1、 INTERNATIONAL TELECOMMUNICATION UNION ITU-T V.42TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (03/2002) SERIES V: DATA COMMUNICATION OVER THE TELEPHONE NETWORK Error control Error-correcting procedures for DCEs using asynchronous-to-synchronous conversion ITU-T Recommendation V.42 ITU-T V-SERIES
2、RECOMMENDATIONS DATA COMMUNICATION OVER THE TELEPHONE NETWORK General V.1V.9 Interfaces and voiceband modems V.10V.34 Wideband modems V.35V.39 Error control V.40V.49 Transmission quality and maintenance V.50V.59 Simultaneous transmission of data and other signals V.60V.99 Interworking with other net
3、works V.100V.199 Interface layer specifications for data communication V.200V.249 Control procedures V.250V.299 Modems on digital circuits V.300V.399 For further details, please refer to the list of ITU-T Recommendations. ITU-T Rec. V.42 (03/2002) i ITU-T Recommendation V.42 Error-correcting procedu
4、res for DCEs using asynchronous-to-synchronous conversion Summary This Recommendation describes error-correcting protocols for use with V-series duplex DCEs to accept start-stop data from the DTE and transmit in synchronous mode. This Recommendation contains and HDLC-based protocol referred to as th
5、e Link Access Procedure for Modems (LAPM). This revised version of the Recommendation adds the capability for suspension and resumption of error correction in support of the V.92 modem-on-hold feature. The negotiation parameters for V.44 data compression are added for reference. This revision also g
6、ives guidance on the use of answerer detection patterns in a new Appendix VI. Annex A and Appendix V of the previous versions are now deleted in the 2002 revision. The remaining annex and appendixes have not been renumbered. Source ITU-T Recommendation V.42 was revised by ITU-T Study Group 16 (2001-
7、2004) and approved under the WTSA Resolution 1 procedure on 29 March 2002. ii ITU-T Rec. V.42 (03/2002) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a
8、permanent organ of ITU. ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Assembly (WTSA), which meets every four years, est
9、ablishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on these topics. The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1. In some areas of information technology which fall within ITU-Ts purview, the necessary s
10、tandards are prepared on a collaborative basis with ISO and IEC. NOTE In this Recommendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. INTELLECTUAL PROPERTY RIGHTS ITU draws attention to the possibi
11、lity that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Rec
12、ommendation development process. As of the date of approval of this Recommendation, ITU had not received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementors are cautioned that this may not represent the latest informati
13、on and are therefore strongly urged to consult the TSB patent database. ITU 2002 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. ITU-T Rec. V.42 (03/2002) iii CONTENTS Page 1 Scope 1 1.1 General . 1 1.2 Relatio
14、nship to other international standards . 1 1.3 References 2 2 Definitions 2 3 Abbreviations 3 4 Establishing an error-corrected connection 4 5 Interchange circuits affected by error correction 4 6 Overview of error-correcting DCE operation. 5 6.1 General . 5 6.2 Overview of the control function . 6
15、6.3 Overview of the error control function. 6 6.4 Communication between the control function and the error control function. 7 7 Operation of the control function . 7 7.1 Physical handshake. 8 7.2 Phases of error-correcting protocol establishment . 8 7.2.1 Detection phase 8 7.2.2 Protocol establishm
16、ent phase . 10 7.3 Data transfer . 10 7.3.1 Flow control across the DTE/DCE interface 11 7.4 Transfer of break signal 11 7.5 Receipt of break 12 7.6 Negotiation/Indication of parameter values and optional procedures 13 7.7 Orderly release of the error-corrected connection 13 7.8 Loop-back test 14 7.
17、9 Operation of the DTE-DCE interface after failure to establish error-correcting operation 14 7.10 Suspension of error correction procedure. 15 7.10.1 Reception of L-SUSPEND request primitive during the detection phase 15 7.10.2 Reception of L-SUSPEND request primitive during the protocol establishm
18、ent phase or after an error correction connection has been established 15 7.11 Resumption of error correction procedure . 15 iv ITU-T Rec. V.42 (03/2002) Page 8 Operation of the error control function: LAPM procedures. 15 8.1 General . 16 8.1.1 Frame structure and fields 16 8.1.2 Format conventions
19、17 8.1.3 Invalid frames. 19 8.1.4 Frame abort. 19 8.1.5 Interframe time fill . 19 8.2 LAPM elements of procedures and field formats. 19 8.2.1 Address field format. 19 8.2.2 Control field format 20 8.2.3 Control field parameters and associated state variables . 21 8.2.4 Frame types 22 8.2.5 Use of ti
20、mers. 28 8.3 Establishment of the error-corrected connection 28 8.3.1 General . 28 8.3.2 Detailed procedures 29 8.4 Transfer of user data from the V.24 interface 30 8.4.1 Transmitting I frames . 30 8.4.2 Receiving I frames 30 8.4.3 Sending and receiving acknowledgements. 31 8.4.4 Receiving REJ frame
21、s 32 8.4.5 Receiving SREJ frames 33 8.4.6 Receiving RNR frames. 34 8.4.7 Own-receiver busy condition . 35 8.4.8 Waiting acknowledgement . 36 8.4.9 Termination of the error-corrected connection. 36 8.5 Exception condition reporting and recovery 37 8.5.1 N(S) sequence error 37 8.5.2 N(R) sequence erro
22、r . 38 8.5.3 Timer-recovery condition. 38 8.5.4 Invalid-frame condition 38 8.5.5 Frame-rejection condition 38 8.5.6 Receipt of an FRMR response frame . 39 8.5.7 Unsolicited response frames. 39 8.6 Transfer of user-control information 39 8.7 Orderly release of an error-corrected connection. 39 8.7.1
23、General . 39 8.7.2 Release procedure. 39 8.7.3 Procedure on expiry of timer T401 40 ITU-T Rec. V.42 (03/2002) v Page 8.8 Disconnected state 40 8.9 Collision of unnumbered commands and responses 40 8.9.1 Identical transmitted and received set-mode commands 40 8.9.2 Different transmitted and received
24、set-mode commands . 40 8.9.3 Unsolicited DM response and SABME or DISC command 40 8.10 Negotiation/indication of parameter values and optional procedures 41 8.10.1 General . 41 8.10.2 Negotiation/Indication procedure. 41 8.10.3 Procedure on expiry of timer T401 41 8.11 Loop-back test 42 8.12 Monitor
25、ing functions 42 8.12.1 General . 42 8.12.2 Supervision during the connected state 42 8.12.3 Connection verification procedures 42 8.13 Transfer of break 43 8.13.1 General . 43 8.13.2 State variables and parameters . 43 8.13.3 Break procedures 44 9 System parameters 45 9.1 Parameters of the control
26、function . 45 9.1.1 Detection phase timer (T400). 45 9.2 Parameters of the error control function. 45 9.2.1 Acknowledgement timer (T401) 45 9.2.2 Maximum number of retransmissions (N400) . 45 9.2.3 Maximum number of octets in an information field (N401) 45 9.2.4 Window size (k) 46 9.2.5 Reply delay
27、timer (T402) Optional . 46 9.2.6 Inactivity timer (T403) Optional . 46 9.2.7 DLCI values 46 9.3 Other parameters 46 10 Negotiation of optional procedures 47 11 Control function-to-control function connection 47 12 Encoding of information fields. 47 12.1 Information fields in I frames. 47 12.2 Inform
28、ation fields in XID frames . 47 12.2.1 General . 47 12.2.2 Encoding for negotiation/indication of parameter values and optional procedures. 48 vi ITU-T Rec. V.42 (03/2002) Page 12.3 Information fields in UI frames 52 12.3.1 Encoding of BRK message. 52 12.3.2 Encoding of BRKACK message 53 12.4 Inform
29、ation fields in TEST frame 53 12.5 Information fields in SREJ frames . 53 Annex A Operation of the error control function Alternative procedure 53 Annex B Mapping of character formats to 8-bit format . 54 Appendix I Interworking with a non-error correcting DCE . 54 I.1 Interworking with a non-error-
30、correcting answerer . 54 I.2 Interworking with a non-error-correcting originator 55 I.3 Disposition of unrecognized bits 55 Appendix II Data forwarding conditions 55 Appendix III Additional information for V.42 implementers regarding robustness of operation . 56 III.1 Transmission of the answerer de
31、tection pattern . 56 III.2 Value of parameter N400 (maximum number of retransmissions) 57 III.3 Incomplete XID exchange 57 III.4 Selective retransmission . 57 III.5 Reject on detection of errored frames 57 III.6 Checkpointing. 57 Appendix IV Factors for determining the acknowledgement timer . 58 App
32、endix V Potential enhancements to LAPM protocol 58 Appendix VI Additional information for V.42 implementers regarding answerer detection patterns 59 VI.1 Alternative answerer detection patterns . 59 VI.2 Skipping of originator/answerer detection patterns 59 ITU-T Rec. V.42 (03/2002) 1 ITU-T Recommen
33、dation V.42 Error-correcting procedures for DCEs using asynchronous-to-synchronous conversion The ITU-T, considering (a) that the use of high-speed DCEs for transmission of asynchronous data on the GSTN is increasing; (b) that there is a demand for an improved error performance on such connections b
34、y the use of an error-correcting protocol; (c) that there is a need to interwork with DCEs not providing such a protocol, declares that the error-correcting procedures to be followed by DCEs using asynchronous-to-synchronous conversion are as specified in this Recommendation. 1 Scope 1.1 General Thi
35、s Recommendation describes error-correcting protocols for use with V-series duplex DCEs to accept start-stop data from the DTE and transmit in synchronous mode. Use in half-duplex DCEs is for further study. This Recommendation contains an HDLC-based protocol referred to as the Link Access Procedure
36、for Modems (LAPM). The principal characteristics of the protocols are as follows: a) interworking in the non-error-correcting mode with V-series DCEs that include asynchronous-to-synchronous conversion according to ITU-T Rec. V.14; b) error detection through the use of a cyclic redundancy check; c)
37、error correction through the use of automatic retransmission of data; d) synchronous transmission through the conversion of start-stop data; e) an initial handshake in start-stop format which minimizes disruption to the DTEs. NOTE Technical changes have been introduced since the 1988 version of this
38、 Recommendation, which make refinements to the operation of the LAPM protocol. The following clauses were affected: 7.2.1.3, 7.3, 7.4, 7.5, 7.9, 8.3, 8.4, 8.5, 8.13, 9.3, 10, 12.2.2, 12.3, A.7.2.1. Implementations conforming to the 1988 version remain fully compatible and conformant with this versio
39、n. 1.2 Relationship to other international standards The error-correcting protocol defined in the main body of this Recommendation can be specified in terms of the High-level Data Link Control (HDLC) formats and procedures. In particular, it makes use of the balanced asynchronous class (BAC) of HDLC
40、 procedures. The basic mode (i.e. without options) of this protocol makes use of HDLC “optional functions“ 1, 2, 4, 6, 8 and 10. (This mode is identical to ITU-T Recs Q.920/Q.921.) When using optional procedures of this error-correcting protocol, HDLC “optional functions“ 3 (for selective retransmis
41、sion), 12 (for loop-back test), and 14 (for 32-bit FCS) are added. 2 ITU-T Rec. V.42 (03/2002) 1.3 References The following ITU-T Recommendations and other references contain provisions, which, through reference in this text, constitute provisions of this Recommendation. At the time of publication,
42、the editions indicated were valid. All Recommendations and other references are subject to revision: users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the curren
43、tly valid ITU-T Recommendations is regularly published. 1 ITU-T Recommendation Q.920 (1993), ISDN user-network interface data link layer General aspects. 2 ITU-T Recommendation Q.921 (1997), ISDN user-network interface Data link layer specification. 3 ITU-T Recommendation V.14 (1993), Transmission o
44、f start-stop characters over synchronous bearer channels. 4 ITU-T Recommendation V.24 (2000), List of definitions for interchange circuits between data terminal equipment (DTE) and data circuit-terminating equipment (DCE). 5 ITU-T Recommendation X.3 (2000), Packet Assembly/Disassembly facility (PAD)
45、 in a public data network. 6 ISO/IEC 3309:1993, Information technology Telecommunications and information exchange between systems High-level data link control (HDLC) procedures Frame structure. 7 ISO/IEC 4335:1993, Information technology Telecommunications and information exchange between systems H
46、igh-level data link control (HDLC) procedures Elements of procedures. 8 ISO/IEC 7809:1993, Information technology Telecommunications and information exchange between systems High-level data link control (HDLC) procedures Classes of procedures. 9 ISO/IEC 8885:1993, Information technology Telecommunic
47、ations and information exchange between systems High-level data link control (HDLC) procedures General purpose XID frame information field content and format. 2 Definitions An error-correcting protocol may be used with a signal converter to create an error-correcting DCE. 2.1 data circuit-terminatin
48、g equipment (DCE): In this Recommendation, a DCE, when used without further qualification, consists primarily of three sections: interchange circuits for the interface to the DTE and signal converters for transmission over telephone circuits. A control function is used to provide a user interface to
49、 coordinate the operation of the interchange circuits and the signal converter. The structure of a DCE is shown in Figure 1. a) The DTE exchanges data with the DCE through a V.24 interface. The data is exchanged in start-stop format. b) The signal converter provides the modulation and demodulation of signals exchanged on the GSTN, or two-wire point-to-point leased circuits. c) The control function provides overall control and coordination between each of the DCE components. Further, the controller provides the s
