RTCM Paper 56-95/SC101-STD, Recommended Minimum Standards for Digital Selective Calling (DSC) Equipment Providing Minimum Distress and Safety Capability, Version 1.0

NOTE:  This 1995 standard is obsolete but is still referenced in the regulations of the U.S. Federal Communications Commission. 

The RTCM SC101 standard applies to VHF, MF and HF maritime ship and coast station radiotelephone transceiver equipment, providing minimum distress and safety capability.
The purpose is to provide basic DSC functionality at minimal cost.
In order to arrive at a low-cost design, a number of performance compromises were made.
One tunable receiver, capable of tuning to DSC or voice channels, is acceptable.
This means that an SC101 radio will not receive a distress call if the receiver is in use for another call.