Bicycles with Motors
One of the difficulties of decentralized computing is that it isolates users from one another and from the data and applications they may need to use in common. When a file is created, how is it shared with Tom, Dick, and Harriet down the hall? The early solution to this was the storied SneakerNet: Put the file on floppy disks and hand carry them to the necessary destinations. But what happens when Tom, Dick, and Harriet modify their copies of the file? How does one ensure that all information in all versions are synchronized? What if those three coworkers are on different floors or in different buildings or cities? What if the file needs to be updated several times a day? What if there are not three coworkers, but 300 people? What if all 300 people occasionally need to print a hard copy of some modification they have made to the file?
The local-area network, or LAN, is a small step back to centralization. LANs are a means of pooling and sharing resources. Servers enable everyone to access a common copy of a file or a common database; no more "walkabouts" with floppies, no more worries about inconsistent information. E-mail furnishes a compromise between phone calls, which require the presence of the recipient, and physical mail service, which is called snail mail for a good reason. The sharing of printers and modem pools eliminates the need for expensive, periodically used services on every desk.
Of course, in their infancy, LANs met with more than a little derision from the mainframe manufacturers. A commonly heard jibe during the early years was, "A LAN is like a bike with a motor, and we don't make Mopeds!" What a difference a few years and a few billion dollars would make.
Physically, a LAN accomplishes resource pooling among a group of devices by connecting them to a common, shared medium, or datalink. This medium may be twisted-pair wires (shielded or unshielded), coaxial cable, optical fiber, infrared light, or whatever. What matters is that all devices attach commonly to the data link through some sort of network interface.
A shared physical medium is not enough. Rules must govern how the data link is shared. As in any community, a set of rules is necessary to keep life orderly, to ensure that all parties behave themselves, and to guarantee that everyone gets a fair share of the available resources. For a local-area network, this set of rules, or protocol, is generally called a Media Access Control (MAC). The MAC, as the name implies, dictates how each machine will access and share a given medium.
So far, a LAN has been defined as being a community of devices such as PCs, printers, and servers coexisting on a common communications medium and following a common protocol that regulates how they access the medium. But there is one last requirement: As in any community, each individual must be uniquely identifiable.
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