Comtec CDM-840 Installation And Operation Manual page 314

CDM-840 Remote Router
Appendix J
Which parameter effect is better for your particular traffic? Sudden severe drops are
resultant with High Drop Precedence (% Full) values, while more gradual distributed drops
result with Low Drop Precedence (% Full) values. Experience indicates that congestion
rarely exists in just the right "sweet spot" for this to become a significant question.
Under heavy congestion, the impact of changing these settings is minimal. Under
severe/sustained congestion, you will get the large discards regardless. The Service Rates
can help to "weight" the priority between AF classes under these congestion states.
J.3 QoS Congestion Avoidance
The QoS system supports Weighted Random Early Detection (WRED) based congestion
avoidance. WRED can be enabled or disabled on any queue. When WRED is disabled, upon
overdriving the queue the packets will be tail-dropped.
For example, using the CDM-840 HTTP Interface 'Configuration | WAN | QoS' page:
1. Select the DiffServ QoS Group Control Mode.
2. Set the modem Tx bandwidth to 10000 kbps.
3. The data rate of the modem drops down from 10000 kbps to 8700 kbps due to Es/No and
ACM MODCOD adjustment.
4. The input data streams to the modem are the same as those used in the Min/Max QoS
example provided in Sect. J.2.2.2.
QUESTION: What is the total output data rate and what are the individual port output data
rates?
ANSWER:
•
The QoS system first drains Priority 1 traffic. Since the management data stream is only 200
kbps, after serving Priority 1 traffic, QoS still has 9800 kbps (10000
•
QoS next drains Priority 2's 800 kbps traffic. After the end of the Priority 2 traffic, QoS still
has 9000 kbps traffic available (9800
•
Similarly, QoS walks through all priorities until it hits the ASFD classes:
The beginning of ASFD Classes QoS leftover bandwidth is 2000 kbps.
o
All ASFD classes have the same Priority of 7, with different service rates.
o
ASFD service rate is a Committed Information Rate (CIR), except serviced if bandwidth
o
available after serving all high priority queues. In this case, 2000 kbps bandwidth is
– 800 = 9000).
J–20
MN-CDM840
Revision 3
– 200 = 9800).