Chapter 9 - Ethernet

Ethernet is a kind of computer network for Local Area Network. It is a communication protocol embedded in software and hardware devices. In Ethernet, the data is divided into individual packets called frames which contains the address of source and destination of the data being transmitted. Ethernet is the original version of the first LAN. It was design by Robert Metcafe and his coworkers more than 30 years ago. It was published and sold in the early 1980s. The first standard for LAN start with the number 802.3. It operates in the lower two layers of the OSI model: the data link layer which is known as the Media Access Control or MAC and the physical layer. If we recall, layer 1 involves signals, bit stream that travel on the media, physical components that put signals on media. Ethernet plays a key role in the communication that takes place between devices, but each of its functions has a limitation. In layer 2, data link layer, all the limitations in layer one fulfilled by this layer. It gives significant to technological compatibility and computer communications. Ethernet layer 2 is divided by MAC sub layer and Logical Link Layer (LLC). In implementing Ethernet physically, a connection in communication media was imposed. It also specifies and implements encoding and decoding scheme that enable frame bits to be carried as signal across the media. The first version of Ethernet used coaxial cable. These versions were known as Thicknet (10BASE5) and Thinnet (10BASE2). The difference between them is that they used coaxial cable but Thicknet allows cabling up to 500 meters while Thinnet allows only 185 meters. Ethernet frame consist of the following headers and trailers: preamble, destination address, source address, type, data and frame check sequence. The preamble and start frame delimiter are used for managing between the sending and receiving devices. Destination MAC addresses is the one that identify the intended recipient. Source MAC addresses is the one that identify the originating interface or the source. The length is the exact length of the frames data field. Frame Check Sequence is the one that detect errors in the frame. Ethernet address is represented by hexadecimal numbering. Ethernet also address in the data link layer and network layer. In Ethernet, MAC addresses are used for layer 2 unicast, broadcast and multicast. Ethernet uses the address resolution protocol to determine the MAC addresses of destination and map them against known layer addresses.