Indicates which physical timing source is bound to this node.
External BITS sources are used by MTA 10's, in T1 based chassis'.
External SETS sources are used by MTA 2X's, in E1 based chassis'.
REFCLK's are based upon timing recovered from a CNS or PNS Server card.
HOLDOVER mimics the last known good source before it failed.
OSCILLATOR is used when no other valid sources exist on the currently active
MTA.