e synchronous digital hierarchy (SDH) transport standard. Network elements based on earlier technology were inflexible to the extent that service provisioning and activation typically took weeks. The predecessor of SDH is called plesiochronous digital hierarchy (PDH). Plesiochronous means "almost synchronous". In particular, the difference between two signals is constrained to some limited time period. The introduction of SDH included new flexible and controllable NEs, such as digi- tal cross-connects (DXCs) and add/drop multiplexers (ADMs). The importance of 1 During 2001, the author participated in a Telemanagement Forum "tiger team" developing guidelines for policy-based management mechanisms for OSS. 2 Network Management OSS Overview Figure 2.9 SDH network. these developments was not lost on OSS vendors such as Clarity International, whose products have notable explicit support for these technologies. This will be seen in discussion of features of the Clarity Configuration Manager product in Chapter 11. Like many modern OSS, Configuration Manager embraces the combination of SDH, DXCs and OSS functionality. The advantages of SDH technology include: fast provisioning and activation of capacity from a central OSS automatic rerouting in case of a cable break, or some other transport problem round-the-clock quality of service (QoS) monitoring the possibility of optimizing route selection in the network after the network has been analysed for bottlenecks, and for over-sensitivity to particular kinds of disturbance. As mentioned in the Introduction, telecom networks nearly always include a combination of technologies through their evolution over several generations of transmission technology. Therefore, PDH often appears alongside SDH in the same network. Modern OSS must support NEs that are based on SDH and DXCs, but they can also communicate with PDH equipment using appropriate mediation devices. In fact, it is even possible to continue using the OSS that was in place before SDH was implemented. In that case, the 