 that new standards were required to meet future requirements. Problems with the PDH standards include the following: Access to a tributary rate requires step-by-step demultiplexing because of stuffing (justification). Optical interfaces are not standardized but vendor specific. To use optical cables, a separate multiplexer for each level (e.g., mul- tiplexing from 2 to 140 Mbps in European PDH requires 21 pieces of multiplexing equipment) and separate line terminals are needed. Transmission American and European standards are not compatible. Network management features and interfaces are vendor dependent. High data rates (above 140 or 274 Mbps) are not standardized. ANSI started to study a new transmission method in the middle of the 1970s to utilize optical networks and modern digital technology more effi- ciently. This system is called the synchronous optical network (SONET) and it is used in the United States. ITU-T made its own worldwide standard, called SDH, by the end of the 1980s. SDH is actually an international extension of SONET and it was based on SONET but adapted to European networks. A subset of SDH rec- ommendations from the ITU-T was selected as a standard for the European SDH by ETSI. You might say that there are two different synchronous opti- cal systems: SONET in the United States and SDH in areas of Europe where European standards have been adapted. The operating principles of SONET and European SDH are quite similar and they use the same data rate at some levels, as shown in Table 4.3. Figure 4.26 shows data rates for European SDH as well as an example of SDH equipment. SDH is a standardized multiplexing system for both ple- siochronous tributaries, for example, 1.5, 2, or 34 Mbps, and synchronous tributaries. The main advantages of SDH over PDH standards are as follows: The data rates for optical transmission are standardized (i.e., vendor independent). Table 4.3 Data Rates of SONET (United States) and Corresponding SDH Data Streams (Europe) STS-N OC-N Optical Electrical Data Rate Carrier 