PPD — An ATM cell discarded in the middle of an AAL5 packet makes the entire packet unusable. The PPD mechanism attempts to minimize the congestion problems that can occur at the higher layers (TCP) because of ATM cell discards.
With PPD, when a cell is discarded by the ATM policing function, no other cells for that PDU are accepted, with exception of the ‟tail cell” that is sent to inform the far end that the end of a frame has arrived.
PPD is enabled on the ATM SAP part of an AAL5 SDU Apipe VLL service and on all services where the ATM SAP cell stream is reassembled. (IES, VPRN, VPLS, Epipe, and Apipe services)
WRED — Congestion and potential discards are performed on per forwarding class basis in the SAP queues in the IOM; packets that are reassembled in the ATM-capable MDAs take advantage of the application of the WRED congestion avoidance service queues associated with the ATM SAP.
Depending on the type of XMA or MDA that supports ATM encapsulation, other XMA- or MDA-specific packet congestion control mechanisms operating on per-SAP queues in the XMA or MDA are also applied.