RS8234 Mindspeed Technologies, RS8234 Datasheet - Page 187

• Manufacturer Part Number: RS8234
• Description: Atm Servicesar Plus With Xbr Traffic Management
• Manufacturer: Mindspeed Technologies

RS8234
ATM ServiceSAR Plus with xBR Traffic Management
28234-DSH-001-B
6.3.6.2 ABR Cell
Table 6-4. ABR Cell Type Decision Vector (ACDV)
3
2
1
0
Bit
Decisions
TRM_EXP
TA_PND
TA_XMIT
RUN
Name
The Cell Decision Tables reside in SAR shared memory as a segment of the Flow
Control Manager’s ABR Instruction Table. Each entry of a table is an ABR Cell
Decision Block (ACDB). Each ACDB contains 16 Cell Type Actions. Cell Type
Actions result in one of four decisions: send in-rate Forward RM cell, send in-rate
Backward RM cell, send user data cell, or no action. A Cell Type Action is chosen
by a 4-bit ABR Cell Type Decision Vector (ACDV). The four bits of the ACDV
correspond to four current ABR VCC conditions.
Initialization Instructions
At system initialization, the Host loads one of more Cell Decision Tables into
SAR shared memory. Mindspeed provides standard Cell Decision Tables. The
system designer either uses these, or customized templates.
at connection setup time. The first field, CELL_INDEX, tracks the current
ACDB position of the VCC. Source Behavior #2 specifies that the first in-rate
cell on a VCC is a Forward RM cell. Therefore, CELL_INDEX should be
initialized to an ACDB position which always decides to send a Forward RM cell.
The second field, FWD_INDEX, provides a reset point for the in-rate cell stream
sequence. Whenever the RS8234 transmits an in-rate Forward RM cell, it copies
FWD_INDEX to CELL_INDEX.
Run-time Operation
Each time an ABR cell transmit opportunity arises, the RS8234 makes a cell
decision. The SAR retrieves CELL_INDEX from SCH_STATE and chooses a
Cell Type Action location within the current ACDB. Table 6-4 shows the four
conditions which are used to form this vector. The presence of a condition sets the
vector bit to a one.
an in-rate Forward RM cell is transmitted, FWD_INDEX is copied to
CELL_INDEX; otherwise, CELL_INDEX is incremented by one
(CELL_INDEX++). Therefore, the number of blocks in a Decision Table is
bounded by the maximum number of cell decisions made between transmitted
Forward RM cells, typically Nrm.
To assign an ABR VCC to a table, the Host initializes two SCH_STATE fields
The SAR updates the CELL_INDEX field after each cell decision is made. If
Time since last forward RM transmitted >= TRM
TA_PND bit set in VCC Table entry
TA_XMIT bit set in VCCTable entry
RUN bit set in VCC Table entry
Mindspeed Technologies
Description
™
6.3 ABR Flow Control Manager
6.0 Traffic Management
6-27