The ifIndex of the interface from which clock can be derived. Only oc3, ds1, and ds3 interfaces can be used for this. This interface will be used to drive the 8khz backplane signal. The interface must be set to loop or local timing in order for it to be considered as the primary clock source. It must also be administratively turned on and be in an operational state. Only one interface in the system can be the primary clock source at any given time. NOTE: This value has been deprecated and should not be used. If specified, the system will use this interface as the clock source and will ignore the provisioning specified in the zhoneSystemClockTable.