csUnchanDs3ConfigClockSource is a read-write setting choosing whether this unchannelized DS3's transmit clock comes from an internal local oscillator or is recovered from the received line signal. Exactly one end of any DS3 link must use internal timing while the other uses line (recovered) timing; getting this backwards on both ends causes clock slips and errored seconds even when the cabling itself is fine. For example, if a customer-premises DS3 device is set to internal and the carrier's demarc is also internal, the mismatch causes clock-slip errors until one side is switched to line.