CDM-550 Satellite Modem is a used SCPC modem manufactured by Comtech EFData. These are low priced basic modems. The IF frequency for transmit and receive is 70 MHz ± 18 MHz.
A typical configuration is 2Mb, QPSK and TPC, but other options can be added using Comtech’s fast key upgrade feature.
A special feature of the CDM-550 is its ability to monitor and control the distant end of a satellite link using a Comtech EF Data proprietary overhead channel. This framed mode is called EDMAC (Embedded Distant-end Monitor and Control).
User data is framed and extra bits are added to pass control, status, and automatic uplink power control information. This process is completely transparent to the user.
Comtech EF Data RF transceivers may be controlled from the front panel of the modem using a low data rate FSK signal on the Rx IF cable. An RF transceiver at the distant end of a satellite link may also be controlled and monitored through the EDMAC channel.