TABLE OF CONTENTS P. Stephan Bedrosian,
Agere Systems
This chapter provides an overview of issues that must be addressed when designing synchronization functionality used in SONET/SDH systems and networks.
In order to understand the place that timing plays in a SONET/SDH network or system, the following questions must be addressed first.
Why is synchronization needed? Synchronization is a means of defining or decoding data. As such, timing imperfections can cause data errors. Therefore, an accurate clock is needed for accurate data recovery.
How is synchronization administered? Synchronization flows through a hierarchical structure. In North America, this hierarchy is divided into discrete categories of accuracy or quality called stratum levels. Clocks of a particular stratum level may provide timing signals to other clocks of an equal or lesser stratum. In this way, the quality of the timing distribution is controlled.
Why is source traceability important? The term traceability is commonly used in timing terminology to describe the synchronous nature of a timing signal. However, traceability alone is not sufficient to establish its suitability for a particular timing application. Rather, the origin or source traceability of the timing information is of great importance. Source traceability provides a means for determining the flow of a timing signal. Such knowledge is important for preventing timing loops, which essentially destroy the integrity of timing signals and lead to the impairment of data transport.
A high-level symbolic representation of synchronization and its role in accurate data recovery is presented...
