Comtech EF Data CDM-625A - Manual (Page 698)

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix M Revision 3
M–18
M.8 Monitoring ACM Performance
The CDM-625A 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’
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.
M.9 ACM Congestion Control
When the ACM controller switches from a lower to a higher ModCod the bandwidth of the
Ethernet link is instantaneously increased. This is not a problem, and the link will adapt to push
more packets/second through the link.
Conversely, when the ACM controller switches from a higher to a lower ModCod the bandwidth
of the Ethernet link is instantaneously reduced. Unless the FIFO in the WAN encapsulator is
configured to be very large, the FIFO will tend to overflow, and packets will be lost before the
network recognizes that there is congestion, and reduces the rate at which packets are sent.
In order to mitigate packet loss when bandwidth is reduced, the CDM-625A ACM system
incorporates a method for congestion control that is illustrated in Figure M-5.