This selects the clock source for a DS1 port: loopTiming derives the transmit clock from the recovered receive clock, localTiming uses a local or externally attached clock source, and throughTiming derives the transmit clock from another interface's recovered receive clock. Getting timing configuration right across a TDM network is critical, since clock mismatches cause slips and errored seconds. An admin would set one DS1 port to localTiming as the timing master and others to loopTiming or throughTiming to follow that reference.