This attribute holds the value 'noPeriodic' if the Periodic
state machine for the Aggregation Port is
in the NO_PERIODIC state, 'fastPeriodic' if the Mux state machine
is in the FAST_PERIODIC state, 'slowPeriodic' if the Mux state
machine for the Aggregation Port is in the SLOW_PERIODIC
state, or 'periodicTx' if the Periodic state machine
for the Aggregation Port is in the PERIODIC_TX state.
This value is read-only.