Motorola MPC8260 - Manual (Page 789)

29-9 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA
Part IV. Communications Processor Module
For information about cell rate pacing, see Section 29.3.5, ÒATM TrafÞc Type.Ó For
information about prioritization, see Section 29.3.6, ÒDetermining the Priority of an ATM
Channel.Ó
29.3.2 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 PHYÕs 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 29.10.4, ÒAPC Data
Structure.Ó)
Each PHYÕs APC parameter table holds parameters that deÞne the priority table location,
the number of priority levels, and other APC parameters. The priority table holds pointers
that deÞne the location and size of each priority levelÕs scheduling table.
Each scheduling table is divided into time slots, as shown in Figure 29-1. 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 channelÕs assigned trafÞc rate (speciÞed in time slots). The PCR parameter
in the TCT, or the SCR or MCR parameters in the TCT extension (TCTE) determine the
channelÕs actual rate.
Table 29-1. ATM Service Types
Service Type Cell Rate Pacing
Real-Time/
Non-Real-Time
Relative Priority
CBR PCR RT 1 (highest)
VBR-RT PCR, SCR (peak-and-sustain) RT 2
VBR-NRT PCR, SCR (peak-and-sustain) NRT 3
ABR 1
1 When ABR ßow control is active, the CP automatically adapts the APC parameters PCR,
PCR_FRACTION. These parameters function as the channelÕs allowed cell rate (ACR).
PCR NRT 4
UBR+ PCR, MCR (peak-and-minimum) NRT 5
UBR PCR NRT 6 (lowest)