Network synchronization—Stand-alone products that support the design of synchronization networks

Network synchronization is a generic concept that depicts a way of distributing common time and frequency references to all the nodes of a network, to align their respective time and frequency scales (Box B). The role of network synchronization, especially in the context of frequency accuracy, has grown in importance since the late 1970s, when digital transmission and switching were introduced. Today it strongly influences the quality of most operator services. The effect of poor synchronization in a telecommunications network is strictly related to the services carried. Digital switching equipment, for example, must be synchronized to avoid slips (Figure 1)—although slips have little effect on plain voice service, they have major impact on circuitdata services (Box C). The deployment of synchronous optical network (SONET) and synchronous digital hierarchy (SDH) technologies in the 1990s put new and more stringent requirements on the stability of synchronization systems. This led to a large standardization effort, resulting in several new or updated recommendations. Network synchronization plays a particularly important role in GSM and WCDMA. Looking ahead, it is also plain to see that it will have a major role in packetswitching environments. Packet switching