This read-write enumerated object (internal/slave/manual) specifies where the T1 interface derives its transmit clock from, defaulting to internal. An admin sets this based on network topology, typically using slave(2) to recover timing from the far-end/network clock and internal(1) only when this device is meant to be the timing source. For example, a branch-office router would typically use slave(2) clock mode to synchronize to the carrier's network clock rather than generating its own.