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CHAPTER 5 SAR FUNCTION
223
5.8.6 Internal Congestion Control
This section explains the internal congestion control that is supported by the
μ
PD98405 using hardware. For
the source and destination, the
μ
PD98405 uses hardware to support internal congestion and "Use-it-or-lose-it"
(source only) functions.
(1) Source internal congestion
The
μ
PD98405 uses hardware to support internal congestion for the source. The internal congestion
supported by the source corresponds to source operations 8 and 14 (see
Section 5.8.2
). If internal
congestion is detected under the conditions for source operation 14, the
μ
PD98405 sets internal flag LCI to
1, and reduces the value of ACR. This reduction is not performed for any VC for which the rate is smaller
than that of the LCR register. The following explains the internal congestion detection conditions for the
source, and ACR reduction.
(Program timer - Service timer) > Time_th0 or sum_of_ACR > C
ABR
then
if ((Program timer - Service timer) <= Time_th1) then ACR = ACR_tmp
if ((Program timer - Service timer) > Time_th1) then ACR = ACR_tmp*
α
ACR_tmp
sum_of_ACR - C
ABR
ACR/16
2*C
ABR
or greater
ACR/8
2*C
ABR
to C
ABR
ACR/4
C
ABR
to (C
ABR
-C
ABR
/4)
ACR/2
(C
ABR
-C
ABR
/4) to C
ABR
/2
ACR-ACR/4
C
ABR
/2 to C
ABR
/4
ACR-ACR/8
C
ABR
/4 to C
ABR
/8
ACR-ACR/16
C
ABR
/8 to C
ABR
/16
ACR
C
ABR
/16 to 0
Program timer:
Internal timer that manages ideal cell transmission
Service timer:
Internal timer that manages actual cell transmission
Time_th0, Time_th1: Register settings that control internal congestion detection and ACR reduction
sum_of_ACR:
Total of ACRs in active ABR VCs
C
ABR
:
General-purpose band that can be used with ABR (band excluding that starting from
the value set with the AUB register, to the band used with the UBR)
ACR_tmp:
Reduced ACR value (value in the above table)
α
:
α
= 1/2
alpha
(alpha: register setting)