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CHAPTER 5 SAR FUNCTION
226
LCR register
Rate reduction based on internal congestion is not applied to any VCs that are transmitting at a rate
less than that set in the LCR register. If VCs that are transmitting at a given rate (about 5% of the
band that can be used by ABR) are not to be subject to any further rate reduction upon the
occurrence of internal congestion, set that rate in LCR.
AUB register
The
μ
PD98405 uses the value set in the AUB register to detect internal congestion. Also, upon the
detection of internal congestion, ACR reduction is performed based on this value. Therefore, it is
necessary for the host to set, in the AUB register, the band that can be used by ABR. For example,
at the start of VC transmission by the VBR, if the band that can be used by ABR is too small, the
value set in the AUB register must exclude the VBR band. However, once the VC of the VBR has
finished transmitting, and the band that can be used by the ABR increases, it becomes necessary to
set a larger value in the AUB register.
When the VBR is not being used, the full band can be used with the ABR. Therefore, a band of
149.76 Mbps can be used with ABR (when using the 155.52 Mbps SONET/SDH framer). At this
time, set AUB to 149.76 Mbps. Subsequently, for example, if the VC of the VBR starts transmitting at
30 Mbps, the setting for AUB must be changed to 110.76 Mbps. Upon the completion of transmission
by that VC, return the setting of AUB to 149.76 Mbps.
When using both ABR and UBR, set the band that ABR and UBR can use in the AUB register. The
μ
PD98405 automatically assigns the band to the ABR and UBR. For details of assigning a band to
the UBR, see
Section 5.9.2
.
(2) Use-it-or-lose-it
The
μ
PD98405 supports a Use-it-or-lose-it function in hardware. The use-it-or-lose-it function supported by
the
μ
PD98405 corresponds to source operation number 5 (see
Section 5.8.2
).
The Use-it-or-lose-it function reduces ACR if the calculated ACR is greater than the actual rate. Upon
detecting Use-it-or-lose-it the
μ
PD98405 reduces ACR to ACR*15/16. The conditions related to Use-it-or-
lose-it are given bleow.
First, in order to display the actual rate, the
μ
PD98405 displays the average in-rate FRM cell interval (Ta).
This value is obtained using the following expression.
Ta(t) = a*T + (1 - a)*Ta(t - 1)
Ta: Average in-rate FRM cell transmission interval (units: cells)
a:
a = 1/2
a0
(a0: register setting)
T:
Interval between current and previous in-rate FRM cell transmission (units: cells)