This read-write enumerated value sets where this modem derives its transmit clock from: its own internal oscillator, an external reference, or the clock recovered from the remote end. Admins configure this to avoid clock islands or timing loops on a link, since typically only one end should be the master, internal or external, clock source while the other tracks it as remote. For example, setting mdmClockSource to remote(3) on the far-end modem makes it slave its timing to the local modem, which itself might be set to internal(2) as the timing master.