The present document specifies the architectural principles that should be applied for the design of synchronization networks that are suitable for the synchronization of Synchronous Digital Hierarchy (SDH) and Plesiochronous Digital Hierarchy (PDH) networks. It supports the construction of synchronization networks that support both the short-term stability requirements of SDH networks and the long-term stability requirements of digital switching networks (e.g. PSTN) connected to the SDH network. It applies to the design of new synchronization networks. It does not characterize existing synchronization networks based on PDH networks.