Page "Asynchronous Transfer Mode" Paragraph 28

If the traffic on a virtual circuit is exceeding its traffic contract, as determined by the GCRA, the network can either drop the cells or mark the Cell Loss Priority ( CLP ) bit ( to identify a cell as potentially redundant ).

Basic policing works on a cell by cell basis, but this is sub-optimal for encapsulated packet traffic ( as discarding a single cell will invalidate the whole packet ).

As a result, schemes such as Partial Packet Discard ( PPD ) and Early Packet Discard ( EPD ) have been created that will discard a whole series of cells until the next packet starts.

This reduces the number of useless cells in the network, saving bandwidth for full packets.

EPD and PPD work with AAL5 connections as they use the end of packet marker: the ATM User-to-ATM User ( AUU ) Indication bit in the Payload Type field of the header, which is set in the last cell of a SAR-SDU.