466 EVOLUTION OF COMPUTER BUILDING BLOCKS

been proposed [Flynn, 1966; Jensen and Anderson, 1977], but this relatively straightforward grouping into three organizations is most suitable for the following discussion.

All of these organizations existed prior to the advent of the microprocessor. The economics of the microprocessor, however, open up the possibilities of using these structures in many new application areas. In our review of these alternative computer organizations, we will reference some older computer systems built with conventional components to help make the discussion more concrete.

Figure 2 shows a computer network. In this type of multiple-processor organization, each processor is embedded in a conventional computer system, and the computers are then inter connected via communication links. The inter-computer communication links are often serial, but in some cases, such as the channel-to-channel adapter of multicomputer IBM S/370 systems, high-bandwidth parallel buses are used.

Figure 2. A network of computers.

Perhaps the most widely known computer network is the ARPA network [Kahn, 1972], but other computer networks have also been implemented and are now in use. These include the Ethernet [Metcalfe and Boggs, 1976], DCS [Farber, 1975], and the Spider network [Fraser, 1975]. Furthermore, most large computer installations are really computer networks. Computer manufacturers are establishing standard network protocols, for example, IBM's system network architecture (SNA) and Digital Equipment Corporation's DECnet protocol, to facilitate the construction of computer networks tailored to individual user needs.

An important attribute of a computer network is the data transmission bandwidth between computers. This bandwidth ranges from a few thousand bits per second up to about 10 Mbits/second. The other important attribute of the inter-computer links is the access or latency time for each unit of information sent between computers. In describing interprocessor communication capability it is common to refer to the degree of coupling between processors in the system. The ARPA network is an example of a loosely coupled (and geographically distributed) computer network because of the 50 Kbit/second links between computers in the network and the 100-250 ms latency times associated with cross-network transmissions of packets of information. A more tightly coupled (and geographically centralized) network is the Ethernet with 3 Mbit/second inter-computer bandwidth and latency times of the order of a small number of milliseconds. As more and more closely coupled computer networks are considered, however, another type of multiple processor structure, the multiprocessor, becomes an increasingly competitive alternative. Multiprocessors will be discussed shortly.

As microprocessors are incorporated into computer terminals, point-of-sale terminals, data acquisition transducers, and other such applications, the natural form of organization will