Friday, February 4, 2011

What is Ethernet

With the fundamentals of networking under our belt, we can now take a closer look at the infrastructure that makes up the networks we all use. This section focuses on the switches and routers that make up networks, along with the protocols that drive them. We start this section with a discussion of the Ethernet protocol, which defines the rules and processes by which computers in a local area communicate. Long before the Internet was in use, computers communicated locally using the Ethernet protocol, and it is still widely used. We then move on to local-area network (LAN) switching, an extension of the Ethernet protocol required when there are more computers in a local segment than can communicate efficiently. Switching is one of the core technologies in networking. One of the necessities in networking is link redundancy, something that makes it more likely that data reaches its intended target. Sometimes, however, link redundancy can create loops in the network, which causes an explosion of administrative traffic that can take down a network in a matter of minutes. Spanning Tree is one of the mechanisms that keeps these “broadcast storms” from wiping out your local network, so we look at how this important protocol works. We end this section with routing, which provides the basis for network communication over long distances. The advent of routing allowed the growth of the Internet and corporate networking as we know it....

History of Ethernet

Robert Metcalfe developed Ethernet at the famous Xerox Palo Alto Research Center (PARC) in 1972. The folks at Xerox PARC had developed a personal workstation with a graphical user interface. They needed a technology to network these workstations with their newly developed laser printers. (Remember, the first PC, the MITS altair, was not introduced to the public until 1975.) Metcalfe originally called this network the Alto Aloha Network. He changed the name to Ethernet in 1973 to make it clear that any type of device could connect to his network. He chose the name “ether” because the network carried bits to every workstation in the same manner that scientists once thought waves were propagated through space by the “luminiferous ether.”
Metcalfe’s first external publication concerning Ethernet was available to the public in 1976. Metcalfe left Xerox, and in 1979 he got Digital Equipment Corporation (DEC), Intel, and Xerox to agree on a common Ethernet standard called DIX. In 1982, the Institute of Electrical and Electronic Engineers (IEEE) adopted a standard based on Metcalfe’s Ethernet. Ethernet took off in academic networks and some corporate networks. It was cheap, and public domain protocols such as Internet Protocol (IP) ran natively on it. However, another company (IBM) wanted the world to adopt its protocol instead, called Token Ring. Before switching was introduced, Ethernet was more difficult to troubleshoot than Token Ring. Although Ethernet was less expensive to implement, larger corporations chose Token Ring because of their relationship with IBM and the ability to more easily troubleshoot problems. Early Ethernet used media such as coaxial cable, and a network could literally be a single long, continuous segment of coax cable tied into many computers. (This cable was known as Thinnet or Thicknet, depending on the thickness of the coax used.) When someone accidentally kicked the cable under his or her desk, this often produced a slight break in the network. A break meant that no one on the network could communicate, not just the poor schmuck who kicked the cable. Debugging usually entailed crawling under desks and following the cable until the break was found. In contrast, Token Ring had more sophisticated tools (than crawling on your knees) for finding the breaks. It was usually pretty obvious where the token stopped being passed and, voilĂ , you had your culprit. The battle for the LAN continued for more than ten years, until eventually Ethernet became the predominant technology. Arguably, it was the widespread adoption of Ethernet switching that drove the final nail in Token Ring’s coffin. Other LAN technologies, such as AppleTalk and Novell IPX, have been and continue to be introduced, but Ethernet prevails as the predominant technology for local high-speed connectivity. Thankfully, we have left behind early media such as coax for more sophisticated technologies.
What Is Ethernet?

Ethernet describes a system that links the computers in a building or within a local area. It consists of hardware (a network interface card), software, and cabling used to connect the computers. All computers on an Ethernet areattached to a shared data link, as opposed to traditional point-to-point networks, in which a single device connects to another single device. Because all computers share the same data link on an Ethernet network, the network needs a protocol to handle contention if multiple computers want to transmit data at the same time, because only one can talk at a time without causing interference. Metcalfe’s invention introduced the carrier sense multiple access collision detect (CSMA/CD) protocol. CSMA/CD defines how a computer should listen to the network before transmitting. If the network is quiet, the computer can transmit its data. However, a problem arises if more than one computer listens, hears silence, and transmits at the same time: The data collides. The collision-detect part of CSMA/CD defines a method in which transmitting computers back off when collisions occur and randomly attempt to restart transmission. Ethernet originally operated at 3 Mbps, but today it operates at speeds ranging from 10 Mbps (that’s 10 million bits per second) to 10 Gbps (that’s 10 billion bits per second).
Evolution of Ethernet

When Metcalfe originally developed Ethernet, computers were connected to a single copper cable. The physical limitations of a piece of copper cable carrying electrical signals restricted how far computers could be from each other on an Ethernet. Repeaters helped alleviate the distance limitations. Repeaters are small devices that regenerate an electrical signal at the original signal strength. This process allows an Ethernet to extend across an office floor that might exceed the Ethernet distance limitations. The addition or removal of a device on the Ethernet cable disrupts the network for all other connected devices. A device called an Ethernet hub solves this problem. First, each port on a hub is actually a repeater. Second, hubs let computers insert or remove themselves nondisruptively from the network. Finally, hubs simplify Ethernet troubleshooting and administration. As networks grow larger, companies need to fit more and more computers onto an Ethernet. As the number of computers increases, the number of collisions on the network increases. As collisions increase, useful network traffic decreases (administrative traffic actually increases because of all the error messages getting passed around). Networks come to a grinding halt when too many collisions occur. Ethernet bridges resolve this problem by physically breaking an Ethernet into two or more segments. This arrangement means that devices communicating on one side of the bridge do not collide with devices communicating on the other side of the bridge. Bridges also learn which devices are on each side and only transfer traffic to the network containing the destination device. A twoport bridge also doubles the bandwidth previously available, because each port is a separate Ethernet. Ethernet bridges evolved to solve the problem of connecting Ethernet networks to Token Ring networks. This process of translating a packet from one LAN technology to another is called translational bridging. As Ethernet networks continue to grow in a corporation, they become more complex, connecting hundreds and thousands of devices. Ethernet switches allow network administrators to dynamically break their networks into multiple Ethernet segments. Initially, switches operated as multiport Ethernet bridges. But eventually, as the cost per port decreased significantly, Ethernet switches replaced hubs, in which each connected device receives its own dedicated Ethernet bandwidth. With switches, collisions are no longer an issue, because connections between computer and switch can be point-to-point, and the Ethernet can both send and receive traffic at the same time. This ability to send and receive simultaneously is called full duplex, as opposed to traditional Ethernet, which operated at half duplex. Half duplex means that a device can receive or transmit traffic on the network, but not at the same time. If both happen at the same time, a collision occurs. This is different from subnetting in a couple of distinct ways. First, Ethernet is a Layer 2 protocol, and subnetting has to do with IP addressing (which is a Layer 3 function). Second, IP addressing is a logical segmentation scheme, and switching is a physical separation, because each end station has a dedicated physical port on the switch.

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