High speed modems are a means for implementing early 100 percent digital connectivity in the transition to total digital networks. The system parameters for high speed data transmission over FM and SSB microwave radio are calculated. It is shown that radio fading is not necessarily a controlling factor. Other impairments such as noise, level, and phase transients are experienced on long terrestrial circuits. The setup for field measurements and results are presented. Technical and economic network criteria are presented that led to the decision to deploy 1.544 Mbit/s modems on analog microwave radio.