This attribute indicates the current number of transmit cells which
are in use for this FP.
On CQC-based ATM FPs, this number indicates the total transmit
cells which are in use for all ports on this FP.
On ATM IP FPs, this number indicates the number of transmit cells
which are in use in the Passport Queue Controller (PQC) ASIC.
Queuing of ATM cells for transmit normally occurs in the ATM
Queue Manager (AQM) ASIC, which is indicated in the Lp Eng Arc
Aqm/x txCellMemoryUsage attribute. Transmit cells should only be
queued in the PQC under circumstances of extreme transient
congestion.