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M.4
QoS with Adaptive Coding and Modulation (ACM)
When ACM is enabled, the modem is configured with symbol rate. However, while the QoS
system recognizes data rate, both Layer 2 QoS and Layer 3 QoS operate on data rate. Therefore,
while the symbol rate will be constant in ACM mode, the data rate will not be constant. As a
result, the modem's data rate can vary "on-the-fly" based on the EbNo. The data rate can be
more or less than when QoS was first configured, or when the modems first boots.
M.4.1
Maximum Clipping
Due to ACM, when in QoS Max-Pri mode and the data rate's available bandwidth exceeds the
maximum clipping rate, the QoS system limits the output rate of that rule to configured maximum
clipping rate.
When the available data rate is less than the maximum clipping rate, the QoS system has nothing
to clip, since it has not exceeded the defined maximum bandwidth rate.
M.4.2
Minimum Data Rate
Due to ACM, when the data rate's available bandwidth exceeds the minimum data rate (per QoS
Min-Max and DiffServ modes' ASFD classes), the QoS system operates as normal; since the
minimum has been met, the available bandwidth is shared among all other same priorities in
round robin fashion.
When the available data rate is less than the minimum data rate, then the QoS system shares
equally among all same priority queues in a round robin fashion until either minimum bandwidth is
met, or no more data is available to drain.
QoS Example 9
SETUP:
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QUESTION:
What will be the total data rate and what will be the individual data rates?
ANSWER:
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Appendix M
DiffServ Layer 3 QoS mode is selected.
ACM is enabled.
The data rate of the modem drops down from 10 Mbps to 8.7 Mbps due to EbNo.
Input data streams are the same as with QoS Example 7 (see Sect. M.3.2).
After serving all high priority queues, only 500 Kbps is left for ASFD classes.
This leftover bandwidth is less than the total minimum bandwidth; it must
therefore share the residual 500 Kbps among all four ASFD classes equally -
i.e., 500/4=175 Kbps per class.
Since ASFD 4 has the service rate 100 Kbps, the excess 75 Kbps will be given
the other queues where the service rate not met.
In this case the 75 Kbps again will share equally among ASFD3, ASFD2, and
ASFD1 since these service rates were not met.
The resulting individual data rate is as follows:
CDM-425 Advanced Satellite Modem
M–14
Revision 0
MN-CDM-425
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