This object selects the SONET transmit clock source: loopTiming(1, the default, synthesized from the recovered receive clock), throughTiming(2, derived from another SONET interface's recovered clock), localTiming(3, from a local or externally attached clock), or external155MHz(4, from an external 155.52 MHz reference, not valid on Sechtor 100). Admins choose loopTiming for a simple line-timed interface, or localTiming/external155MHz when the device needs to be the master timing source for the network. Setting this to localTiming(3) makes the interface transmit using the box's local clock source instead of deriving timing from the received signal.