The source of Transmit Clock. For the logicalDs1, the
timing will be throughTiming. This is not currently a
configurable parameter.
The options are:
'loopTiming' indicates that the recovered receive clock
is used as the transmit clock.
'localTiming' indicates that a local clock source is used
or when an external clock is attached to the box containing
the interface.
'throughTiming' indicates that recovered receive clock from
another interface is used as the transmit clock.