e 1980s, the industry started to under- stand that standardization across vendors was desirable. In addition to being based on copper/coaxial systems, the traditional digital hierarchy suf- fered two additional drawbacks: 1. The speeds (up to DS3C or 90 Mbps) were not adequate for fiber. 2. The hierarchy was asynchronous (that is, it did not rely on a master clock). SONET (North America) and Synchronous Digital Hierarchy (SDH) (the rest of the world) were developed to address these issues. SONET/SDH standards define equipment interoperability and help facilitate the trans- lation of electrical signals to optical signals for transmission over an optical waveguide. The standards require SONET/SDH equipment to include essential multiplexing features, transmission features, and Operations, Administration, Management, and Provisioning (OAM&P) features, such as restoration, signaling, performance monitoring, and fault management. The widespread use of fiber has been made possible in part by the indus- try's acceptance of SONET/SDH as the standard for transmission functions (user plane) and OAM&P functions (management plane). Work on SONET/SDH standards started in the mid-1980s, but it was only in the late 1980s and early 1990s that they achieved market acceptance. Traditional SONET Architectures Throughout the 1990s, with the commercialization of the Internet, there was a major demand for additional bandwidth. (The growth of the Internet has been variously quoted as "100 percent a quarter"-the so-called Inter- net year-or more conservatively as "100 percent a year.") Using SONET/SDH standards, telecommunication companies have gradually expanded their capacity by increasing data transmission rates to the point that many carriers now routinely transport 2.5 Gbps on fiber (STM-16/OC- 48), and the deployment of 10 Gbps (STM-64/OC-192) is now underway. Work on 40-Gbps STM-256/OC-768 systems is just beginning. During the late 1990s, bandwidth demand was approaching the maxi- mum capacity available in typical networks and car