cn8237 Mindspeed Technologies, cn8237 Datasheet - Page 194

• Manufacturer Part Number: cn8237
• Description: Atm Oc-12 Servicesar Plus With Xbr Traffic Management
• Manufacturer: Mindspeed Technologies

6.0 Traffic Management
6.3 ABR Flow Control Manager
6-38
Turnaround RM Cells
Implicit ER Modification
6.3.7.6 Rate
Adjustment in
The system designer has various options of effecting a lowered explicit rate in
Turnaround RM cells.
Added to word 10 of the RSM VCC table entry are EN_IMP_CHG and
CONG_ID. Added to word 9 is ERS_INDEX.
SCH_CNG(CONG_ID) is checked. If a logic high, the ER field is modified per
the mapping mechanism described below and written into TA_ER field of the
SEG VCC table entry. This mechanism provides an efficient per-connection
hardware reduction of ER values when turning around RM cells.
ER_HC_RATE table to provide programmable scaled reduction of the ER value
based on the value’s range. In addition, multiple ER_SHIFT tables and
ER_HC_RATE tables may be used to allow different VCCs to utilize different
mappings to ER_HC_RATE table entries. For example, different mappings may
be associated with various ABR templates. A 6-bit index, ERS_INDEX, located
in the RSM VCC table entry indicates which ER_SHIFT table to use. The
exponent portion of the ER value is used as an index into the ER_SHIFT table.
Thus, there are 32 entries in each ER_SHIFT table (see
contains two values, the first, HC_SHIFT, specifies the mantissa shift mask, and
the second, HC_INDEX, specifies the address of an entry into the ER_HC_RATE
table. For HC_SHIFT values larger than nine, no ER_HC_RATE table lookup is
performed, and the ER value from the incoming RM cell is written into the
TA_ER field in the SEG VCC table.
determined by:
mantissa at its bit position HC_SHIFT is zero, and the left otherwise. The new
reduced rate is then:
ER_SHIFT_B base address. Multiple ER_HC_RATE tables might be used.
(ER_SHIFT_B << 5 + HC_INDEX) + RM_CELL mantissa [8:1] >> HC_SHIFT
When EN_IMP_CNG is a logic high and a forward RM cell is received,
The implicit ER reduction mechanism uses a ER_SHIFT table and the
The number of rates in the ER_HC_RATE table for each exponent value is
The address of an ER_HC_RATE table entry is calculated as:
The right entry for this address is chosen, if the bit value of the RM_CELLS
The maximum index into the rate table has an offset of 4096 words to the
Mindspeed Technologies
2 ^^ HC_EXP × (1 + HC_MANT/512)
ATM OC-12 ServiceSAR Plus with xBR Traffic Management
2^^(9 >> HC_SHIFT)
™
Figure
28237-DSH-001-C
6-22). Each entry
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