This read-write value forces a specific Synchronization Status Message as the received SSM on this interface, causing the interface to ignore the actual SSM in the incoming S1 byte; setting it to none clears any forced value. Admins use this during testing or to work around a misbehaving upstream timing source, by overriding what SSM value this interface reports as received. For example, an admin sets this to force a low-quality SSM value to prevent the switch from selecting this interface's timing as its primary reference during a lab test.