Computer Networking: Internet Protocols in Action

Chapter 5.2. Ethernet

INTRODUCTION

The majority of desktop computers sold today come equipped with Ethernet interfaces. Ethernet is actually a family of local area network technologies. Variants of Ethernet vary widely in the format of the link layer frame headers, the type of cabling used and the speed of transmission.

The first Ethernet was developed in 1972 by Robert Metcalf at Xerox PARC to connect Alto computers to each other and other local resources like printers. In the original Ethernet, all connected devices tapped into a single shared network segment called the "Ether." When any device transmitted data, all the other devices would receive the transmission. Typically, a device simply ignores the data sent if the transmission was not intended for them (i.e. the destination address does not match their own). However, when put into promiscuous mode, devices will report all traffic seen if it was sent to another destination. This is the origin of the "capture packets in promiscuous mode" option in the Ethereal Capture dialog.

Only one device could transmit at a time because simultaneous transmissions would corrupt each other. The Ethernet specification included rules to govern how to avoid these conflicting transmissions. These rules are often referred to as CSMA/CD, which stands for Carrier Sense Multiple Access/ Collision Detection. This means that before a device can transmit data it must sense or listen to the channel to make sure that no other device is currently transmitting. ...