Notes And Recommendations - Comtec CDM-840 Installation And Operation Manual

CDM-840 Remote Router
Appendix D
D.5

Notes and Recommendations

•
VersaFEC is for point-to-point applications. It is required that both directions on the link run in
ACM mode, although the symbol rates do not need to be equal. For VersaFEC, if you wish to
constrain one direction to run in CCM, simply set the Min and Max ModCod to be equal.
•
ACM constitutes a closed-loop control system and it should be remembered that like all
control systems, the speed at which the system can react is governed by a number of factors,
including the time taken to estimate SNR to the required accuracy, and the transport delay
over the satellite. Realistically, it can cope with fading and other link impairments that do not
exceed 1 dB/second (more if Target Eb/No margin is increased).
•
The ACM controller algorithm that resides in the CDM-840 modem exhibits a different
response depending on whether SNR is increasing or decreasing. If the SNR is improving, the
ACM controller switches to higher ModCods sequentially until the highest possible ModCod
has been reached. It may, therefore, take several seconds for the link to reach maximum
throughput. Conversely, if SNR is degrading rapidly (in a fade) then it can change, if needed,
directly from ModCod11 to ModCod0 (VersaFEC example). This gives the fastest possible
response and helps keep the link intact under fast fading events.
•
While ACM can do remarkable things, the fundamentals still apply. Don't expect the demod to
run at a 16-ARY ModCod if the SNR instantaneously drops to 0 dB – the demod will lose lock
and the system will recover by switching to ModCod0 (if so configured, in VersaFEC).
To achieve the most robust link performance, Comtech EF Data strongly
recommends that you set the Minimum ModCod to 0 (the ModCod of last
resort), and set the Unlock Action to 'Go to minimum ModCod'.
• For VersaFEC running in ACM mode the demodulator is performing blind acquisition –
meaning that it has no a priori knowledge of the modulation type or code rate. For this reason
the demodulator acquisition time will be slower than in CCM mode. However, the acquisition
time is typically under 1 second for all symbol rates and noise conditions.
•
Running the ACM link with the Target Eb/No Margin set to 0 dB will give the best utilization
of link power, but in conditions of fast fading may cause demod unlock events, or highly
degraded BER just prior to the switch to a lower ModCod. In order to mitigate this, we
recommend a Target Eb/No Margin of at least 0.5 dB – more if the fading events are
particularly severe and/or frequent.
•
The BER versus Eb/No performance of the ModCods is identical to the VersaFEC CCM modes
described in Appendix B. FEC (FORWARD ERROR CORRECTION) OPTIONS.
•
To achieve minimum latency, set the WAN buffer to the smallest practical value This will
depend on how well the connected equipment implements rate limiting, and the variation in
IP packet size. The default setting is 20 ms.
•
All IP features that are available in the CDM-840 (VLAN, QoS, etc) are available while in ACM
mode.
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MN-CDM840
Revision 3