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