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31 Days Before Your CCENT Exam: Ethernet and Media Access Control

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To help you prepare for your CCENT exam. this chapter reviews Ethernet technologies and operation at both the data link and physical layers.
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CCENT 100-101 ICND1 Exam Topics

  • Determine the technology and media access control method for Ethernet networks.

Key Topics

Ethernet has continued to evolve from the 10BASE2 flavor, capable of speeds up to 10Mbps, to the newest 10GigE (10 Gigabit Ethernet), capable of speeds up to 10Gbps. Since 1985, the IEEE has continued to upgrade the 802.3 standards to provide faster speeds without changing the underlying frame structure. This feature, among others, has made Ethernet the choice for LAN implementations worldwide. Today we review Ethernet technologies and operation at both the data link and physical layers.

Ethernet Overview

802.3 is the IEEE standard for Ethernet, and both terms are commonly used interchangeably. The terms Ethernet and 802.3 both refer to a family of standards that together define the physical and data link layers of the definitive LAN technology. Figure 29-1 shows a comparison of Ethernet standards to the OSI model.

Figure 29-1

Figure 29-1 Ethernet Standards and the OSI Model

Ethernet separates the functions of the data link layer into two distinct sublayers:

  • Logical Link Control (LLC) sublayer: Defined in the 802.2 standard
  • Media Access Control (MAC) sublayer: Defined in the 802.3 standard

The LLC sublayer handles communication between the network layer and the MAC sub-layer. In general, LLC provides a way to identify the protocol that is passed from the data link layer to the network layer. In this way, the fields of the MAC sublayer are not populated with protocol type information, as was the case in earlier Ethernet implementations.

The MAC sublayer has two primary responsibilities:

  • Data encapsulation: Includes frame assembly before transmission, frame parsing upon reception of a frame, data link layer MAC addressing, and error detection.
  • Media Access Control: Because Ethernet is a shared media and all devices can transmit at any time, media access is controlled by a method called Carrier Sense Multiple Access with Collision Detection (CSMA/CD) when operating in half-duplex mode.

At the physical layer, Ethernet specifies and implements encoding and decoding schemes that enable frame bits to be carried as signals across both unshielded twisted-pair (UTP) copper cables and optical fiber cables. In early implementations, Ethernet used coaxial cabling.

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