This read-write value selects whether this DS1/T1 channel's transmit clock is generated internally or recovered from the received line signal, applicable only for T1. As with the parent DS3-level clock setting, exactly one end of a timing chain should typically be internal while others follow line clock to avoid slips. For example, an admin sets csDs3Ds1ClockSource to internal on the channel acting as the network's timing master, while downstream devices are set to line to follow that reference.