Clock Types

 There is a hierarchy of three types of clock that have been defined by ETSI [1]. These are:

• Primary Reference Clocks
• Synchronisation Supply Units
• SDH Equipment Clocks

Primary Reference Clocks are at the top of the synchronisation network. They have the most stringent performance characteristics. There is typically one clock per network with perhaps a backup. Synchronisation Supply Units are generally present at major network nodes and are responsible for selecting one of a number of synchronisation signals, cleaning up the selected signal and then distributing the signal to elements within a node. This normal mode of operation is known as locked mode. SDH Equipment Clocks are the internal clocks for individual SDH elements. They are used to provide timing for the outgoing STM-N interfaces in the network. As with Synchronisation Supply Units, they select one of a number of possible synchronisation signals and filter the signal before distributing it to individual network interfaces. Synchronisation can also be delivered via a PDH network link. The standards for the delivery of synchronisation signals fall out of the scope of the standards being examined.