Ethernet has pretty much taken over the wired LAN market. In the 1980s and the early 1990s, Ethernet faced many challenges from other LAN technologies, including token ring, FDDI, and ATM. Some of these other technologies succeeded in capturing a part of the LAN market for a few years. But since its invention in the mid-1970s, Ethernet has continued to evolve and grow and has held on to its dominant position. Today, Ethernet is by far the most prevalent wired LAN technology, and it is likely to remain so for the foreseeable future. One might say that Ethernet has been to local area networking what the Internet has been to global networking.

There are many reasons for Ethernets success. First, Ethernet was the first widely deployed high-speed LAN. Because it was deployed early, network administrators became intimately familiar with Ethernet - its wonders and its quirks - and were reluctant to switch over to other LAN technologies when they came on the scene Second, token ring, FDDI, and ATM were more complex and expensive than Ethernet, which further discouraged network administrators from switching over. Third, the most compelling reason to switch to another LAN technology (such as FDDI or ATM) was normally the higher data rate of the new technology; however, Ethernet always fought back, producing versions that operated at equal data rates or higher. Switched Ethernet was also introduced in the early 1990s, which further increased its effective data rates. Finally, because Ethernet has been so popular, Ethernet hardware (in particular, adapters and switches) has become a commodity and is outstandingly cheap.

The original Ethernet LAN was invented in the mid-1970s by Bob Metcalfe and David Boggs. Figure 1 shows Metcalfes schematic for this invention. In the figure youll notice that the original Ethernet LAN used a coaxial bus to interconnect the nodes. Bus topologies for Ethernet actually persisted throughout the 1980s and into the mid-1990s. Ethernet with a bus topology is a broadcast LAN - all transmitted frames travel to and are processed by all adapters connected to the bus.

By the late 1990s, most companies and universities had replaced their LANs with Ethernet installations using a hub-based star topology. As shown in Figure 5.21, in such an installation the hosts (and router) are directly connected to a hub with twisted-pair copper wire. A hub is a physical-layer device that acts on individual bits rather than frames. When a bit, representing a zero or a one, arrives from

The original Metcalfe design led to the 10BASE5 Ethernet standard

Star topology for Ethernet

the interface, the hub simply re-creates the bit, boosts its energy strength, and transmits the bit onto all the other interfaces. Thus, Ethernet with a hub-based star topology is also a broadcast LAN - whenever a hub receives a bit from one of its interfaces, it sends a copy out on all of its other interfaces. In particular, if a hub receives frames from two different interfaces at the same time, a collision occurs and the nodes that created the frames must retransmit. In the early 2000s Ethernet experienced yet another major evolutionary change. Ethernet installations continued to use a star topology, but the hub at the center was replaced with a switch. Well be examining switched Ethernet in depth later in this section. For now, we only mention that a switch is not only "collision-less" but is also a bona-fide store-and-forward packet switch; but unlike routers, which operate up through layer 3, a switch operates only up through layer 2.


global networking, bus topology, hub, nodes, star topology, routers

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