MPC8280 datasheet(969/1386 Pages) FREESCALE

969 / 1386 page

ATM Controller and AAL0, AAL1, and AAL5

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

Freescale Semiconductor

31-9

For information about cell rate pacing, see Section 31.3.5, “ATM Traffic Type.” For information about

prioritization, see Section 31.3.6, “Determining the Priority of an ATM Channel.”

31.3.2

APC Unit Scheduling Mechanism

The APC unit consists of an APC data structure in the dual-port RAM for each PHY and a special

scheduling algorithm performed by the CP. Each PHY’s APC data structure includes three elements: an

APC parameter table, an APC priority table, and cell transmission scheduling tables for each priority level.

(See Section 31.10.4, “APC Data Structure.”)

Each PHY’s APC parameter table holds parameters that define the priority table location, the number of

priority levels, and other APC parameters. The priority table holds pointers that define the location and

size of each priority level’s scheduling table.

Each scheduling table is divided into time slots, as shown in Figure 31-1. The user determines the number

of ATM cells to be sent each time slot (cells per slot). After a channel is sent, it is removed from the current

time slot and advanced to a future time slot according to the channel’s assigned traffic rate (specified in

time slots). The PCR parameter in the TCT, or the SCR or MCR parameters in the TCT extension (TCTE)

determine the channel’s actual rate.

Figure 31-1. APC Scheduling Table Mechanism

Table 31-1. ATM Service Types

Service Type

Cell Rate Pacing

Real-Time/

Non-Real-Time

Relative Priority

CBR

PCR

1 (highest)

VBR-RT

PCR, SCR (peak-and-sustain)

VBR-NRT

PCR, SCR (peak-and-sustain)

NRT

ABR1

When ABR flow control is active, the CP automatically adapts the APC parameters PCR,

PCR_FRACTION. These parameters function as the channel’s allowed cell rate (ACR).

PCR

NRT

UBR+

PCR, MCR (peak-and-minimum)

NRT

UBR

PCR

NRT

6 (lowest)

Number of Slots

Current Slot

Cell Rescheduling

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