The reason for the most recent change of a source of
network clock, as indicated by the change in the value
of cncSourceChangeTimeStamp.
'none' indicates that the source of network clock
has not changed.
'lossOfLock' indicates that the clock source was
changed because the network clocking hardware lost
lock on the previous network clock source.
'lossOfActivity' indicates that the clock source was changed
because the network clocking hardware detected a loss of
activity on the previous network clock source.
'ncdpRestructure' indicates that the NCDP entity has changed
the clock source as a result of a network-wide network clock
distribution tree restructuring process.
When the reason for a clock switchover is none of the
above, the value of this object is 'other'.