MPC8280 datasheet(980/1386 Pages) FREESCALE

980 / 1386 page

ATM Controller and AAL0, AAL1, and AAL5

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

31-20

Freescale Semiconductor

(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.

Figure 31-10. MPC8280’s ABR Basic Model

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.

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).

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

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