01-05-2012, 10:53 AM
Ethernet technology
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A Brief History
In the early 1960's large mainframes dominated the computer industry. It wasn't until the late 1960s and early 1970s, when local area network (LAN) technology development began. A shift away from mainframes to a more decentralized approach seemed more feasible. Instead of upgrading the mainframe, why not distribute the processing power to individual users? The major hurdle that this generated was how to link all the distributed computers.
It became apparent immediately that there was a need for a communications network. Since there were no standards for local area networks in the 1970s, it was practically impossible to use equipment from different vendors when designing a LAN. Administrators and users saw that compatibility was a major problem. It was imperative to create an Open System. In other words, create a decentralized, distributed, multivendor approach to data processing and networking.
The first Ethernet specification was published in September 1980, by Digital, Intel, and Xerox. The specification was known as DIX, short for Digital, Intel, and Xerox. In 1982, the Institute of Electrical and Electronic Engineers (IEEE) formed a committee responsible for designing new local area network standards. The committee was called Project 802. A subcommittee, 802.3, was formed from the original one which was responsible for the international standard for Ethernet. The International Standards Organization (ISO) reviewed the standard and adopted it in 1985. Ethernet became the preferred method for connecting LANs and is still recognized today as the dominant leader.
Goals of Ethernet
• Understand the required and optional MAC frame formats, their purposes, and their compatibility requirements.
• List the various Ethernet physical layers, signaling procedures, and link media requirements/limitations.
• Describe the trade-offs associated with implementing or upgrading Ethernet LANs—choosing data rates, operational modes, and network equipment.
Evolution
Ethernet evolved to include higher bandwidth, improved media access control methods, and different physical media. The coaxial cable was replaced with point-to-point links connected by Ethernet repeaters or switches to reduce installation costs, increase reliability, and improve management and troubleshooting. Many variants of Ethernet remain in common use.
Ethernet stations communicate by sending each other data packets: blocks of data individually sent and delivered. As with other LANs, each Ethernet station is given a 48-bit MAC address. The MAC addresses are used to specify both the destination and the source of each data packet. Ethernet establishes link level connections, which can be defined using both the destination and sources addresses. On reception of a transmission, the receiver uses the destination address to determine whether the transmission is relevant to the station or should be ignored. Network interfaces normally do not accept packets addressed to other Ethernet stations. Adapters come programmed with a globally unique address. An Ethertype field in each frame is used by the operating system on the receiving station to select the appropriate protocol module (i.e. the Internet protocol module). Ethernet frames are said to be self-identifying, because of the frame type. Self-identifying frames make it possible to intermix multiple protocols on the same physical network and allow a single computer to use multiple protocols together.
Repeaters and hubs
For signal degradation and timing reasons, coaxial Ethernet segments had a restricted size. Somewhat larger networks could be built by using an Ethernet repeater. Early repeaters had only 2 ports, but they gave way to 4, 6, 8, and more ports as the advantages of cabling in a star network were recognized. Early experiments with star topologies (called "Fibernet") using optical fiber were published by 1978.
Ethernet on unshielded twisted-pair cables (UTP) began with StarLAN at 1 Mbit/s in the mid-1980s. SynOptics introduced the first twisted-pair Ethernet at 10 Mbit/s in a star-wired cabling topology with a central hub, later called LattisNet. These evolved into 10BASE-T, which was designed for point-to-point links only, and all termination was built into the device. This changed repeaters from a specialist device used at the center of large networks to a device that every twisted pair-based network with more than two machines had to use. The tree structure that resulted from this made Ethernet networks easier to maintain by preventing most faults with one peer or its associated cable from affecting other devices on the network.
Despite the physical star topology, repeater based Ethernet networks still use half-duplex and CSMA/CD, with only minimal activity by the repeater, primarily the Collision Enforcement signal, in dealing with packet collisions. Every packet is sent to every port on the repeater, so bandwidth and security problems are not addressed. The total throughput of the repeater is limited to that of a single link, and all links must operate at the same speed.
Bridging and switching
While repeaters could isolate some aspects of Ethernet segments, such as cable breakages, they still forwarded all traffic to all Ethernet devices. This created practical limits on how many machines could communicate on an Ethernet network. The entire network was one collision domain, and all hosts had to be able to detect collisions anywhere on the network. This limited the number of repeaters between the farthest nodes. Segments joined by repeaters had to all operate at the same speed, making phased-in upgrades impossible.
Router with shared Internet connection
In this network, your Internet service provider allows you one Internet connection that is shared among computers by a router. In the example shown here, the router is an AirPort base station. An Ethernet cable connects a DSL or cable modem to the base station's WAN port. Another Ethernet cable connects the base station's LAN port to a wired computer. Where the illustration shows one wired computer, you could connect a hub to the LAN port to accommodate many wired computers.
conclusion
Ethernet has survived as the major LAN technology (it is currently used for approximately 85 percent of the world's LAN-connected PCs and workstations) because its protocol has the following characteristics:
• Is easy to understand, implement, manage, and maintain
• Allows low-cost network implementations
• Provides extensive topological flexibility for network installation
• Guarantees successful interconnection and operation of standards-compliant products, regardless of manufacturer.