ecommunication 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 ADVANCED BROADBAND DIGITAL TRANSPORT FORMATS 19.2 SONET 19.2.1 Introduction and Background The SONET standard was developed by the ANSI TIX1 committee with first publication in 1988. The standard defines the features and functionality of a transport system based on the principles of synchronous multiplexing. In essence this means that individual tributary signals may be multiplexed directly into a higher rate SONET signal without intermediate stages of multiplexing. DS1 and E1 digital hierarchies had rather limited overhead capabilities for network management, control, and monitoring. SONET (and SDH) provides a rich built-in capac- ity for advanced network management and maintenance capabilities. Nearly 5% of the SONET signal structure is allocated to supporting such management and maintenance procedures and practices. SONET is capable of transporting all the tributary signals that have been defined for the digital networks in existence today. This means that SONET can be deployed as an overlay to the existing network and, where appropriate, provide enhanced network flexibility by transporting existing signal types. In addition, SONET has the flexibility to readily accommodate the new types of customer service signals such as SMDS (switched multimegabit data service) and ATM (asynchronous transfer mode). Actually, it can carry any octet-based binary format such as TCP/IP, SNA, OSI regimes, X.25, frame relay, and various LAN formats, which have been packaged for long-distance