Motorola MPC8260 User Manual

29-21

MOTOROLA

The MPC8260 ABR ßow control implements both source and destination behavior. The
MPC8260Õs ABR ßowchart is described in Section 29.5.1.3, ÒABR Flowcharts.Ó

The MPC8260Õs implementation of ABR ßow control for end-system sources is described
in the following steps:

1. An ABR channelÕs 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) Þeld 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 Þrst 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 Þrst 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 notiÞcation), 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.

The MPC8260Õs implementation of ABR ßow 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 Þeld of the received F-RM cell is changed from 0 to 1 (backward DIR).