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Introduction to Digital Transport Systems

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The next generation of digital communication is here and taking over a network near you. Explore the history, the benefits, and the future of digital communication by examining the Synchronous Optical Network (SONET).

This chapter introduces the Synchronous Optical Network (SONET) and T1 technology. A brief history of SONET is provided, as well as the reasons that SONET came into existence. We also provide a brief history of the T1 system and make a general comparison of T1 and SONET. As a prelude to later chapters, a general description is provided of the major features of SONET.

What are SONET and T1

Digital carrier systems, such as the well-known T1 technology, have served the telecommunications industry well for over 40 years, and they shall continue to do so for quite some time. T1 was first installed in 1962 to provide a high-speed (1.544 Mbit/s) digital carrier system for voice traffic. It was modified later to support data and video applications.

In terms of communications technology, 1962 is a very long time ago. Since this date, extraordinary progress has been made in the fields of computers and communications. Many of the technical underpinnings of SONET exploit this new technology.

We have cited T1 in a previous paragraph to help explain the nature of SONET. T1 and associated systems (such as T3 and similar technology in other parts of the world) are first-generation digital transport systems. SONET is a second-generation digital transport system. Like T1, its purpose is to transport, multiplex, and switch digital signals representing voice, video, and data traffic to and from users' applications.

However, T1 and SONET differ significantly in how they accomplish these functions. Many facets of the T1 architecture are based on technology that is over four decades old. In contrast to T1, the SONET architecture is based on the technology of today. With this brief comparison in mind, let us take a look at how SONET came into existence and then examine some of the major features of SONET.

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