28236-DSH-001-B Datasheet, PDF (209/443 Pages) M/A-COM Technology Solutions, Inc. – ATM ServiceSAR Plus with xBR Traffic Management

CN8236
ATM ServiceSAR Plus with xBR Traffic Management
6.0 Traffic Management
6.3 ABR Flow Control Manager
6.3.8 Boundary Conditions and Out-of-Rate RM Cells
6.3.8.1 Calculated Rate
Boundaries
When the system designer pre-calculates the rates for the ARDB tables, those
rates must 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.
6.3.8.2 Out-of-Rate
Forward RM Cell
Generation
6.3.8.3 Out-of-Rate
Backward RM Cells
A mechanism must exist to restart scheduling of a VCC once the rate on that
channel has dropped to 0, or is below the schedule table minimum rate (that is, the
rate is less than one schedule slot on the schedule table). Out-of-rate Forward RM
cell generation provides this mechanism.
To globally enable out-of-rate Forward RM cell generation, set
SCH_ABRBASE(OOR_ENA) to a logic high.
The system designer can set the SET_OOR bit to a logic high in any ABR
Rate Decision Block (ARDB). This enables out-of-rate Forward RM cell
generation for that rate.
When the actual cell rate on a channel has lowered to the point where
scheduling of the VCC has halted, and SET_OOR at that rate = 1, the CN8236
sets the SCH_OOR bit in the VCC’s SCH_STATE to a logic high. The CN8236
then generates an out-of-rate Forward RM cell on that channel.
The CN8236 calculates the rate for generation of out-of-rate Forward RM
cells as follows:
R = (maximum scheduled rate) = sysclk ⁄ SLOT_PER generated rate
= R ⁄ (OOR_INT + 1) ⁄ (VCC_MAX ⁄ 2 + 1)
The system designer can set the TA_OOR bit to a logic high in any ABR Rate
Decision Block. This enables the CN8236 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.
28236-DSH-001-B
Mindspeed Technologies™
6-41