Comtech EF Data CDM-625 - Manual (Page 410)

CDM-625 Advanced Satellite Modem Revision 15
Adaptive Coding and Modulation (ACM) MN-CDM625
17–14
• SNR is the preferred metric for driving the adaptation – this is the value displayed on the
monitor screens. If you want to convert this to Eb/No then remember that the relationship is
simply Eb/No = SNR - 10log (Spectral Efficiency).
• To achieve minimum latency, set the WAN buffer to the smallest practical value. The default
setting is 20ms, and we recommend keeping it at this level.
• At this time Comtech EF Data has chosen to disable AUPC while ACM is active. This may
change in the future, but for now, ACM should be considered to be a constant power,
constant symbol rate scheme.
• All IP features that are available in the CDM-625 (VLAN, QoS, etc) are available when in IP-
ACM mode. The Sub-Mux feature, however, is not available.
• VersaFEC ACM is 100% compatible with Carrier-in-Carrier.
• If required, VersaFEC ACM may be used in conjunction with any of the EDMAC modes, either
for serial remote control of the remote modem, or for SNMP proxy. It should be emphasized,
however, that unlike AUPC, a framing mode is not required for SNR reporting.
• ACM maximizes throughput not only when Eb/No varies due to atmospheric conditions, but
will also mitigate the effects of other impairments, such as antenna pointing error, excessive
phase noise and certain types of interference. However, rapidly fluctuating impairments (~
less than 1 second) such as scintillation at low antenna look-angles at C-band will generally not
be improved by ACM.
• VersaFEC ACM modes are not compatible with VersaFEC CCM modes, due to differences in
frame preambles.
• The CDM-625 was purposely architected to provide the platform for VersaFEC ACM, and has
required new approaches to the signal processing employed in both modulator and
demodulator. It is the intention of Comtech EF Data to include VersaFEC ACM in future
modem platforms.