Motorola PowerQUICC II MPC8280 Series Reference Manual page 1036

Available Bit Rate (ABR) Flow Control
• Explicit rate (ER) feedback. The network carries explicit bandwidth information, to
allow the source to adjust its rate. The source sends forward RM cells specifying its
chosen transmit rate (source ER). A congested switch along the network may
decrease ER to the exact rate it can support. The destination receives forward RM
cells and returns them to the source as backward RM cells. The MPC8280
implements source behavior by adjusting the rate according to each returning
backward RM cell's ER.
Explicit rate feedback has several advantages over binary feedback (EFCI). Explicit rate
feedback allows immediate source rate adaptation, eliminating rate oscillation caused by
incremental rate changes. Using the information in RM cells, the network can allocate
bandwidth evenly among active ABR channels.
31.5.1 The ABR Model
Figure 31-10 shows the MPC8280's ABR model.
Source Behavior
The MPC8280 ABR flow control implements both source and destination behavior. The
MPC8280's ABR flowchart is described in Section 31.5.1.3, "ABR Flowcharts."
31.5.1.1 ABR Flow Control Source End-System Behavior
The MPC8280's implementation of ABR flow control for end-system sources is described
in the following steps:
1. An ABR channel's allowed cell rate (ACR) lies between the minimum cell rate
(MCR) and the peak cell rate (PCR).
2. ACR is initialized to the initial cell rate (ICR).
3. An F-RM (Forward-RM) cell is sent for every Nrm data cell sent. If more than
Mrm cells are sent and the time elapsed since the last F-RM exceeds Trm, an F-RM
cell is sent.
31-22
Freescale Semiconductor, Inc.
Nrm Data Cells
F-RM Cell
CCR,ER
B-RM Cell
Update Rate
ER, CI, NI
Figure 31-10. MPC8280's ABR Basic Model
MPC8280 PowerQUICC II Family Reference Manual
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Destination Behavior
Turn-around
F-RM Cell
B-RM Cell
Set CI, NI
Reduce ER
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