The North American public switched telecommunications network (PSTN) is 100% digital. The interexchange carrier (IXC) portion has been 100% digital for many years. The world network is expected to be all-digital by the first decade of the twenty-first century. That network is still basically hierarchical, and the structure changes slowly. There are possibly only two factors that change network structure:

1. Political

2. Technological

In the United States, certainly divestiture of the Bell System/AT&T affected network structure with the formation of LECs (local exchange carriers) and IXCs. Outside North America, the movement toward privatization of government telecommunication monopolies in one way or another will affect network structure. As mentioned in Section 1.3, and to be discussed further in Chapter 8, there is a decided trend away from strict hierarchical structures, particularly in routing schemes; less so in topology.

Technology and its advances certainly may be equally or even more important than political causes. Satellite communications, we believe, brought about the move by CCITT away from any sort of international network hierarchy. International high-usage and direct routes became practical. We should not lose sight of the fact that every digital exchange has powerful computer power, permitting millisecond routing decisions for each call. This was greatly aided by the implementation of CCITT Signaling System No. 7 (Chapter 13). Another evident factor certainly is fiber-optic cable for a majority of trunk routes. It has also forced the use of geographic route diversity to improve survivability and availability. What will be the impact of the asynchronous transfer mode (ATM) (Chapter 18) on the evolving changes in network structure (albeit slowly)? The Internet certainly is forcing changes in data route capacity, right up to the subscriber. Privatization schemes now being implemented in many countries around the world will indeed have impact, as well, on network structure.

In the following section we discuss the digital network from the perspective of the overall PSTN. Certainly the information is valid for private networks as well, particularly if private networks are backed up by the local PSTN.

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