Description: This attribute defines the type of clocking source used for synchronizing the OC-3 transmit clock: local means direct use of the local clock oscillator at its centre frequency; line means the use of a signal phase-locked to the incoming OC-3 line rate; module means the use of a signal phase-locked to the CP reference clock. When adding a Sonet/Sdh component, the clockingSource attribute is set to module if the NetworkSynchronization component has been added and is set to local otherwise. Note that the multiport OC-3 cell-based cards has one common clock source for each of the three ports when either the clocking source local or module is selected. For this reason, it is not allowed to have at the same time the clockingSource attribute set to local on one port and module on another port. Furthermore, the clockingSource attribute cannot be set to module if the NetworkSynchronization component has not been added.