The method used to distribute network clock for the device.
When the mode of operation is 'ncdp', the services provided
by the NCDP entity are in use. When this is the case, NCDP
entities residing on adjacent nodes exchange protocol
messages that are used by the resident NCDP entity to select
clock source such that a network-wide clock distribution
tree is constructed. When the mode of operation is manual,
the NCDP entity is not in use and sources of network clock
are statically configured. When this is the case, the
construction of a network-wide clock distribution tree is
done manually by a network administrator or NMS
application.