The highest stability synchronization reference is the Primary Reference Clock (PRC) specified by the ITU-T in Recommendation G.811 with a frequency accuracy of `10^-11`. This highly stable clock signal is distributed throughout the SDH network. The synchronization signal is distributed through the Synchronization Supply Units( SSU) and the Synchronization Equipment Clock( SEC). The clock used in the SDH hierarchy has different levels.
The primary reference( RPC) is standardized in ITU recommendation G.811, SSU in G.812 and SEC in G.813.