Monitoring Acm Performance; Notes And Recommendations - Comtech EF Data CDM-570A Installation And Operation Manual

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Monitoring ACM Performance

The CDM-570A/570AL-IP provides several ways to determine the current state of the ACM
system.
Use the front panel menu to select MONITOR: ACM. The current Tx and Rx ModCod, along with
the Local and Remote SNR, are displayed here. The SNR displays values between -3.0 dB and
+22.0 dB, with a resolution of 0.1 dB. If either the local or remote demod is unlocked, the SNR will
show 'No Sync'. Of course, under MONITOR: Rx PARAMETERS, the Eb/No continues to be
displayed corrected for modulation type and code rate, in case you do not wish to deal with SNR
values.
If you wish to see the exact detail of the ModCod (data rate, modulation, code rate), then these
parameters can be seen under INFO: TX or INFO: Rx. Furthermore, if you are in a CONFIG: TX
or CONFIG: RX screen, both the symbol rate and data rate are displayed. All of these screens
update dynamically, so if a ModCod changes, the parameters are refreshed.
This information is also available through the Remote Control (serial interface), as well as the
Web Server, SNMP and Telnet interfaces.
If you wish to use the 'Constellation over Ethernet' application that comes with the released code,
it is also informative to see the demodulator changing type 'on the fly'. With no noise, and a
modem in a loop on itself, you may wish to experiment with Min and Max ModCod values to drive
the adaptation.
Alternately, if you have access to an Oscilloscope in X-Y mode, the Alarms connector provides
analog voltages to monitor the constellation. This has the advantage that it will show a change in
modulation type instantaneously, unlike the 'Constellation over Ethernet' application, which only
updates once per second.
If you wish to verify that link performance is meeting the required level, the internal BERT tester is
an excellent tool. When using the IP interface it is not an easy matter to verify BER performance,
but the internal BERT will not only do this, but is also tolerant of the change in bit rate that
accompanies a change in ModCod. The BERT can be used to confirm that there are no sync
losses or bursts of bit errors when a ModCod changes.
P.9

Notes and Recommendations

•
VersaFEC and VersaFEC-2 ACM are for point-to-point applications. It is required that both
directions on the link run in IP-ACM mode, although the symbol rates do not need to be
equal. If you wish to constrain one direction to run in CCM, simply set the Min and Max
ModCod to be equal. Note, however, that VersaFEC and VersaFEC-2 cannot be mixed on
the same bi-directional link.
•
ACM constitutes a closed-loop control system (similar in concept to AUPC) 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 1dB/second (more if Target Eb/No margin is
increased).
Adaptive Coding and Modulation (ACM) Option
CDM-570A/570AL Satellite Modem with Optional Packet Processor
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Revision 5
MN-CDM570A