Comtech EF Data CDM-840 - Manual (Page 230)

CDM-840 Remote Router Revision 2
Appendix F MN-CDM840
F–10
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.
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Contact Comtech EF Data Product Support to obtain the ECM Calculator.
F.3.2.2 ECMv2 Backoff Algorithm
The ECMv2 Backoff Algorithm optimizes between quick entry in a quiet network and minimal
collisions in an active network. Its key feature is the concept of fixed-length data slots within a
repeating frame (or cycle). You can control the number of slots in a frame based on the known
number of CDM-840s trying to gain entry into the network. Each total slot consists of three
components:
• Acquisition Preamble
• Data Segment for Management Messages
• Guard Band to resolve timing uncertainty