RS8234EBGC Mindspeed Technologies, RS8234EBGC Datasheet - Page 195

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

RS8234
ATM ServiceSAR Plus with xBR Traffic Management
6.3.8 Boundary Conditions and Out-of-Rate RM Cells
28234-DSH-001-B
6.3.8.1 Calculated Rate
6.3.7.6 Optional Rate
6.3.8.2 Out-of-Rate
6.3.8.3 Out-of-Rate
Backward RM Cells
Adjustment Due to
Use-It-Or-Lose-It
Forward RM Cell
Boundaries
Generation
Behavior
The Use-It-Or-Lose-It behavior defined in TM 4.1 describes a rate adjustment for
a VCC that has a high ACR but is not actually using that bandwidth in
transmission. In this case, the ACR is lowered to the level of ICR for that channel.
allows the system designer to exactly tailor its operation. It uses a simple timeout
mechanism, dictatable for each explicit rate.
setting the ACR_ENA bit to a logic high in each of the corresponding ABR Rate
Decision Blocks. As a general guideline, this bit would only be set in those
ARDBs dictating rates above ICR.
two other values: ACR_TO (the trigger time for the timeout value when the rate
adjustment function activates), and ACR_INDEX (the new lower explicit rate
index to be established when the use-it-or-lose-it behavior is triggered).
Time Factor (ATDF), and ACR_INDEX should be patterned on the rate index for
ICR.
compares RM_TIME (the schedule slot count at the time the last Forward RM
cell was generated) with ACR_TO. If the trigger point for the timeout has been
reached, the RS8234 generates a new Forward RM cell with the ACR_INDEX
value as the assigned new explicit rate.
When the system designer pre-calculates the rates for the ARDB tables, the
designer must tailor those rates to fall within the two obvious upper and lower rate
boundaries—the lower rate boundary should not fall below MCR, and the upper
rate boundary should not be calculated above PCR.
A mechanism must exist to restart scheduling of a VCC once the rate on that
channel has dropped to zero, or is below the schedule table minimum rate (i.e.,
the rate is less than one schedule slot on the schedule table). Out-of-rate Forward
RM cell generation provides this mechanism.
SCH_ABRBASE(OOR_ENA) to a logic high.
Rate Decision Block (ARDB). This enables out-of-rate Forward RM cell
generation for that rate.
scheduling of the VCC has halted, and SET_OOR at that rate = 1, the RS8234
sets the SCH_OOR bit in the VCC’s SCH_STATE to a logic high. The RS8234
will then generate an out-of-rate Forward RM cell on that channel.
as follows:
The system designer can set the TA_OOR bit to a logic high in any ABR Rate
Decision Block. This enables the RS8234 sending a Backward RM cell
out-of-rate when there is a pending Turnaround RM cell scheduled at that rate but
not yet sent, and another Forward RM cell is received.
The RS8234’s implementation of the TM 4.1 Use-It-Or-Lose-It behavior
The designer selectively enables this function for the rate(s) he chooses by
In those ARDBs which have this function enabled, the designer also specifies
As a general guideline, ACR_TO should be patterned on the ACR Decrease
During run time on those ARDBs with the ACR_ENA bit set, the RS8234
To globally enable out-of-rate Forward RM cell generation, set
The system designer can set the SET_OOR bit to a logic high in any ABR
When the actual cell rate on a channel has lowered to the point where
The RS8234 calculates the rate for generation of out-of-rate Forward RM cells
Mindspeed Technologies
R = (maximum scheduled rate) = sysclk/SLOT_PER
generated rate = R / (OOR_INT + 1) / (VCC_MAX/2 + 1)
™
6.3 ABR Flow Control Manager
6.0 Traffic Management
6-35