SDH combines n signals with a bit rate of b to form data streams with bit rates of n x b on a synchronously clocked network. Unlike PDH, the individual transmission paths have minimal clock discrepancies. The synchronous mode of operation allows low-order multiplex systems, such as communication links for telephone systems, to be inserted in higher hierarchy levels and then removed again via add and drop. In accordance with the levels defined by the standard, SDH recognises different hierarchies such as STM-1, STM-4, STM-16 or STM-64 (155 MBits, 622 MBits, 2,488 MBits, 9,953 MBits) and more. About five per cent of the gross data rate is reserved for OAM tasks (Operations, Administration and Maintenance). The data is transparently transported over the SDH network in containers.