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 28-1 shows a comparison of Ethernet standards to the OSI model.
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Figure 28-1 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 sublayer. 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).
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.