Comtech EF Data CDM-625 - Manual (Page 407)

CDM-625 Advanced Satellite Modem Revision 15
Adaptive Coding and Modulation (ACM) MN-CDM625
17–11
The 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 highly
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 Sect. 17.11 Notes and Recommendations.
To continue with the configuration:
Step Task Front Panel Mnemonic
6
(SELECT) CONFIG: IP  Switch-set-up 
WAN
Enter the desired size of the WAN Buffer. The
minimum size is 20 milliseconds, and is referred to
the data rate corresponding to ModCod0. In order
to achieve minimum system latency, do not make
this value unnecessarily large.
Ethernet Switch: Mode WAN PerPortCnfg
MAC-Learning VLAN Qos Stats ()
17.9 Monitoring ACM performance
The CDM-625 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.