The CCSDS has developed seven “restructured” Recommendations for space link protocols. These restructured Recommendations are reorganized versions of the CCSDS Telemetry (TM), Telecommand (TC), and Advanced Orbiting Systems (AOS) specifications, which were originally developed in the 1980s before many of the modern conventions for protocol specification had been formalized. The restructured Recommendations do not change the major technical content of the original Recommendations, but the presentation of the specifications has been changed so that:

• all CCSDS space link protocols and services can be used to transfer any data over any space link in either direction;
• all CCSDS space link protocols and services are specified in a unified manner;
• the layered model matches the Open Systems Interconnection (OSI) Basic Reference Model.

Together with the change in presentation, a few technical specifications have been modified in order to define the Data Link layer protocols in a unified way. Also, some technical terms have been changed in order to unify the terminology used in all the CCSDS Recommendations that define space link protocols and to define them as general communications schemes. Each restructured Recommendation contains an annex that identifies particular changes in terminology.

The restructured Recommendations are expected to supersede the original TM, TC, and AOS Recommendations after a period of acclimation; currently they are being maintained in parallel.

Correspondence of Restructured Specifications with Source Specifications