chapter 9

ETHERNET

   

      The first LAN in the world was the original version of Ethernet and it is now the prime LAN technology in the world that being designed by Metcalfe and his coworkers at Xerox for more than thirty years ago. The first Ethernet standard was published in 1980 by a association of Digital Equipment Corporation, Intel, and Xerox (DIX). This can be understood by the OSI model of its comprised standards at these lower layers. The OSI model separates the Data Link layer functionalities of addressing, framing and accessing the media from the Physical layer standards of the media.

      Ethernet operates across two layers of the OSI model. the Layer 1 involves signals and Layer 2 involves addresses limitations. Ethernet separates the functions of the Data Link layer into two distinct sublayers: the Logical Link Control (LLC) sublayer that implemented in software, and its implementation is independent of the physical equipment. And Media Access Control (MAC) sublayer is implemented by hardware, typically in the computer Network Interface Card (NIC).It is the lower part of Data Link layer.

      Ethernet was designed to accommodate multiple computers that were Interconnected on a shared bus topology. The first version of Ethernet incorporated a media access method known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD). And it can managed the problems that result multiple devices attempt to communicate over a shared physical medium. Each computer was directly connected to the backbone.Hub was in the Legacy Ethernet that typically at the central point of the network segment. And this is created as a shared media. The Ethernet Beyond the LAN in the increased cabling distances enabled by the use of fiber-optic cable in Ethernet-based networks has resulted in a blurring of the distinction between LANs and WANs. Ethernet was initially limited to LAN cable systems within single buildings, and then extended to between buildings. It can now be applied across a city in what is known as a Metropolitan Area Network (MAN).

      The original Ethernet standard defined the minimum frame size as 64 bytes and the maximum as 1518 bytes is Ethernet Frame Size. This includes all bytes from the Destination MAC Address field through the Frame Check Sequence (FCS) field. While the MAC address value is a direct result of IEEE-enforced rules for vendors to ensure globally unique addresses for each Ethernet device.

      OSI Data Link layer (Layer 2) physical addressing, implemented as an Ethernet MAC address, is used to transport the frame across the local media. Although providing unique host addresses, physical addresses are non-hierarchical. They are associated with a particular device regardless of its location or to which network it is connected. In Ethernet, different MAC addresses are used for Layer 2 unicast, multicast, and broadcast communications. A unicast MAC address is the unique address used when a frame is sent from a single transmitting device to single destination device. With a broadcast, the packet contains a destination IP address that has all ones (1s) in the host portion. This numbering in the address means that all hosts on that local network (broadcast domain) will receive and process the packet. Multicast addresses allow a source device to send a packet to a group of devices. Devices that belong to a multicast group are assigned a multicast group IP address.