The virtual channel function accepts service data units from high layer functions and builds them into space link protocol data units (VCDU or CVCDU). VCDUs are virtual channel data units and CVCDUs are coded virtual channel data units. CVDUs may be used to insure a lower probability of the data units containing errors.
The CCSDS AOS Recommendations identify three classes of service. Grade-1 service operates with a space link ARQ procedure. The sequence of the data units is preserved with a high probability of no errors. Grade-2 service is Reed-Solomon block code protected. Grade-2 service is appropriate for compressed audio or video data. Grade-2 service is possibly incomplete but sequence is preserved with a high probability of no errors. Grade-3 service is intended for redundant data sources where bit errors are not critical. Grade-3 service data units are possibily incomplete with a moderate probability of containing errors. The sequence of the data units is preserved. Grade-3 might be used for redundant video or audio transmissions.
CCSDS AOS can provide video links. The CCSDS AOS recommendation provides for both video and audio capability. The CCSDS bit stream service should be used to transmit compressed video. Class-2 service should be provided. The CCSDS AOS recommendation class-2 type service provides Reed-Solomon forward error correction to correct transmission errors. Compressed video and voice signals require protection from error.