Motorola PowerQUICC II MPC8280 Series Reference Manual page 1037

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).
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.
MOTOROLA Chapter 31. ATM Controller and AAL0, AAL1, and AAL5
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Available Bit Rate (ABR) Flow Control
31-23