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CHAPTER 5 SAR FUNCTION
217
The FRTT and TBE parameters are used to determine the other parameters. The
μ
PD98405 has no field for
setting these parameters. The host must determine the other parameters from the FRTT and TBE parameters
that are obtained through negotiation. The parameters determined from FRTT and TBE are as follows:
CRM = TBE/Nrm
ICR = min(ICR, TBE/FRTT)
5.8.5 ABR Service Processing
This section describes the processing that the host must perform to enable the use of the ABR service. It
focuses on processing unique to the ABR service. The host should also perform the same processing as that
performed when using the VBR service. Before attempting to use the ABR service, read and become familiar
with the contents of
Chapter 5
.
Remark
The terms "transmission" and "reception," as used in this section, have the same meanings as the
source and destination, respectively.
(1) Setting control memory
To use the ABR service, it is necessary to allocate an ABR lookup table in control memory (see
Section
5.2
). The host sets the start address of the ABR lookup table in the ALA register. An area from the ALA
register to the TOS register is assigned as the ABR lookup table. This table can consist of up to 8 blocks
(each block having 16 words). The actual number of blocks varies with the number of active ABR channels
that are supported. The
μ
PD98405 can support up to 256 active ABR channels. When 256 active ABR
channels are supported, the ABR lookup table will consist of 8 blocks. An active ABR channel is a channel
that is currently transmitting data (or that is active). The
μ
PD98405 can support up to 256 active ABR
channels simultaneously. Similarly to the VBR service, the ABR service can support 32K channels (VC) if
they are open and placed in the idle state by setting a VC table for them.
The ABR lookup table is an area used by the
μ
PD98405. The host need not access this area.
Active channel support for 256 and more VCs
For the
μ
PD98405, a maximum of 256 active ABR channels can be registered in the ABR lookup table.
Therefore, no more than 256 VCs can be supported. This band is used, by the ABR scheduler, to
perform ABR scheduling. Even if more than 256 VCs are active, the
μ
PD98405 will not malfunction.
Those VCs in excess of 256 are made to wait to be registered in the ABR lookup table and, when a VC
becomes inactive, one of the waiting VCs is registered and data transmission starts. Note, however,
that there will be a period during which those VCs that are being made to wait are not subject to control
by the ABR scheduler. As a result, rate control can not be performed successfully. In other words, if
more than 256 VCs are active, the
μ
PD98405 will not actually malfunction, but rate control will not be
applied during data sending.