiation

Automatic configuration of Ethernet equipment is provided by the Auto-Negotiation protocol, which is defined in the Ethernet standard. This chapter describes the Auto-Negotiation protocol, and shows how automatic configuration functions. The need for an automatic configuration system becomes obvious when you consider the challenge facing someone who is installing a desktop computer and connecting it to an Ethernet system.

Among the things the installer needs to know are which speed should be set on the Ethernet interface and whether full-duplex mode should be enabled. However, these features are embedded in the network equipment and are invisible to the installer. One twisted-pair port looks a lot like another, and it is not obvious which network options may be supported. The Auto-Negotiation protocol allows Ethernet equipment to automatically select the correct speed and other features, thus relieving the installer of this configuration task.

Development of Auto-Negotiation

The specifications for Auto-Negotiation were first published in 1995 as part of the 802.3u Fast Ethernet supplement to the IEEE standard. These specifications were based on an automatic configuration system called NWay, which was invented by National Semiconductor. Engineers working on the standard found that an automatic configuration signaling system could be readily developed that would work on twisted-pair links. Therefore, all Ethernet media systems that use twisted-pair media ...