This read-write value selects where the modem derives its timing clock from: its own internal oscillator, the remote end of the link, or an external clock source. Getting this right matters because only one end of a point-to-point E1 or T1 link should be the timing master, so admins set it to avoid clock slips that cause bit errors or dropped frames. For example, an admin might configure the local modem's clock source to internal(2) while setting the remote modem to remote(3), so the local unit acts as the timing master for the circuit.