This attribute is HISTORICAL and is replaced by
txCellMemoryUsage.
This attribute is only valid for AtmIf's on ATM IP FPs. It is not
displayed for AtmIf's on other card types.
This attribute indicates the current amount of transmit cells
available for this AtmIf. Transmit cell memory is the memory
available for storing ATM cells to be transmitted by connections
under this AtmIf.