Comtech EF Data CDM-625A - Manual (Page 701)

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix M Revision 3
M–21
• For VersaFEC, the value of Max ModCod may be limited by other FAST codes installed in the
CDM-625A. For example, suppose the 4100 ksps FAST option is installed, and the symbol
rate set to 4100 ksps, the theoretical maximum data rate would be 14 Mbps at ModCod 11.
However, is CnC is being used, with a 10 Mbps FAST limit, the ACM Max ModCod will be
limited to ModCod 7, or 9.6 Mbps. Therefore, if it is not possible to set the Maximum
ModCod to the desired value for a given symbol rate, you should check to determine what
other FAST codes may be limiting it. The same applies for VersaFEC-2.
• The BER versus Eb/No performance of the ModCods is identical to the VersaFEC and
VersaFEC-2 CCM modes described in Appendix B. FEC (FORWARD ERROR CORRECTION)
OPTIONS.
• 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 - 10
log
(Spectral Efficiency).
• 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 20ms for VersaFEC, and 100ms for VersaFEC-2.
• All IP features that are available in the CDM-625A (VLAN, QoS, etc) are available when in IP-
ACM mode. The Sub-Mux feature, however, is not available.
• VersaFEC and VersaFEC-2 ACM are 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. However, VersaFEC-2 ACM and CCM modes are compatible.