Why Isn't A Local Area Network Merely a "Big Bus"?
Distinctions Between Local Area Networks and Computer Buses
One distinguishing feature of local area networks is the geographic restrictions that permit them to utilize low-cost but very high-bandwidth transmission media. That characterization also applies to the bus structure of a computer.
A computer bus is usually conceived as connecting together the components of a single system; it's difficult to imagine a computer continuing to operate in the absence of its bus. In contrast, a network is understood to connect together a number of autonomous nodes, each capable of operating by itself in the absence of the network..
1) Defensiveness: The control and management strategies of a network are much more defensive than the equivalent strategies of a bus are required to be. A bus is not designed to operate while one of the devices fail. Whereas the network will continue to operate despite arbitrary failures of one or more nodes.
Than handling of traffic overloads is another example of the defensive nature of a network. A network is designed with anticipation that independently initiated transfers will occasionally demand more bandwidth than the network has available, at which time the network itself must mediate gracefully between these conflicting demands. There is usually no such concern for a computer bus.
2) Generality: the local area network protocols are designed with explicit intention that messages can be exchanged between a local network and a long-haul network, the idea that is usually missing from the addressing and control structure of a computer bus. Networks usually transmit variable size messages, while buses often transfer single, fixed-size words.
A computer bus often has a specialized interface, oriented towards the addressing and control architecture of a particular computer. A network usually attempts to provide an interface equally suitable for a wide variety of computers and other devices.
3) Minor distinctions: Buses are often more "local" than our definition of a local area network. A computer bus is often highly parallel, with separate control, data and address lines; networks tend to carry this information serially over a single set of lines.