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.