RS8234EBGC Mindspeed Technologies, RS8234EBGC Datasheet - Page 194

• Manufacturer Part Number: RS8234EBGC
• Description: RS8234EBGC ATM XBR SAR
• Manufacturer: Mindspeed Technologies

6.0 Traffic Management
6.3 ABR Flow Control Manager
6.3.7.4 Forward RM Cell
6-34
Transmission Decisions
Based on Congestion in
6.3.7.5 Optional Rate
Turnaround RM Cells
Adjustment in
the Switch
The RS8234 also adjusts the transmission rates of an ABR in-rate cell stream
before sending a Forward RM cell. This adjustment utilizes a Rate Decision
Vector selection process. As stated above, four of the eight ARDB vectors are
used for CI/NI rate adjustment. The other four are used for Forward RM cell rate
adjustments.
the ADTF timer and the Crm counter. These internal measures may force the
connection to decrease its rate when sending an RM cell.
table entry (FWD_ER). The normal value for FWD_ER is PCR. When this field
is initialized in the VCC Table entry, it should be set to the rate specified by the
exponent table entries so that the resulting selected rate is PCR. This is because
when a Forward RM cell is eventually received as a Backward RM cell, the
RS8234 will map the available cell rate (ACR) to a rate specified by the exponent
table. If PCR falls between two exponent table rates and FWD_ER is set to PCR,
the ACR of the connection will be limited to the lower of the two exponent table
rates, thereby lowering the rate below PCR.
The system designer has the option of dictating a lowered explicit rate in
Turnaround RM cells, on any VCC which encounters congestion in the free buffer
queue for that VCC.
FBQ_CNG[31:0]; one bit for each of the 32 free buffer queues. The Host sets the
appropriate bit in this register field for any free buffer queue experiencing
congestion (i.e., an Empty condition).
the CONG_ID field in the VCC’s SCH_STATE to the value of that FBQ ID.
NOTE:
that rate’s ABR Rate Decision Block (ARDB) to one. As a general rule, the
designer would set the CONG_ENA bit in those ARDBs describing higher rates,
when switch congestion is more likely to occur.
assigns a new lower explicit rate for congestion condition, and stores this rate in
the CONG_ER field of the ARDB.
for that rate, the RS8234 uses the CONG_ID value to look at the corresponding
SCH_CNG(FBQ_CNG[31:0]) bit to determine if it is set to logic high. If it is, the
RS8234 assigns the lowered rate in the CONG_ER field as the new explicit rate
in the Turnaround RM cell.
Instead of the CI/NI bits, the Forward RM cell vector selection is based upon
The ER for Forward RM cells on any channel is set in word 18 of the VCC
The Schedule Congestion register, SCH_CNG, includes a 32-bit field,
The system designer can map each VCC to its free buffer queue ID by setting
The designer enables this option for any rate by setting the CONG_ENA bit in
In each of the ARDBs that have the CONG_ENA bit set, the designer also
During operation, when a Forward RM cell is received and CONG_ENA=1
employ some other mapping strategy than the above.
This is a suggested mapping only. The system designer may wish to
Mindspeed Technologies
ATM ServiceSAR Plus with xBR Traffic Management
™
28234-DSH-001-B
RS8234