5-181/19/05 ATM Services Configuration Guide for CBX 3500, CBX 500, GX 550, and B-STDX 9000
ATM FCP Discard Mechanisms
Beta Draft Confidential
ATM FCP Discard Mechanisms
The ATM FCP supports three mechanisms for discarding cells:
Early Packet Discard (EPD) — The ATM FCP performs EPD for UBR, ABR, and
VBR-NRT VCs. If a cell causes the queue for a VC to exceed the discard thresholds,
the VC enters the EPD state. The cells in the current packet of the VC are admitted to
the queue. However, when the end of the current packet is detected, all of the cells in
the next packet are discarded for that VC.
Selective Cell Loss Priority 1(CLP1) Discard — Selective CLP1 discard can be
provisioned for UBR, ABR, and VBR-NRT VCs. If the current cell causes the queue
for a VC to exceed the discard thresholds and the cell has CLP set to 1, then the cell is
discarded. Note that EPD is not performed in this case.
Partial Packet Discard (PPD) — If the global CLP0+1 threshold for a port is
reached, PPD is performed for circuits that are configured for EPD. However, unlike
EPD, all of the remaining cells in the current packet are discarded. Note that the End
of Frame (EOF) cell is discarded as well. This results in the loss of the next packet
even if the packet is transmitted.
Note – The PPD results in no further throughput for this circuit if both of the
following conditions occur:
• A circuit is set for EPD and does not send ATM Adaptation Layer 5 (AAL-5)
protocol data units (PDUs) (for example, AAL0 data)
• A port becomes sufficiently congested (CLP 0+1 threshold is reached)
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