Freescale Semiconductor MPC8260 - Manual (Page 941)

ATM Controller and AAL0, AAL1, and AAL5
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 30-21
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.
30.5.1.2 ABR Flow Control Destination End-System Behavior
The PowerQUICC II’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.
30.5.1.3 ABR Flowcharts
The PowerQUICC II’s ABR transmit and receive flow control is described in the following flowcharts.
See
Figure 30-11 , Figure 30-12 , Figure 30-13 , and Figure 30-14 .