MPC8280 datasheet(981/1386 Pages) FREESCALE

981 / 1386 page

ATM Controller and AAL0, AAL1, and AAL5

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

Freescale Semiconductor

31-21

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).

7. Before sending an F-RM cell, if more than ADTF (ACR decrease time factor) has elapsed since

sending the last F-RM cell, ACR is reduced to ICR. In other words, if the source does not fully use

its gained bandwidth, it loses it and resumes sending at its initial cell rate.

8. Before sending an F-RM cell and after action 7, if more than Crm F-RM cells were sent since the

last B-RM cell was received with BN=0 (backward notification), the ACR is reduced by ACR

CDF (cutoff decrease factor).

9. A source whose ACR is less than the tag cell rate (TCR) sends out-of-rate cells at the TCR. This

behavior is intended for sources whose rates were set to zero by the network. These sources should

periodically sense the network state by sending out-of-rate RM cells. In this case data cells will not

be sent.

10. An RM cell with an incorrect CRC10 is discarded and the UNI statistics tables are updated.

31.5.1.2

ABR Flow Control Destination End-System Behavior

The MPC8280’s implementation of ABR flow control for end-system destinations is described in the

following steps:

1. A received F-RM cell is turned around and sent as a B-RM cells.

2. The DIR field of the received F-RM cell is changed from 0 to 1 (backward DIR).

3. The CCR and MCR fields are taken from the F-RM and is not changed.

4. The CI bit of the B-RM cell is set if the previous data cell arrived with EFCI = 1 (congestion bit in

the ATM cell header).

5. The ER field of the turn around B-RM cells is limited by TCTE[ER-BRM].

6. If a F-RM cell arrives before the previous F-RM cell was turned around (for the same connection),

the new RM cell overwrites the old RM cell.

31.5.1.3

ABR Flowcharts

The MPC8280’s ABR transmit and receive flow control is described in the following flowcharts. See

Figure 31-11, Figure 31-12, Figure 31-13, and Figure 31-14.

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