Maximum Clipping; Minimum Data Rate - Comtec CDM-840 Installation And Operation Manual

CDM-840 Remote Router
Appendix J
J.4 QoS with ACM/VCM (Adaptive/Variable Coding and Modulation)
When ACM is enabled, the modem is configured with symbol rate, while QoS operates on data
rate. Although the symbol rate will be constant in ACM/VCM mode, the data rate will not be
constant. As a result, the modem's data rate can vary "on-the-fly" based on the Es/No. The data
rate can be more or less than when QoS was first configured, or when the modems first boots.
J.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.
J.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.
For example, using the CDM-840 HTTP Interface 'Configuration | WAN | QoS' page:
1. Select the DiffServ QoS Group Control Mode.
2. Enable ACM.
3. The data rate of the modem drops down from 10000 kbps to 8700 kbps due to Es/No.
4. Input data streams are the same as with the QoS Group Min/Max Mode example (see Sect.
J.2.2.2).
QUESTION: What will be the total data rate and what will be the individual data rates?
ANSWER:
•
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.
J–22
MN-CDM840
Revision 3