Motorola MPC8260 - Manual (Page 801)

29-21 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA
Part IV. Communications Processor Module
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
29.5.1.1 ABR Flow Control Source End-System Behavior
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.
29.5.1.2 ABR Flow Control Destination End-System Behavior
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).