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Definition and Architecture

Ethernet is a local area network (LAN) architecture that uses the Carrier Sense Multiple Access/Collision Detection (CSMA/CD) access method. Ethernet was originally developed by Digital Equipment Corporation, Intel, and Xerox (DIX). This standard is known as Ethernet Version II. Later, the Institute of Electrical and Electronics Engineers (IEEE) included Ethernet as part of the IEEE 802 standard. This standard is known as IEEE 802.3. To make things more confusing, the generic term "Ethernet" is used to describe both standards.

The IEEE 802.3 standard corresponds to layer 1 and part of layer 2 of the Open Systems Interconnect (OSI) Reference Model. IEEE 802.2 Logical Link Control (LLC) and IEEE 802.1d Bridging, also known as Spanning Tree Protocol (STP), provide the missing parts of OSI layer 2. STP is described further in Chapter 6, "LAN Switching." The Ethernet Version II standard corresponds to layers 1 and 2 of the OSI/RM. The relationship between the OSI/RM, Ethernet Version II, and IEEE 802.3 is described in Figure 3.1.

Figure 3.1 Relationship between OSI/RM, Ethernet Version II, and IEEE 802.

Key Concept

IEEE 802.3 supports all of layer 1 and part of layer 2 of the OSI model. Ethernet Version II supports both layer 1 and layer 2.

IEEE 802.3 has defined several media speeds as part of the physical layer. These speeds are as follows:

  • 10Mb for Ethernet

  • 100Mb for Fast Ethernet

  • 1000Mb or 1Gb for Gigabit Ethernet

Just as there are different media speeds there are different types of physical media. IEEE 802.3 uses a designation for each combination of speed and physical media type. Appending the speed in megabits to the word BASE and then adding a designation for the physical media type forms this designation. As an example, 10BASE-T designates 10Mb Ethernet over twisted-pair telephone wire. Following is a list of the commonly used physical media for Ethernet and Fast Ethernet:

  • 10BASE-2 and 10BASE-5 use coaxial cable (also called thinnet and thicknet).

  • 10BASE-T uses twisted-pair telephone wire with an RJ-45 connector.

  • 100BASE-T is Fast Ethernet over twisted-pair telephone wire with an RJ-45 connector.

  • 100BASE-FX is Fast Ethernet over optical fiber.

It is impractical to have a connector on the router for every possible physical media type. Hence, generic interfaces were developed that supported several physical interfaces. A transceiver is used to convert from the generic interfaces that follow to a specific physical interface:

  • Attachment Unit Interface (AUI) for 10Mb Ethernet

  • Media Independent Interface (MII) for 10Mb Ethernet and Fast Ethernet

  • Gigabit Media Independent Interface (GMII) for Gigabit Ethernet

Key Concept

IEEE 802.3 supports multiple speeds (for example, 10Mb Ethernet, 100Mb Fast Ethernet, and 1000Mb Gigabit Ethernet). Ethernet Version II supports only 10Mb operation. Both standards support multiple physical media types.

Following is a sample router configuration for a 10Mb Ethernet interface with an AUI connector and a 100Mb Fast Ethernet interface with an MII connector:

interface Ethernet0 
  media-type aui 
interface FastEthernet0 
  media-type mii
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