Page 70 - Optical Switching And Networking Handbook

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SONET 55

the Exchange Carriers Standards Association (ECSA) for the Amer-
ican National Standards Institute (ANSI). This standard defines an
optical telecommunications transport for U.S. telecommunications.
The SONET standard provides an extensive set of operational para-
meters for optical transmission systems throughout the industry.
The North American industry uses the SONET specifications,
whereas the rest of the world uses a close “cousin” called the Syn-
chronous Digital Hierarchy (SDH), covered in Chapter 4, “Synchro-
nous Digital Hierarchy.” Between the two sets of standards, the
industry attempted to define the roles of transport for the telecom-
munications providers using optical fibers as the transport medium.
SONET provides more, though. It defines a means to increase
throughput and bandwidth through a set of multiplexing parame-
ters. These roles provide certain advantages to the industry, such as

Reduced equipment requirements in the carriers’ networks
Enhanced network reliability and availability

Conditions to define the overhead necessary to facilitate
managing a network better

Definitions of multiplexing functions and formats to carry lower-
level digital signals (such as DS-1, DS-2, and DS-3)

Generic standards encouraging interoperability between
different vendors’ products

A flexible means of addressing current as well as future
applications and bandwidth usage

SONET defines the optical carrier (OC) levels and electrical equiv-
alent rates in synchronous transport signals (STS) for the fiber-based
transmission hierarchy.

Background Leading to SONET

Development

Prior to the development of SONET, the initial fiber-based systems
used in the public switched telephone network (PSTN) were all
highly proprietary. The proprietary items included