Synchronization Processes and Jitter Generation along a SDH Transmission Chain: a Review and Measurement Results

New timing issues have been raised by Synchronous Digital Hierarchy (SDH), which implies new phenomena generating jitter and wander compared to those well known in legacy networks based on the Plesiochronous Digital Hierarchy (PDH). First, this paper reviews the bit and byte synchronization processes that take place in a SDH transmission chain: the mapping of PDH tributaries into SDH frames, the re-synchronization of Virtual Containers (VCs) in intermediate nodes, the demapping of PDH tributaries from SDH frames. Then, this paper reviews the main causes of jitter and wander in a SDH transmission chain, by outlining the different phenomena that may affect the phase of the tributaries transported. First, the diurnal and annual wander due to variations of environmental conditions (temperature) is considered. Then, the jitter and wander caused by inclusion of additional bits in the mapping of tributaries into STM frames, by bit justification and by pointer justification is addressed. Finally, real measurement results of phase hits caused by AU4 pointer adjustments on a commercial SDH system are also provided, to provide experimental support to the general treatise. Key-Words: digital communication, jitter, pointer, SDH, SONET, synchronization, wander.