In the industrialized countries, the evolutionary nature of the ISDN concept has been a matter of economic practicality considering the vast telephone infrastructure already in place. The primary emphasis, however, has been the enhancement of services rather than a simple replacement of existing equipment. This evolutionary approach has also merit for most of the developing countries, although in some instances where analogue facilities do not exist presently or where they are very limited, the direct installation of digital facilities could allow a direct leapfrog to ISDN capabilities.
在一些工业化国家，ISDN发展的本质一直是一个经济实用性问题，因为要考虑到现有的庞大电话设施。然而，主要方面一直是提高业务能力而不是简单的取代现有的设备。这一发展方式对多数发展中国家大有好处。因为在一些国家，目前的模拟设备不存在或很有限，所以直接安装数字设备可以直接跨越到ISDN的通信能力。
