Motorola PowerQUICC II MPC8280 Series Reference Manual page 1024

ATM Pace Control (APC) Unit
• Priority level—Real-time channels (CBR or VBR-RT) are scheduled at
high-priority levels; non-real-time channels (VBR-NRT, ABR, UBR) are scheduled
at low-priority levels. Up to eight priority levels are available.
31.3.1 APC Modes and ATM Service Types
The ATM Forum (http://www.atmforum.com) defines the service types described in
Table 31-1.
Service Type
CBR
VBR-RT
VBR-NRT
1
ABR
UBR+
UBR
1
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).
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.
31-10
Freescale Semiconductor, Inc.
Table 31-1. ATM Service Types
Cell Rate Pacing
PCR
PCR, SCR (peak-and-sustain)
PCR, SCR (peak-and-sustain)
PCR
PCR, MCR (peak-and-minimum)
PCR
MPC8280 PowerQUICC II Family Reference Manual
For More Information On This Product,
Go to: www.freescale.com
Real-Time/
Relative Priority
Non-Real-Time
RT 1 (highest)
RT
NRT
NRT
NRT
NRT 6 (lowest)
2
3
4
5
MOTOROLA