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5.0 Reassembly Coprocessor
5.5 Global Statistics
RS8234
Datasheet for RS8234 xBR ServiceSAR
N8234DS1B
then discards cells on that channel, until the channel has recovered from the fire-
wall condition.
All AAL5 PDUs discarded under the firewall condition cause the
AAL5_DSC_CNT counter to be incremented. Recovery occurs only on a BOM
or SSM cell when the credit is rechecked.
5.4.11.3 Credit Return
The user returns credit, at the same time the buffer is recovered to the free buffer
queue, by writing the third word of the free buffer queue. The VCC_INDEX is
written to the channel to which credit is returned. The FWD_VLD bit is set to a
logic high, and the QFC bit is set to a logic low. The Rsm coprocessor increments
the RX_COUNTER[15:0] of the applicable channel. For proper operation of the
update interval function, buffers must be returned at the same time as credits are
returned.
Credits are returned to VCCs through Bank 0 free buffer queues. In order to
return buffer credits independently from buffer usage, the RS8234 maintains a
separate read pointer into free buffer queues which return credits. This pointer
name is FORWARD, in the free buffer queue base table entry. The Host deter-
mines the number of Bank 0 free buffer queues which return credits by setting
FWD_EN in the RSM_FQCTRL register.
The RS8234 “snoops” writes to free buffer queues that return firewall credits.
When a write completes, the RS8234 will begin processing firewall return credits
on that queue. The third word of each entry will be read, and if FWD_VLD is set,
a credit will be added to the VCC_INDEX indicated. The RS8234 will continue
to process credit return entries until FWD_VLD is zero. Multiple free buffer
queues can have credit return entries outstanding at one time. The RS8234 will
process the entries according to the priority set in FWD_RND in the
RSM_FQCTRL register. If FWD_RND is a logic low, the RS8234 will exhaust
the credit returns on the highest number active queue before proceeding to other
queues. Otherwise, it will service the queues in round-robin order.
Before the reassembly coprocessor is enabled, the host must initialize the
FORWARD read pointer to the first entry where credit will be returned. Typically,
this will be the first entry after the initial buffers placed on the queue.
5.5 Global Statistics
To meet the requirements of ILMI (ATM Forum) and AToM (RFC1695) docu-
ments, three register based counters are implemented. They are as follows:
CELL_RCVD_CNT - Number of cells received that map to active chan-
nels.
CELL_DSC_CNT - Number of cells received that map to inactive chan-
nels. This includes idle cells, since those channels will be turned off.
AAL5_DSC_CNT - Number of AAL5 CPCS-PDUs discarded due to per
channel firewall, buffer queue underflow, FIFO full packet discard, status
queue overflow, or maximum CPCS-PDU length exceeded on non-EOM
cells.