The system uses a maximum on-air data rate of 3600 bps with fallback in stages ultimately to a robust 53 bps under extremely adverse propagation conditions or interference.

The Medium Speed Modem (MSM) uses 10 parallel data channels in the audio baseband of the transceiver. Each of the data channels is modulated at 125 bps, providing a total capacity of 1250 bps. Advanced digital signal processing techniques are employed to generate the tones and recover the data from each tone. The most dominant effect of ionospheric propagation is the selective fading of frequencies within the channel baseband.