Comtec CDM-840 Installation And Operation Manual page 312

CDM-840 Remote Router
Appendix J
The Service Rate is the minimum rate, in kbps, that the QoS engine attempts to provide the AF
class if the capacity exists during congestion. It is similar to a CIR but it is not absolute and it is
NOT guaranteed over the Priority 1 through 7 classes.
If there is no congestion, there is no maximum limit on the AF class. However, during
congestion, an AF class with Service Rate of "0" may experience near 100% discard or very high
levels of discard as compared with other AF Classes with greater Service Rates. The QoS engine
will not attempt to maintain a minimum rate for that class; however, Assured Forwarding
algorithms may not cut that class off completely.
AF dropping is NOT controlled by data rate, but by the status of the system buffer. Any time
there is "buffering" of the data, there is risk of AF traffic dropping. The maximum data rate
without dropping is always the size of the capacity of the link (minus the demand of Classes 1
through 7).
There is NO maximum data rate for the AF Class except for the total capacity rate, regardless of
the configured "service rate" or buffer settings. These values are significant ONLY when there is
enough WAN congestion to force the system to start buffering data.
For example:
If the Service Rate for Assured Forwarding Class 3 is 500 kbps, what is the maximum data rate
permitted – without dropping – for Low, Medium and High Drop Precedence if the Service Rate
we have is 100 kpbs, 90 kbps, or 80 kbps?
First, consider this common configuration:
• AF4 Service Rate 500 kbps Drop Precedence: Low=100% Med=75% High=50%
•
AF3 Service Rate 500 kbps Drop Precedence: Low=100% Med=75% High=50%
•
WAN Capacity = 7000 kbps
•
AF4 traffic = 3000 kbps demanded
•
AF3 traffic = 3000 kbps demanded
Here, because the total traffic is less than the WAN capacity, there is no dropping regardless of
the Service Rate or Drop Precedence percentage. The low Service Rates have no impact at all on
the data rate that is passed.
Next, using the same configuration parameters, consider the following:
•
WAN Capacity = 7000 kbps
• Class 6 traffic = 5000 kbps demanded
3000 kbps passed
3000 kbps passed
5000 kbps passed
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MN-CDM840
Revision 3