Overview

Once upon a time, standards were set in North America by the Bell System, in particular, by AT&T. As the prime carrier for network services, AT&T set the rules on how the network operated. In 1958, the designers at AT&T developed the North American Digital Hierarchy as a means of delivering high-speed digital communications. This worked throughout the network because at the time, only one provider existed. Because there was no competition, AT&T needed only to satisfy itself on the operation of the network. Although the North American Digital Hierarchy worked, many limitations also existed that curtailed the overall operation of the network. One such problem was in the asynchronous nature of many of the transmission systems. The industry needed some critical changes in the network to accommodate growth and competition. Moreover, other changes were underway. In the late 1960s, competition began to appear in the long-distance network. This competition began to bring additional constraints inside the network to the surface.

Until recently, signal transmission consisted of voice communications, dial-up data communications, and leased lines. These variations all depended on the voice concept of analog communications. However, modern networks must go beyond just a voice networking standard and support many different types of signals. These demands required the development of new standards that used synchronous transmission capabilities throughout. The services also required openness, including

• Multivendor support

• Multivendor compatibility

• Ease of adding new services when they develop

• Enhancements to operation, administration, maintenance, and provisioning

• Midspan...