This variable identifies the type of a clock source that
does not have an 'ifIndex' associated.
This value is 'none(1) if 'cwnInterfaceIndex'
contains a non-zero value.
The value 'internalOscillator(1) indicates a clock source
is an oscillator local to the device.
The value 'bitsClockE1(2) indicates a Building Integrated
Timing Supply (BITS) clock source on an E1 port.
The value 'bitsClockT1(3) indicates a BITS clock source
on a T1 port.