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CDD-880 Multi Receiver Router
Appendix H
H.4.4
MODCOD Switch Points
The Figure H-7 graph shows the switch points with the Target Eb/No margin set to 0dB.
However, the switch points can be moved (increased) by configuring the Target Eb/No margin
parameter, which can vary from 0 to 4.5 dB, in 0.5dB steps. In a fading environment, it is
strongly recommended to add sufficient margin to maintain an adequate link quality (and to
maintain demod lock) during the interval between the Eb/No degrading and the ACM controller
responding by lowering the MODCOD. See Section H.6 Notes and Recommendations.
H.5
ACM Congestion Control
When the ACM controller switches from a lower to a higher MODCOD the bandwidth of the
Ethernet link is instantaneously increased. This is not a problem, and the link will adapt to push
more packets/second through the link.
Conversely, when the ACM controller switches from a higher to a lower MODCOD the
bandwidth of the WAN link is instantaneously reduced. If the MODCOD changes down, the
datarate will be reduced and the system will continue to send the highest priority traffic first. If
the traffic demand exceeds the available bandwidth, the system will start dropping packets in
the lowest Qos Priority / non-CIR protected QoS ques first.
Figure H-7. CDM-840 – ACM MODCOD Switch Points
H–11
MN-CDD880
Revision 2
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