Introduction to SONET:

Synchronous optical network (SONET) is a standard for optical telecommunications transport. It was formulated by the ECSA for ANSI, which sets industry standards in the United States for telecommunications and other industries. The comprehensive SONET/synchronous digital hierarchy (SDH) standard is expected to provide the transport infrastructure for worldwide telecommunications for at least the next two or three decades.

The increased configuration flexibility and bandwidth availability of SONET provides significant advantages over the older telecommunications system. These advantages include the following:

· Reduction in equipment requirements and an increase in network reliability.

· Provision of overhead and payload bytes-the overhead bytes permit management of the payload bytes on an individual basis and facilitate centralized fault sectionalization

· Definition of a synchronous multiplexing format for carrying lower level digital signals (such as DS-1,DS-3) and a synchronous structure that greatly simplifies the interface to digital switches, digital cross-connect switches, and add-drop multiplexers

· Availability of a set of generic standards that enable products from different vendors to be connected

· Definition of a flexible architecture capable of accommodating future applications, with a variety of transmission rates

In brief, SONET defines optical carrier (OC) levels and electrically equivalent synchronous transport signals (STSs) for the fiber-optic-based transmission hierarchy.