Synchronization is based on a global timer .
All single medium objects are attached to a time axis that represents an abstraction of real-time. Each objects can map this world time to its local time and moves along its local time axis. If skews occur, then we need resynchronization mechanism with the world time. This type of synchronization specification is very good for closed LDUs, since synchronization can only be defined based on fixed points of time, problems arise when open LDUs are used.