This Annex describes an extension of Recommendation H.223. It defines specific adaptation layers to allow the use of H.324 terminals in highly error-prone transmission environments. These adaptation layers include specific options for H.324 terminals, e.g: error detection and correction, sequence numbering, automatic repeat request, retransmission capabilities (hybrid ARQ-type I & II), segmentation procedure to transmit frames which shall be transmitted unframed, support of framed in unframed transmission mode.