1 Scope
2 DTE/DCE physical interface elements
3 Alignment of call control characters and error checking
4 Elements of the call control phase for circuit switched service
5 Data transfer phase
6 Clearing phase
7 Test loops
Annex A   Interface signalling state diagrams
Annex B   Interface signalling sequence diagrams and time-out operations
Annex C   DTE time-limits and DCE time-outs
Annex D   Formats of selection, call progress and DCE-provided information signals
Annex E   Interworking between DTEs conforming to Recommendations X.21 and X.21 bis
Annex F   Coding of call progress signals and DTE provided information
Annex G Facility request, indicator and parameter coding
Annex H   Information content of DCE-provided information
Annex I    Reference and transition tables