Comtech EF Data CDM-625 - Manual (Page 446)

CDM-625 Advanced Satellite Modem Revision 15
Quality of Service (QoS) MN-CDM625
20–14
20.3.3 Layer 3 QoS DiffServ Mode
Layer 3 DiffServ QoS is fully compliance to RFC standards. When the Layer 3 QoS mode is set to
DiffServ, the system automatically configures the rules with DSCP code points, priority values,
and WRED. You can only configure the service rate and drop precedence levels for Assured
Forwarding (ASFD) classes.
20.3.3.1 Layer 3 QoS Congestion Avoidance
The Layer 3 QoS system supports Weighted Random Early Detection (WRED) based congestion
avoidance. WRED can be enable or disabled on any queue. When WRED is disabled, upon
overdriving the queue the packets will be tail dropped.
QoS Example 8
1. DiffServ Layer 3 QoS mode is selected.
2. The modem Tx bandwidth is set to 10 Mbps.
3. The data rate of the modem drops down from 10 Mbps to 8.7 Mbps due to EbNo.
4. The input data streams to the modem are the same as those of QoS Example 7.
QUESTION: What is the total output data rate and what are the individual port output data
rates?
ANSWER:
•
The Layer 3 QoS system first drains Priority 1 traffic. Since the management data stream is
only 200 Kbps, after serving Priority 1 traffic, QoS still has 9.8 Mbps (10.0-0.2=9.8).
•
QoS next drains Priority 2’s 800 Kbps traffic. After the end of the Priority 2 traffic, QoS still
has 9 Mbps traffic available (9.8-0.8=9).
•
Similarly, QoS walks through all priorities until it hits the ASFD classes:
o
The beginning of ASFD Classes QoS leftover bandwidth is 2 Mbps.
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
available after serving all high priority queues. In this case, 2 Mbps bandwidth is
available, serving all service rates first. The total of all ASFD service rates are 1.6 Mbps,
which is less than the 2 Mbps leftover bandwidth.
o
After serving the service rates, the leftover bandwidth is 400 Kbps. This 400 Kbps will be
distributed to all ASFD classes equally in round robin fashion until either there is no
more bandwidth, or the input streams have no data.