In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet.

With the implementation of (1) , the so - called network has become a reality. The provision of such facilities is the most important part of the network requirements. However, in many applications the communicating computers may be of different types. This means that they may use different programming languages and, more importantly, different forms of (2) interface between user(application) program, normally referred to, as application processes, and the underlying communication services may be (3) . For example, one computer may be a small single - user computer, while another may be a large (4) system. In the earlier days of computer communication, these issues meant that only closed communities of computers (that is, from the same manufacturer) could communicate with each other in a meaningful way. IBM’s Systems Network Architecture (SNA) and DEC’s Digital Network Architecture (DNA) are just two examples of communication software packages produced by manufacturers to allow their systems to be interconnected together. These proprietary packages, however, of which there am still many inexistence, do not address the problem of universal interconnect ability, or open systems interconnection. In an attempt to alleviate this problem, (5) , in the late 1970s, formulated a reference model to provide a common basis (or the coordination of standards developments and to allow existing and evolving standards activities to be placed into perspective with one another.

1（）

A．communication channel

B．protocols

C．data channel

D．public communication

简述门店退货的定义。