This selects the source of the transmit clock for a SONET interface, either a locally generated clock or the clock recovered from the incoming receive signal. Admins typically set exactly one device in a point-to-point SONET link to local(1) as the clock master and the other to recovered(2) to avoid clock domain conflicts; this object is marked deprecated in the MIB. Configuring qbSonetPortTxClockSource as recovered(2) on a remote-site interface makes it slave its transmit timing to the clock received from the central-office device.