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CDD-880 Multi Receiver Router
Appendix D
D.2.2
Scenario 2 – Reverted or Auto-Recovered Messages
"Revert" or "Auto Recover" of an already registered CDM-840 is required when:
•
The CDM-840 was forced into or dropped back to ECM mode;
•
The CDM-840 transmits into an SAS setting due to a "Need to be Switched" flag in the RID
message;
•
The HCC sends ECM switch requests to the VMS on behalf of the CDM-840 (Standard
Processing).
D.2.2.1 ECM Revert Cycle Timing
When a CDM-840 reverts from dSCPC to ECM, there is typically no wait because the CDM-840 is
always monitoring TAP cycles to synchronize. It then randomly selects a slot in the next cycle
and transmits. If the probe is not detected at the corresponding CDD-880 (as indicated by the
next TAP to the CDM-840), it backs off by doubling the cycle and selecting another random slot.
If the revert happened near the end of the cycle the CDM-840 wait for next TAP.
The Slots in Frame setting is determined by the number of remotes in the cluster that are either
waiting in the channel or put back into the channel for recovery. Example, if there was only one
CDM-840 attempting to enter the network, it would be simple to determine the number of
required slots would be two – one for contention, and one for assignment. However, in any
given network there may be many CDM-840s attempting to pass through the ECM channel
depending on their state or network environment. Other catastrophic and potential real world
conditions may include hub outage (although unlikely), sun outages, rain fade, or regional
nightly power-down schedules.
In normal operations, only one to a few CDM-840s would be accessing the channel. The worst-
case scenario is where all CDM-840s are placed in ALOHA on the ECM channel and depending on
slot count the contention will be high until the ECM controller starts to cycle down through the
reception list and all CDM-840s are cleared.
In any case, it is important to accurately assign the total number of number of slots in order to
reduce contention and reduce the time it takes for all CDM-840s to re-enter the network dSCPC
resources. While it might seem obvious to adjust the Slots in Frame setting to a large number,
however this could have an inverse effect, because adding more slots increases the cycle time
adding potentially more delays-per-CDM-840 attempting access the channel.
While trial and error may be used to determine the correct settings (based on the actual
network parameters), Comtech EF Data enables you to avoid such a tedious process with its
available ECM Calculator. This tool, provided upon request, presents theoretical analysis,
number of cycles and time to capture all waiting CDM-840s.
Contact Comtech EF Data Product Support to obtain the ECM Calculator.
D–8
MN-CDD880
Revision 2
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