cn8237 Mindspeed Technologies, cn8237 Datasheet - Page 197

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

CN8237
ATM OC-12 ServiceSAR Plus with xBR Traffic Management
6.3.8 Boundary Conditions and Out-of-Rate RM Cells
6.4.1 A Brief Overview of GFC
28237-DSH-001-C
6.3.8.1 Calculated Rate
6.3.8.2 Out-of-Rate
6.3.8.3 Out-of-Rate
Backward RM Cells
Forward RM Cell
Boundaries
Generation
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.
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.
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 CN8237
sets the SCH_OOR bit in the VCC’s SCH_STATE to a logic high. The CN8237
then generates an out-of-rate Forward RM cell on that channel.
cells as follows:
The system designer can set the TA_OOR bit to a logic high in any ABR Rate
Decision Block. This enables the CN8237 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.
6.4 GFC Flow Control Manager
Generic Flow Control (GFC) is a one-way control mechanism which allows the
network equipment to control the input from an end station, for the class(es) of
traffic defined as controlled. This mechanism does not allow the end station to
exert any control on traffic from the network.
side of the network switch buffers. This allows a much higher degree of
multiplexing than is otherwise possible, and allows the network-side costs of
connections to be significantly reduced. By overbooking the input bandwidth, a
high degree of sharing is possible, and the buffer system can be used by more end
nodes than full bandwidth input would allow. GFC is used to coordinate access to
that bandwidth when temporary conflicts occur.
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 CN8237 calculates the rate for generation of out-of-rate Forward RM
GFC is useful because it allows overbooking of the bandwidth on the input
Mindspeed Technologies
R = (maximum scheduled rate) = sysclk / SLOT_PER
generated rate = R / (OOR_INT + 1) / (VCC_MAX / 2 + 1)
™
6.4 GFC Flow Control Manager
6.0 Traffic Management
6-41