In the early 1980s, fiber-optic transmission links burst upon the telecommunication transport scene. The potential bit rate capacity of these new systems was so great that there was no underlying digital format to accommodate such transmission rates. The maximum bit rate in the DS1 family of digital formats was DS4 at 274 Mbps; and for the E1 family, E4 at 139 Mbps. These data rates satisfied the requirements of the metallic transmission plant, but the evolving fiber-optic plant had the promise of much greater capacity, in the multigigabit region.
In the mid-1980s ANSI and Bellcore began to develop a new digital format standard specifically designed for the potential bit rates of fiber optics. The name of this structure is SONET, standing for Synchronous Optical Network.
As the development of SONET was proceeding, CEPT1 showed interest in the development of a European standard. In 1986 CCITT stepped in proposing a singular standard that would accommodate U.S., European, and Japanese hierarchies. This unfortunately was not achieved due more to time constraints on the part of U.S. interests. As a result, there are two digital format standards: SONET and the synchronous digital hierarchy (SDH) espoused by CCITT.
It should be pointed out that these formats are optimized for voice operation with 125-^sec frames. Both types commonly carry plesiochronous digital hierarchy (PDH) formats such as DS1 and E1, as well as ATM cells.2
In the general scheme of things, the interface from one to the other will take place at North American gateways. In other words, international trunks are SDH-equipped, not SONET-equipped. The objective of this chapter is to provide a brief overview of both SONET and SDH standards.
1CEPT stands for Conference European Post & Telegraph, a European telecommunication standardization agency based in France. In 1990 the name of the agency was changed to ETSI—European Telecommunication Standardization Institute.
2Held (Ref. 9) defines plesiochronous as "a network with multiple stratum 1 primary reference sources." See Section 6.12.1. In this context, when transporting these PCM formats, the underlying network timing and synchronization must have stratum 1 traceability.
Fundamentals of Telecommunications, Second Edition, by Roger L. Freeman ISBN 0-471-71045-8 Copyright © 2005 by Roger L. Freeman
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