Ethernet

Ethernet standart and implementation

IEEE Standards
The first LAN in the world was the original version of Ethernet. Robert Metcalfe and his coworkers at Xerox designed it more than thirty years ago. The first Ethernet standard was published in 1980 by a consortium of Digital Equipment Corporation, Intel, and Xerox (DIX). Metcalfe wanted Ethernet to be a shared standard from which everyone could benefit, and therefore it was released as an open standard. The first products that were developed from the Ethernet standard were sold in the early 1980s.

In 1985, the Institute of Electrical and Electronics Engineers (IEEE) standards committee for Local and Metropolitan Networks published standards for LANs. These standards start with the number 802. The standard for Ethernet is 802.3. The IEEE wanted to make sure that its standards were compatible with those of the International Standards Organization (ISO) and OSI model. To ensure compatibility, the IEEE 802.3 standards had to address the needs of Layer 1 and the lower portion of Layer 2 of the OSI model. As a result, some small modifications to the original Ethernet standard were made in 802.3.

Ethernet operates in the lower two layers of the OSI model: the of the Data Link layer and the Physical layer.

Ethernet Layer 1 and Layer 2
Ethernet operates across two layers of the OSI model. The model provides a reference to which Ethernet can be related but it is actually implemented in the lower half of the Data Link layer, which is known as the Media Access Control (MAC) sublayer, and the Physical layer only.

Ethernet at Layer 1 involves signals, bit streams that travel on the media, physical components that put signals on media, and various topologies. Ethernet Layer 1 performs a key role in the communication that takes place between devices, but each of its functions has limitations.

As the figure shows, Ethernet at Layer 2 addresses these limitations. The Data Link sublayers contribute significantly to technological compatibility and computer communications. The MAC sublayer is concerned with the physical components that will be used to communicate the information and prepares the data for transmission over the media..

The Logical Link Control (LLC) sublayer remains relatively independent of the physical equipment that will be used for the communication process.

Ethernet separates the functions of the Data Link layer into two distinct sublayers: the Logical Link Control (LLC) sublayer and the Media Access Control (MAC) sublayer. The functions described in the OSI model for the Data Link layer are assigned to the LLC and MAC sublayers. The use of these sublayers contributes significantly to compatibility between diverse end devices.

For Ethernet, the IEEE 802.2 standard describes the LLC sublayer functions, and the 802.3 standard describes the MAC sublayer and the Physical layer functions. Logical Link Control handles the communication between the upper layers and the networking software, and the lower layers, typically the hardware. The LLC sublayer takes the network protocol data, which is typically an IPv4 packet, and adds control information to help deliver the packet to the destination node. Layer 2 communicates with the upper layers through LLC.

LLC is implemented in software, and its implementation is independent of the physical equipment. In a computer, the LLC can be considered the driver software for the Network Interface Card (NIC). The NIC driver is a program that interacts directly with the hardware on the NIC to pass the data between the media and the Media Access Control sublayer.

http://standards.ieee.org/getieee802/download/802.2-1998.pdf

http://standards.ieee.org/regauth/llc/llctutorial.html

http://www.wildpackets.com/support/compendium/reference/sap_numbers