• CCITT Rec. X.21 (11/1988) – INTERFACE BETWEEN DATA TERMINAL EQUIPMENT (DTE) AND DATA CIRCUIT-TERMINATING EQUIPMENT (DCE) FOR SYNCHRONOUS ...
  • CONTENTS
  • Preface
  • 1 Scope
  • 2 DTE/DCE physical interface elements
    • 2.1 Electrical characteristics
    • 2.2 Mechanical characteristics
    • 2.3 Functional characteristics of interchange circuits
    • 2.4 Physical link control conditions
    • 2.5 Quiescent phase
    • 2.6 Failure detection
  • 3 Alignment of call control characters and error checking
    • 3.1 Character alignment
    • 3.2 Error checking
  • 4 Elements of the call control phase for circuit switched service
    • 4.1 Events of the call control procedures (see Figure A-2/X.21)
    • 4.2 Unsuccessful call
    • 4.3 Call collision (state 15)
    • 4.4 Direct call
    • 4.5 Facility registration/cancellation procedure
    • 4.6 Selection, call progress and DCE provided information formats
  • 5 Data transfer phase
    • 5.1 Circuit-switched service
    • 5.2 Leased circuit service - point-to-point (see Figure A-3/X.21) and packet-switched service (Recommendation X.25, level 1)
    • 5.3 Leased circuit service - centralized multipoint (see Figure A-3/X.21)
    • 5.4 Circuit-switched service - Point-to-multipoint service
  • 6 Clearing phase (see Figure A-4/X.21)
    • 6.1 Clearing by the DTE (states 16, 17, 21)
    • 6.2 Clearing by the DCE (states 19, 20, 21)
  • 7 Test loops
    • 7.1 DTE test loop - type 1 loop
    • 7.2 Local test loop - type 3 loop
    • 7.3 Network test loop - type 2 loop
    • 7.4 Subscriber-line test loop - type 4 loop
    • 7.5 Signal element timing provision
  • 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
  • C.1 DTE time-limits
  • C.2 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
  • ANNEX G – Facility request, indicator and parameter coding
  • G.1 Multiple address calling
  • G.2 Charging information
  • G.3 Redirection of call
  • G.4 Reverse charging
  • G.5 Direct call on a per call basis
  • G.6 Abbreviated address calling
  • G.7 Closed user group selection
  • G.8 DTE inactive registration/cancellation
  • ANNEX H – Information content of DCE provided information
  • H.0 General
  • H.1 Information content of calling and called line identification
  • H.2 General coding of the DCE-provided information
  • H.3 Information content of charging information
  • H.4 Sub-addressing information
  • H.5 Date and time indication
  • H.6 Characteristics of the call
  • H.7 Type of call indication
  • H.8 Closed user group indication
  • ANNEX I – Reference and transition tables