This read-write enumeration selects the clock source, such as internal or external, used to time a WAN port, though on the processor/LAN interface board's first two ports the clock source is fixed by the unit and cannot be changed. An administrator configures this on other ports so the timing source matches whichever end of the link is expected to provide clocking. For example, a port connected to a network-provided clock source would be set to external rather than internal to avoid clock conflicts.