Local area network

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Local area network

The local area network (LAN) evolved as a means to interconnect a variety of computing platforms in a high-speed, reliable, and robust environment. First and foremost, a LAN is a network. Furthermore, the network serves as a communications infrastructure for computerized systems. This includes computers, printers, storage systems, and any other device that has a processor, and an ability and need to communicate with another system.

It is helpful to note how the definition of a LAN has changed over the years. The traditional definition of a LAN, as described by the Institute of Electrical and Electronics Engineers (IEEE), includes the concepts of high speed, low error rate, private ownership, and small geographic area. Some of these concepts have become somewhat less important in recent years.

In today's networks, high speed is no longer a characteristic restricted to LANs, so it is not a good distinguishing factor. When the public communications network was offering transmission rates across the wide area from 300 bps to 48 kbps, the 256 kbps to 10 Mbps rates typical of a LAN were comparatively high-speed. Today, the wide area network (WAN) can offer rates measured in tens of gigabits per second (Gbps). LAN transmission rates have also increased, with 100 Mbps LANs the norm, Gbps technologies common, and 10 Gbps being deployed. Clearly, the faster LANs operate at the same speeds as modern wide area services. That being said, it should be noted that although both LANs and WANs are now capable of high speeds, the vast majority of deployed LANs are still operating at rates measured in hundreds of megabits per second or more; the vast majority of WANs are still operating at rates at or significantly under 10 Mbps.

The concept of low error rate has seen a similar shift. LANs typically carry digital signals over baseband facilities. When the public network was still largely carrying analog information over broadband facilities, the bit error rates (BER) on LANs were extremely low when compared to the WAN. Today, however, the North American public networks are largely digital, so the BER of a LAN is comparable to the BER we have come to expect from the public network.

The description of a LAN as occupying a small geographic area has always been somewhat tenuous. Even the original Ethernet specifications supported distances as great as 2.5 kilometers. Today, LAN technologies are stretching out across the wide area. So the distinction between LAN, MAN, and WAN is no longer one of technology alone, but also geographic scope. The term LAN reflects a network measured in tens or hundreds of meters and typically exists within a single location (i.e., a single room, floor, building, or campus).

Private ownership is perhaps the one characteristic of LANs that is clear and has not changed significantly over the years. When a network is deployed on premises, it is most commonly private to the premises owner. Sometimes exceptions exist for multi-tenant dwellings, and technologies have emerged to provide for “virtual privacy” over even these shared network environments.

Physical and Logical Topologies

Physical and Logical Topologies

The type of media used within a LAN is only one of the Physical Layer issues to be considered. Another consideration is the fundamental organization of the medium. In a LAN, this organization can be described by the physical topology and the logical topology, although advances in LAN technology are rendering the latter less critical. The physical topology of a network literally describes where the wires go (i.e., through which walls, between which points, etc.) and their general configuration. The logical topology of a LAN describes the organization of the attached devices and the flow of signals between attached devices. Both the physical topology and the logical topology of a LAN are Physical Layer issues!

Why would the topology of a LAN be constrained to a small set of topologies? To see why, we must remember the context in which these technologies emerged. In the 1970s, data switching equipment was relatively crude, expensive, and slow. To provide for a low cost, high-speed, multi-system transmission environment, LANs emerged as broadcast environments. It is the broadcast nature of these networks that limited the choices of topology and the scale of the network.

The three physical topologies that were long associated with LANs include the star, the ring, and the bus. With the advent of high-speed, low cost switching technologies, some of the initial constraints on LANs could be relaxed, so the point-to-point topology emerged and has grown steadily in importance since.

In the days when the physical star dominated the LAN world, two logical topologies were commonly found as well: the ring and the bus.