This attribute indicates the current number of receive frames which
are in use for this FP.
On CQC-based ATM FPs, this number indicates the total receive
frames which are in use for all ports on this FP.
On ATM IP FPs, this number indicates the number of receive
frames which are in use in the Passport Queue Controller (PQC)
ASIC. Queuing of received frames only occurs in the PQC.