1、附录 DIGITAL NETWORK The ISDN will be a worldwide public telecommunications network that will deliver a wide variety of services. The ISDN will be defined by the standardization of user interfaces, and will be implemented as a set of digital switches and paths supporting a broad range of traffic types
2、 and providing value-added processing services. In practice, there will be multiple networks, implemented within national boundaries, but from the users point of view there will be a single, uniformly accessible worldwide network. There are two key aspects to ISDN: universal access and user services
3、. By standardizing the interfaces to ISDN, all ISDN-compatible equipment (e. g. ,telephones computer terminals, personal computers )will be able to attach to the network anywhere in the world and connect to any other attached system. This can lead to extraordinary flexibility. For example, telephone
4、 numbers could be assigned in the same fashion as U.S. social security numbers, good for a lifetime. No matter where you lived, or how often you moved, dialing the number permanently assigned to you would always ring your telephone. A digitized network is a prime requirement for integrating a variet
5、y of services that the network will support. It is appropriate to view ISDN as being evolutionary in nature and scope. The first step in the process toward realizing an ISDN is end-to-end digitization. Digitization of the telephone network in the United States began in the early 1960s. The early dig
6、itized program largely addressed the inter exchange transmission system of the metropolitan network. Use of the time division multiplexing technique, rather than the traditional frequency division multiplexing technique, resulted in an enhanced capacity of the existing telephone plant at minimal cos
7、t. Further development in transmission systems, especially those based on fiberoptic transmission techniques, have pushed the digitization program well beyond the most optimistic projections made just a few years ago. Compared to other forms of information transport, lightware systems have proved th
8、emselves economically and will continue to be the technology of choice in the foreseeable future. Form an initial transmission capability of 45 Mbps ten years ago , the capacity of lightware systems has doubled every 18 months. Presently ,1.8Gbps systems are available, and by mid-1990, 2.2Gbps syste
9、ms will be on the commercial market. End-to-end digitization is available in limited pockets in the United States today but is expected to grow sharply during the next few years. This will provide the hardware base for realizing the ISDN capability. ISDN goes much beyond having digital switching and
10、 transmission hardware on a ubiquitous basis. The main focus of ISDN is on the support of a wide range of voice and nonvoice or integrated voice/nonvoice applications over the same network. A basic tenet of ISDN is that this service integration be achieved through a limited set of usernetwork interf
11、ace arrangements. Heretofore, private line and switched offerings have been made by telecommunications administrations using functionally separate networks, for example , are often isolated from one another , and interworking between them is usually a difficult technical issue. ISDN provider for the
12、se services within a common framework in order to benefit the end users , particularly from the point of view of assuring them, through easy interworking among different services, of the continuing usage of their applications well-specified and functional user-network interface characteristics and t
13、he assurance that the network transport function can be achieved using whatever technique best meets the end users needs, but resulting in no additional interface problems , these goals of ISDN will not be realized. Recognizing the continuing growth in digitization of the telecommunications network
14、and the advantages and additional capabilities of an end-to-end digital network , the concept of a single integrated network was born in the 1970s. Serious efforts were soon started in the CCITT, the organization that establishes international standards for public telecommunications, on a global bas
15、is. The first set of standards for ISDN were issued by the CCITT in 1984. As ISDN is established on a global basis, a variety of voice and nonvoice services can be supported over a single , homogeneous infrastructure. Analog communication will eventually disappear , resulting in enhanced quality at
16、reduced costs. An integrated access mechanism is a basic tenet of ISDN. The ISDN framework requires that a variety of services be accessed using common access arrangements with a limited net of access interfaces. It is easy to recognize that there are very large fixed costs in the global telecommuni
17、cations network today. Since a large discontinuity in service features and pricing will be unacceptable in the telecommuni8cations marketplace , it is easy to see that it may be infeasible to move immediately to ISDN in many areas of the world. The full implementation of some ISDN concepts may take
18、several years, possibly through the whole decade of the 1990s. In the meanwhile , ISDN systems and services will interoperate with the existing services. ISDN is thus an evolutionary concept from the standpoint of services to the end user. From a networking perspective, ISDN is a new network infrast
19、ructure that will provide a single point of access to multiple networks and to different kinds of networks. ISDN exchanges will be able to interconnect with each other. In addition, ISDN exchange will be able to connect to other existing networks, for example, X.25 packet networks. They will able to
20、 connect to other existing voice telephone networks (public not disrupted). There will also interconnect with ensure that the presently available services are not disrupted while new services employing the additional capabilities of ISDN are made available to user. Before the advent of ISDN, the pre
21、mises equipment and network services provide by the common user network were assumed to be distinct elements cooperating in only limited ways in providing an end-to-end service. The concept of ISDN specifically provides for the extension of network functions into the premises equipment, and vice ver
22、sa, depending on the need of the application. This requires a rich set of signaling mechanisms between the premises equipment and the network, higher speeds of transmission that both the premises equipment and the network services must support, and an integrated set of end-to-end network management
