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5.0 Reassembly Coprocessor
5.4 Buffer Management
RS8234
Datasheet for RS8234 xBR ServiceSAR
N8234DS1B
bits set to a logic high. EPD functions are described in Section 5.4.8. Early Packet
Discard. Also, if a buffer queue empty condition initially occurs at the beginning
of a BOM cell, a status queue entry is written with UNDF set to a logic high and
BD_PNTR null. In both cases, the RSM_HF_EMPT bit is set in the
HOST_ISTAT1 and LP_ISTAT1 registers if the BD_LOCAL bit is a logic low in
the free buffer queue base table, or the RSM_LF_EMPT bit is set if BD_LOCAL
is a logic high.
All cells of a PDU up to and including the next EOM cell are discarded. Upon
receiving a BOM or SSM cell, the reassembly coprocessor checks the queue indi-
cated by BFR0 for a valid free buffer. If a free buffer exists, the Rsm coprocessor
stores the cell in the assigned buffer.
For AAL5 channels, the AAL5_DSC_CNT counter is incremented for each
CPCS_PDU discarded during this error condition.
Note that channels that have outstanding buffers from an empty queue are not
affected until they need a new buffer. Once the Host has written more free buffers
on the queue with VLD bit set to a logic high, the reassembly coprocessor will
automatically recover from the empty condition.
5.4.8 Early Packet Discard
The packet discard feature provides a mechanism to discard complete or partial
CPCS-PDUs, based upon service discard attributes or error conditions.
5.4.8.1 General
Description
The EPD feature performs these basic functions:
Halts reassembly of the CPCS-PDU marked for discard until the next
BOM cell and the error condition has cleared.
Writes a status queue entry with the EPD bit set and other appropriate
STATUS and PDU_CHECKS bits set, based on the reason for the discard.
5.4.8.2 Frame Relay
Packet Discard
The frame relay discard attribute is contained in the BOM cell of a CPCS-PDU. If
the FRD_EN bit in the Rsm VCC Table is a logic high, the frame relay packet dis-
card function for that VCC is enabled, and the functions below are performed.
When the reassembly coprocessor receives a BOM cell on a VCC with this
feature enabled, it checks the 1-bit Discard Eligibility (DE) field in the frame
relay header. If this bit is a logic high, and the channel priority (the DPRI in the
Rsm VCC Table) is less than or equal to the global priority, RSM_CTRL
(GDIS_PRI), then the Rsm coprocessor discards the cell, marks the rest of the
packet for discard, and increments the SERV_DIS counter in the VCC Table. All
cells on that channel up to the next BOM are discarded.
If the SERV_DIS counter rolls over, the CNT_ROVR bit in the next status
entry for this channel will be set to a logic high. The CNT_ROVR bit in the VCC
Table holds this flag information until a status is sent.
5.4.8.3 CLP Packet
Discard
If the CLPD_EN bit in the Rsm VCC Table is a logic high, the Cell Loss Priority
packet discard function for that VCC is enabled, and the functions below are per-
formed.
When the Rsm coprocessor receives a cell and this function is enabled, it
checks the 1-bit CLP field in the ATM header. If this bit is a logic high, and the
channel priority is less than or equal to the global priority, RSM_CTRL
(GDIS_PRI), then the Rsm coprocessor discards the cell, marks the rest of the