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CHAPTER 5 SAR FUNCTION
211
5.8 ABR FUNCTION
5.8.1 Overview
The
μ
PD98405 supports the ABR service by hardware. It controls the resource management cell (RM cell)
according to ATM Forum Traffic Management Specification Version 4.0. Upon receiving a forward RM cell
(FRM cell), the
μ
PD98405 automatically returns a backward RM cell (BRM cell) using an in-rate or out-of-rate
stream. Upon receiving a BRM cell, the
μ
PD98405 automatically changes the data cell transmission rate
according to the congestion information (CI, NI, and ER) of the BRM cell.
The features of the ABR service supported by the
μ
PD98405 are as follows:
Complies with ATM Forum Traffic Management Specification Version 4.0 (af-tm-0056.000).
Supports the source/destination behavior of the ATM Forum's ABR flow control.
Supports 256 active ABR channels.
Contains an in-rate buffer and out-of-rate queue for the ABR service.
Supports local congestion management (source/destination).
Remarks
1
.
A BRM cell that is sent back is called a turn-around BRM cell.
2
.
A cell stream with CLP = 0 is called an in-rate stream, while one with CLP = 1 is called an
out-of-rate stream.
5.8.2 Source/Destination Behavior
This section describes the behavior of the source/destination supported by the
μ
PD98405 by hardware.
(1) Source behavior
1.
The ACR value will never exceed the PCR, nor become smaller than the MCR. The
μ
PD98405 will
not transmit in-rate cells at a rate higher than the ACR. The
μ
PD98405 transmits in-rate cells at the
ACR rate or slower.
2.
Once a connection has been set up, the ACR value is set in the ICR before the
μ
PD98405 transmits
the first cell. The first in-rate cell to be transmitted is an FRM cell.
3.
Once the first in-rate FRM cell has been transmitted, the other in-rate cells are transmitted according
to the rules stated below:
a) When at least Mrm in-rate cells have been transmitted, and a period at least equal to Trm has
elapsed, or when Nrm - 1 in-rate cells have been transmitted since the latest FRM cell transmission,
the next in-rate cell to be transmitted will be an FRM cell.
b) When condition a) does not apply, a BRM cell is waiting to be transmitted, and no in-rate BRM cell
has been transmitted since the latest FRM cell transmission, or no data cell is waiting to be
transmitted, the next in-rate cell to be transmitted will be a BRM cell.