This attribute defines the type of clocking source used for
synchronizing the E3 transmit clock: local means direct use of the
local clock oscillator at its centre frequency; line means use of a
signal phase-locked to the incoming E3 line rate; module means the
use of a signal phase-locked to the CP reference clock.
Note that the multiport cell-based cards have one common clock
source for each of the three ports. For this reason, the following
rules apply for the value of clockingSource across the board's three
ports:
All three clockingSource values must be 'local' OR
All three clockingSource values must be 'module' OR
One clockingSource value must be 'line' and the other two must be
'otherPort'.
The default value for this attribute will be module if a CP clock is
available, and local otherwise.