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CHAPTER 5 SAR FUNCTION
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prevent this, when N VCs are linked to one shaper, and if the set rate for that shaper exceeds 1/(N
+ 1), set C to at least 2. Note that setting C to 0 totally disables cell transmission. Therefore,
always set C to at least 1.
(b) Setting the scheduler register for CBR
When 0 is set in the priority field for a VBR shaper, according to the highest priority, that shaper can
be used as a CBR. Essentially, there is no difference between the operation of a shaper that is used
as a CBR and a VBR shaper. Setting the maximum priority results only in any cell delay being
eliminated.
When using a shaper as a CBR, to prevent burst transmission at the peak rate, set P = 0 for the peak
rate, C = 1 for the credits, and the transmission rate for I/M.
Note that, in the above case where N VCs are linked to 1 shaper, if the transmission rate (I/M) for
that shaper exceeds 1/(N + 1) when C = 1 has been set, transmission at this rate is not possible. In
this case, it is necessary to set C = 2 but this results in burst transmission occurring. Therefore,
when I/M > 1/(N + 1), full CBR support can not be provided. At the head of a transmission packet,
therefore, burst transmission is likely to occur.
(5) Shaper link list
The user selects the link destination shaper for each transmit VC, after which the host sets the
corresponding 4-bit "Shaper No." field in the transmit VC table. Several VCs can be linked to a specific
shaper. The
μ
PD98405 automatically links or unlinks a transmit VC to or from a shaper.
The
μ
PD98405 creates a link list of VC tables for each shaper to enable the management of multiple active
VCs. The link list of the shaper is managed by using the fifteenth word of the VC table. This word has a
"Forward Pointer" and "Backward Pointer," as well as an "Lst" bit that indicates the end of the list. A "VC
NUMBER" code is written to the pointers to indicate the beginning of the VC table. By changing this code
as necessary, the
μ
PD98405 can link or unlink a VC and shaper.
Figure 5-23. Pointers Used for Linking
Transmit VC table
Word 15
A
Backward Pointer
Lst
Forward Pointer