As detailed above, the normal mode of operation, known as locked mode is where a clock has its frequency phaselock to an input reference signal. It is this mode of operation that has been modelled, If the quality of the synchronisation inputs falls below accepted levels, Synchronisation Supply Units and SDH equipment clocks first use stored data to control phase and frequency variations. This mode of operation is known as holdover mode. If Synchronisation Supply Units and SDH equipment clocks cannot access stored data, or have never had a reference input, they are able to use their own internal clocks to generate the synchronisation signal until the network is restored. This is known as free running mode.