 ers are through-timed. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Telephony Telephony One Final Thought: Next-Generation SONET As we have discussed, SONET and SDH were introduced in the 1980s as an optical solution for high-bandwidth vendor interoperability in optical networks. SONET transmission rates have steadily increased to OC-768 (40 Gbps), and the protocol was developed primarily to transport voice in fixed timeslots through the network, in retrospect making SONET inef- ficient for the transport of the increasing volumes of bursty traffic. Beginning in the 1990s, the Internet arrived, and with it a profusion of data transferred over the existing network infrastructure. STS-1 (51.84 Mbps) is the basic rate at which SONET operates, and higher- bandwidth services are delivered as multiples of STS-1. This technique, however, is less than ideal for the transport of multimedia data payloads, and when it is used, a variety of challenges appear. These challenges include: the need to identify techniques to "rescue" the stranded band- width that occurs when SONET is used for data transport without hav- ing to upgrade existing SONET ADMs and cross-connect boxes; identification of the most effective way to carry data traffic over SONET; the creation and deployment of a reasonable technique for dynamically and efficiently resizing allocated bandwidth without traffic loss; methods and procedures to ensure survivability of link groups in the event of a link loss; and processes to support multiple traffic types across the same physical infrastructure. The solution to this plethora of chal- lenges came in the form of Next-Generation SONET (NGS), and with NGS came a new set of terms: virtual concatenation, link capacity adjust- ment scheme (LCAS), and generic framing procedure (GFP). Virtual concatenation is based on the concept that a group of s