Freescale Semiconductor MPC8260 - Manual (Page 940)

ATM Controller and AAL0, AAL1, and AAL5
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
30-20 Freescale Semiconductor
support. The destination receives forward RM cells and returns them to the source as backward RM
cells. The PowerQUICC II 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.
30.5.1 The ABR Model
Figure 30-10 shows the PowerQUICC II’s ABR model.
Figure 30-10. PowerQUICC II’s ABR Basic Model
The PowerQUICC II ABR flow control implements both source and destination behavior. The
PowerQUICC II’s ABR flowchart is described in
Section 30.5.1.3, “ABR Flowcharts.”
30.5.1.1 ABR Flow Control Source End-System Behavior
The PowerQUICC II’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.
4. When sending an F-RM cell, the current ACR is written in the CCR (current cell rate) field of the
RM cell.
5. When B-RM (backward-RM) cell is received with CI = 1 (congestion indication), ACR is reduced
by ACR
× RDF (rate decrease factor). After the reduction, the new ACR is determined first by
letting ACRtemp be the min of (ACR, ER), and then taking the max of (ACRtemp, MCR).
6. When B-RM is received with CI=0 and NI=0 (no increase), ACR is increased by RIF
× PCR (rate
increase factor). The new ACR is determined first by letting ACRtemp be the min of (ACR, ER),
and then taking the max of (ACRtemp, MCR).
Source Behavior Destination Behavior
Nrm Data Cells
F-RM Cell
B-RM Cell
Update Rate
CCR,ER
ER, CI, NI
F-RM Cell
B-RM Cell
Set CI, NI
Turn-around
Reduce ER