CONTENTS

1 Scope
1.1 General
1.2 Relationship to other international standards
1.3 References
2 Definitions
3 Abbreviations
4 Establishing an error-corrected connection
5 Interchange circuits affected by error correction
6 Overview of error-correcting DCE operation
6.1 General
6.2 Overview of the control function
6.3 Overview of the error control function
6.4 Communication between the control function and the error control function
7 Operation of the control function
7.1 Physical handshake
7.2 Phases of error-correcting protocol establishment
7.2.1 Detection phase
7.2.2 Protocol establishment phase
7.3 Data transfer
7.3.1 Flow control across the DTE/DCE interface
7.4 Transfer of break signal
7.5 Receipt of break
7.6 Negotiation/Indication of parameter values and optional procedures
7.7 Orderly release of the error-corrected connection
7.8 Loop-back test
7.9 Operation of the DTE-DCE interface after failure to establish error-correcting operation
7.10 Suspension of error correction procedure
7.10.1 Reception of L-SUSPEND request primitive during the detection phase
7.10.2 Reception of L-SUSPEND request primitive during the protocol establishment phase or after an error correction connection has been established
7.11 Resumption of error correction procedure
8 Operation of the error control function: LAPM procedures
8.1 General
8.1.1 Frame structure and fields
8.1.2 Format conventions
8.1.3 Invalid frames
8.1.4 Frame abort
8.1.5 Interframe time fill
8.2 LAPM elements of procedures and field formats
8.2.1 Address field format
8.2.2 Control field format
8.2.3 Control field parameters and associated state variables
8.2.4 Frame types
8.2.5 Use of timers
8.3 Establishment of the error-corrected connection
8.3.1 General
8.3.2 Detailed procedures
8.4 Transfer of user data from the V.24 interface
8.4.1 Transmitting I frames
8.4.2 Receiving I frames
8.4.3 Sending and receiving acknowledgements
8.4.4 Receiving REJ frames
8.4.5 Receiving SREJ frames
8.4.6 Receiving RNR frames
8.4.7 Own-receiver busy condition
8.4.8 Waiting acknowledgement
8.4.9 Termination of the error-corrected connection
8.5 Exception condition reporting and recovery
8.5.1 N(S) sequence error
8.5.2 N(R) sequence error
8.5.3 Timer-recovery condition
8.5.4 Invalid-frame condition
8.5.5 Frame-rejection condition
8.5.6 Receipt of an FRMR response frame
8.5.7 Unsolicited response frames
8.6 Transfer of user-control information
8.7 Orderly release of an error-corrected connection
8.7.1 General
8.7.2 Release procedure
8.7.3 Procedure on expiry of timer T401
8.8 Disconnected state
8.9 Collision of unnumbered commands and responses
8.9.1 Identical transmitted and received set-mode commands
8.9.2 Different transmitted and received set-mode commands
8.9.3 Unsolicited DM response and SABME or DISC command
8.10 Negotiation/indication of parameter values and optional procedures
8.10.1 General
8.10.2 Negotiation/Indication procedure
8.10.3 Procedure on expiry of timer T401
8.11 Loop-back test
8.12 Monitoring functions
8.12.1 General
8.12.2 Supervision during the connected state
8.12.3 Connection verification procedures
8.13 Transfer of break
8.13.1 General
8.13.2 State variables and parameters
8.13.3 Break procedures
9 System parameters
9.1 Parameters of the control function
9.1.1 Detection phase timer (T400)
9.2 Parameters of the error control function
9.2.1 Acknowledgement timer (T401)
9.2.2 Maximum number of retransmissions (N400)
9.2.3 Maximum number of octets in an information field (N401)
9.2.4 Window size (k)
9.2.5 Reply delay timer (T402) Optional
9.2.6 Inactivity timer (T403) Optional
9.2.7 DLCI values
9.3 Other parameters
10 Negotiation of optional procedures
11 Control function-to-control function connection
12 Encoding of information fields
12.1 Information fields in I frames
12.2 Information fields in XID frames
12.2.1 General
12.2.2 Encoding for negotiation/indication of parameter values and optional procedures
12.3 Information fields in UI frames
12.3.1 Encoding of BRK message
12.3.2 Encoding of BRKACK message
12.4 Information fields in TEST frame
12.5 Information fields in SREJ frames
Annex A Operation of the error control function Alternative procedure
Annex B Mapping of character formats to 8-bit format
Appendix I Interworking with a non-error correcting DCE
I.1 Interworking with a non-error-correcting answerer
I.2 Interworking with a non-error-correcting originator
I.3 Disposition of unrecognized bits
Appendix II Data forwarding conditions
Appendix III Additional information for V.42 implementers regarding robustness of operation
III.1 Transmission of the answerer detection pattern
III.2 Value of parameter N400 (maximum number of retransmissions)
III.3 Incomplete XID exchange
III.4 Selective retransmission
III.5 Reject on detection of errored frames
III.6 Checkpointing
Appendix IV Factors for determining the acknowledgement timer
Appendix V Potential enhancements to LAPM protocol
Appendix VI Additional information for V.42 implementers regarding answerer detection patterns
VI.1 Alternative answerer detection patterns
VI.2 Skipping of originator/answerer detection patterns