This read-only value reports the source of the transmit clock for this DS1 interface: loopTiming (the recovered receive clock is reused as the transmit clock), localTiming (a local clock source is used), or throughTiming (a recovered receive clock from another interface is used as the transmit clock). Clock source selection is critical in DS1 networks because exactly one device in a chain should be the timing master; getting this wrong across multiple devices causes slips (the dsx1TotalCSSs counter) and bit errors from clock drift. An engineer chasing a DS1 line showing periodic Controlled Slip Seconds would check dsx1TransmitClockSource to confirm the timing hierarchy is correctly configured, e.g., a CPE device correctly set to loopTiming(1) so it derives timing from the network rather than conflicting with it via localTiming(2).