A network consists of several computers linked by communication lines. The machines can function independently; but their activities can also be coordinated. In the supermarket network, the checkout stations collect data, the minicomputers perform store-level processing, and the large mainframe handles chain wide inventory and distribution tasks．In another distributed network, the accounting, sales, production, and engineering departments of a busy-ness firm might have their own computers, with althea machines linked to a centralized mainframe housing the organizations database. With such a system, the individual departments have the flexibility to do their own processing, while the central machine en-sores their of use common data, and helps to monitor and control their activities。
Although the physical separation of the communicating computers may vary considerably from one type of application to another，in general a computer communication network can be described: at the heart of any computer communication networks is the data communication facility which, as has just been outlined ．may be a PSDN，a private LAN or perhaps number of such networks interconnected together. However ,irrespective of the type of data communication facility, an amount of hardware and softwares required within each attached computer to handle the appropriate network-dependent protocols．Typically, these arc concerned with the establishment of communication channel access to the network and with the controls of the flow of messages across this channel. The provision of such facilities is only part of the network requirements, however, since in many applications the communicating computers may be different types ．This means that they may use differ-net programming languages and, more importantly, different forms of data representation interface be-teen user application programs, normally referred to as application processes or Apes，and the underlying communication services may be different. For example, one computer may be a small single user computer, while another may be large multi-user system。
In the earlier days of computer communication, these issues meant that only closed communities of computers (that is, from the same manufacture) could communicate with each other in a meaningful way. IBM's Systems Network Architecture（SNA) adder’s Digital Networks Architecture (DNA) are just two examples of communication software pack-ages produced by manufacturers to allow their sys-teems to be interconnected together. These proprietary packages, however，of which there are still many inexistence, do not address thus problems of universal interconnectivity, or open systems interconnection. Inane attempt to solve this problem, the ISO, in the late1970s，formulated a reference model to provide common basis for the coordination of standards de-elopements and to allow existing and evolving standards activities to be placed into perspective with one another. The ultimate aim was to allow an application process in any computer that supported the applicable standards to freely communicate with an application process in any other computer supporting the same standard, irrespective of its origin of Manu-facture ．This model was termed the ISO Reference Model for Open System Interconnection。
It should be stressed, however, that this model isn't concerned with specific applications of computer communication networks．Rather，it is concerned with the structuring of the communication software that is needed to provide a reliable, data transparent, communication service (which is independent of any specific manufactures equipment or conventions) to support a wide range of applications。
Networks are not limited to such hierarchical structures, however. For example, many large or-generations with computers spread all over the world link their machines t0 form a ring network. Obviously, with all the computers linked, the various divisions and offices can exchange information. Less obvious is the backup provided by such a network; if one computer fails, its work can be switched to the others．
Not all networks are private. For example, any one with a microcomputer, a modem, the right software, and a telephone can link to any number of computers and access a variety of data ranging from stock quotations to library catalogs, to electronic bulletin boards. More sophisticated systems even allow users all over the world to exchange electronic mail ．Such as，we have just begun to tap the potential of computer networks．
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