![](http://datasheet.mmic.net.cn/380000/-PD98405_datasheet_16745025/-PD98405_220.png)
CHAPTER 5 SAR FUNCTION
220
(3) Setting the ABR scheduler
The
μ
PD98405 links a VC that uses the ABR service to the ABR scheduler that performs automatic rate
control. The host need only specify the priority of the ABR scheduler. (See
Section 7.3
.)
The ABR scheduler is handled as shaper 16. The
μ
PD98405 has an in-rate buffer and out-of-rate queue in
the ABR scheduler. The in-rate buffer and out-of-rate queue have their own priority registers, and the host
specifies priority in these registers. Usually, the in-rate buffer has a higher priority than the out-of-rate
queue. The host must assign the ABR scheduler (for both the in-rate buffer and out-of-rate queue) a
priority lower than that of the shaper used for VBR but higher than that of the shaper used for UBR.
The assigned priority can be changed at any time. To change it, set the B3 bit (byte enable) of the
Indirect_Access command to 1 without overwriting the other fields of the priority register.
(4) Open channel
ABR channels are opened using an Open_Channel command, similarly to VBR channels. The host opens
two channels at a time. To enable the use of the ABR service, the
μ
PD98405 must have two channels, a
transmit VC and receive VC. The ABR service always uses both a transmit VC and a receive VC. A
receive VC is needed even if data is only transmitted, because it is used to receive a BRM cell. If data is
only received, a transmit VC is required to send back a BRM cell. If data is both transmitted and received,
the same VC can be used for both data reception and turn-around BRM cell reception, and the same VC
can be used for both data transmission and BRM cell return. To sum up, data transmission and reception
only require one transmit VC and one receive VC. No extra VC is required for turn-around BRM cell
reception or BRM cell return.
(5) Setting the VC table
The host sets initial values for the transmit and receive VCs which it opened. For details of the format of
the VC table, see
Sections 5.4.3
and
5.5.3
.
Setting the transmit VC table
The host sets the transmit VC table with its initial values. In addition to the settings for the VBR VC
table, the host must set values that are specific to the ABR service. The settings specific to the ABR
transmit VC are as follows:
<1>
Set the ABR bit to 1 to indicate that the VC uses the ABR service. The SHAPER NO. field has no
meaning for the ABR service, because the ABR service uses the ABR scheduler. This field must
always be set to 0.
<2>
Set, in the Rx VC TABLE POINTER field of word 7, the VC NUMBER of a receive VC (VC used to
receive a turn-around BRM cell) that is paired with this transmit VC.
<3>
Set the ABR parameter obtained through negotiation. (See
Section 5.8.4
.)