1、 INTERNATIONAL TELECOMMUNICATION UNION ITU-T V.44 TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (11/2000) SERIES V: DATA COMMUNICATION OVER THE TELEPHONE NETWORK Error control Data compression procedures ITU-T Recommendation V.44 (Formerly CCITT Recommendation) ITU-T V-SERIES RECOMMENDATIONS DATA
2、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 networks V.100V.199 Inte
3、rface 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 V.44 (11/2000) i ITU-T Recommendation V.44 Date compression procedures Summary This Recommend
4、ation describes a data compression algorithm for use in DCEs. It achieves a better performance than V.42 bis on many types of data. In addition to the normal stream method, the algorithm has a method which can compress data already contained in packets in an efficient way. Source ITU-T Recommendatio
5、n V.44 was prepared by ITU-T Study Group 16 (2001-2004) and approved under the WTSA Resolution 1 procedure on 17 November 2000. ii ITU-T V.44 (11/2000) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications. The ITU Teleco
6、mmunication Standardization Sector (ITU-T) is a 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 As
7、sembly (WTSA), which meets every four years, establishes 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 w
8、hich fall within ITU-Ts purview, the necessary standards 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 PR
9、OPERTY RIGHTS ITU draws attention to the possibility 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 asse
10、rted by ITU members or others outside of the Recommendation development process. As of the date of approval of this Recommendation, ITU had received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementors are cautioned that
11、 this may not represent the latest information and are therefore strongly urged to consult the TSB patent database. ITU 2001 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, wit
12、hout permission in writing from ITU. ITU-T V.44 (11/2000) iii CONTENTS Page 1 Scope. 1 1.1 General 1 1.2 Requirements for error-correcting procedures 1 1.3 A DCE employing data compression 1 2 References. 2 3 Definitions 2 4 Abbreviations 4 5 Functional description of a DCE 4 5.1 General 5 5.2 DTE/D
13、CE interchange circuits . 5 5.3 Signal converter 5 5.4 Control function 5 5.5 Error-control function . 5 5.6 Data compression function . 5 6 Procedures of the data compression function . 6 6.1 Overview of the data compression function . 6 6.2 Dictionary structure 6 6.2.1 Encoder dictionary. 6 6.2.2
14、Decoder dictionary 7 6.3 Encoding . 8 6.3.1 String-matching procedure 8 6.3.2 String-extension procedure 9 6.3.3 Creating string-segments . 10 6.3.4 Encoding summary 11 6.4 Decoding. 11 6.4.1 Processing codes 11 6.4.2 Creating new strings 12 6.5 Transparent mode . 13 6.5.1 Transition from compressed
15、 mode to transparent mode . 13 6.5.2 Transition from transparent mode to compressed mode . 13 6.6 Transfer. 13 6.6.1 Transfer of control codes, ordinals, and codewords 14 6.6.2 Transfer of string-extension length 14 6.6.3 Code prefixes. 15 6.6.4 Example of Transfer 15 iv ITU-T V.44 (11/2000) Page 7
16、Operations of data compression 16 7.1 Communication between the control and data compression functions. 16 7.2 Communications between peer data compression functions 17 7.3 Negotiation of V.44 capability 17 7.4 Negotiation of data compression parameters 17 7.4.1 Negotiation through XID. 18 7.4.2 Neg
17、otiation after link establishment 18 7.5 Initialization of the data compression function. 19 7.5.1 Initial state of the encoder dictionary 20 7.5.2 Initial state of the decoder dictionary 20 7.6 Establishment of error-controlled connection. 20 7.7 Transfer of data between the DTE/DCE interface and t
18、he data compression function . 20 7.8 Encoding . 20 7.9 Transfer of data between the data compression function and the error-control function . 20 7.10 Decoding. 20 7.11 Autonomous adjustments 21 7.11.1 Ordinal size and STEPUP 21 7.11.2 Codeword size and STEPUP . 21 7.11.3 Node-tree full. 21 7.11.4
19、History full. 21 7.11.5 Data compressibility monitoring . 21 7.12 Dictionary reinitialization. 22 7.13 Expedited data transfer and FLUSH. 22 7.14 ESCAPE command sequence . 22 7.15 Action on detection of C-ERROR 22 8 Parameters. 23 Annex A XID information field for negotiating V.44 capability when us
20、ed with V.42 25 Annex B Operation of V.44 in Packet Networks 26 B.1 Packet method operation of V.44 . 27 B.1.1 General description 27 B.1.2 Default values of data compression parameters for packet method 28 B.2 Multi-packet method of operation of V.44 . 28 B.2.1 General description 28 B.2.2 Default
21、values of data compression parameters for multi-packet method 29 ITU-T V.44 (11/2000) v Page Appendix I Notes on Implementation. 29 I.1 Selection of N2: the total number of codewords. 29 I.2 Selection of N7: maximum string length 30 I.3 Selection of N8: data structures and length of history 30 I.4 E
22、fficient compression of Unicode data. 31 I.5 Applicability of transparent mode 31 I.6 Calculation of compression performance . 31 I.7 Differences between V.44 and V.42 bis . 31 Appendix II Illustration of operation of V.44 algorithm 32 II.1 Compression and decompression of “ABCDEXABCDEYABCDE FFH AC“
23、. 32 II.2 Compression b) a decoding procedure that recovers the strings of characters from received binary codes of variable length; c) a string-building mechanism that rapidly extends existing strings; d) an automatically invoked transparent mode of operation when uncompressible data are detected.
24、Annex B describes the implementation of this data compression procedure in packet networks. A summary of the set of parameters used in this Recommendation is given in clause 8. Clause I.7 summarizes the main differences between the algorithms of ITU-T V.44 and V.42 bis. This Recommendation contains
25、examples for illustrative purposes; in any case in which an example appears to contradict the normative text, the normative text shall take precedence. 1.2 Requirements for error-correcting procedures For correct operation of the data compression function, it is necessary that an error-correcting pr
26、ocedure be implemented between the two entities using this Recommendation. In the case of V-series Recommendations, the Link Access Procedure for Modems (LAPM) error-correcting procedures defined in ITU-T V.42 or the error-correcting procedures in ITU-T V.76 or V.120 must be implemented. NOTE Undete
27、cted bit errors will cause mis-operation of the data compression function. Use of a 32-bit frame check sequence (FCS), as defined in ISO/IEC 13239, substantially reduces the possibility of such errors, and may be desirable in environments with severe impairments. This 32-bit FCS is an option in V.42
28、 LAPM. 1.3 A DCE employing data compression The data compression function may be used with an error-correcting DCE, as shown in Figure 1 and described in clause 5. The elements of an error-correcting V-series DCE are specified in ITU-T V.42. 2 ITU-T V.44 (11/2000) T1609040-00Control FunctionGSTNChar
29、acter-oriented InterfaceInterchangecircuitsData CompressionFunctionError ControlFunctionSignalConvertertoDTEFigure 1/V.44 DCE employing data compression and error control 2 References The following ITU-T Recommendations contain provisions which, through reference in this text, constitute provisions
30、of this Recommendation. At the time of publication, 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
31、 other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. ITU-T V.42 (1996), Error-correcting procedures for DCEs using asynchronous-to-synchronous conversion. ITU-T V.42 bis (1990), Data compression procedures for data circuit-terminating equipment
32、(DCE) using error correction procedures. ITU-T V.120 (1996), Support by an ISDN of data terminal equipment with V-series type interfaces with provision for statistical multiplexing. ISO/IEC 13239:2000, Information technology Telecommunications and information exchange between systems High-level data
33、 link control (HDLC) procedures. 3 Definitions This Recommendation defines the following terms: 3.1 alphabet: The set of all possible characters. In this Recommendation, the characters are contiguous from 0 to N4 1 (see clause 8). 3.2 append: To create a new string using an unmatched character. If t
34、he unmatched character follows a string match, the new string consists of the string match with the unmatched character added to the end; if the unmatched character follows a character, the new string consists of both characters. 3.3 character: A single data element input from the DTE to the encoder
35、 using a predefined number of bits N3(see clause 8). 3.4 code: A sequence of bits output by the encoder that represents control or information. Defined code types are: control codes, ordinals, codewords, and string-extension lengths. 3.5 code prefix: One or two bits preceding a code that indicates t
36、he type of code that follows. These are defined in 6.6.3. 3.6 codeword: A binary number in the range 4 to N2 1 that represents a string of consecutive characters. In the encoder, it corresponds to a particular string-segment. In the decoder, it corresponds to an entire string (see clause 8). ITU-T V
37、.44 (11/2000) 3 3.7 compressed mode: The mode of operation in which data from the DTE are transmitted as binary codes. 3.8 compressed operation: The state of the DCE in which the data compression function is active. Compressed operation has two modes: compressed mode and transparent mode. Transition
38、s between these modes may be automatic, and based on the content of the data input from the DTE (see 6.5). 3.9 control code: A binary number in the range 0 to 3, reserved for use in DCE-to-DCE signalling of control information related to the compression function while in the compressed mode of opera
39、tion (see 7.2). 3.10 decoder: The data compression subfunction that decompresses the output of an encoder. 3.11 dictionary: The data structure that represents the strings created while encoding or decoding. For the encoder, it consists of the root array, node-tree, and history; for the decoder, it c
40、onsists of the string collection and the history. 3.12 encoder: The data compression subfunction that compresses data. 3.13 ESCAPE: In transparent mode, the indication of the beginning of a command sequence to the decoder. 3.14 extend: To create a new string, consisting of a string match with one or
41、 more characters added to the end. 3.15 flush: The encoder completes processing of characters input up to that point, transfers any codes that result, updates the dictionary as appropriate, and establishes octet alignment. The decoder subsequently establishes octet alignment. 3.16 history: The histo
42、ry buffer, herein called “the history“, is the data structure that contains the characters input to the encoder (or equivalently, decoded by the decoder) since the most recent reinitialization of the data-structures. Characters are entered into the history in the order in which they are input (or de
43、coded). 3.17 multi-packet method: The compression of data that are packetized (or framed) such that both ends of the data transmission can identify packet (or frame) boundaries, and several packets, or portions of packets, are processed as continuation. The multi-packet method is described in Annex
44、B. 3.18 node: A node is a point in a tree-structure that represents a string-segment. It corresponds to a codeword in the encoder. 3.19 node-tree: The data structure in the encoder that represents the set of string-segments and their interrelationships. Combined with the root array, it represents th
45、e encoder tree-structures. A codeword corresponds to a particular string-segment and to a particular node. 3.20 ordinal: The ordinal of a character is the numerical equivalent of the character. 3.21 packet method: The compression of data that are packetized (or framed) such that both ends of the dat
46、a transmission can identify packet (or frame) boundaries, and each packet is processed separately. The packet method is described in Annex B. 3.22 parameter mode: The mode of operation in which compression parameters are transferred between the data compression peer. 3.23 receive: In parameter negot
47、iation, the direction corresponding to an entitys decoder. 3.24 root: A root is the point at the base of a tree-structure that represents, within the context of this Recommendation, the first character of a string (see Figure 2 and 6.2.1). Each character in the alphabet has a corresponding root. 3.2
48、5 root array: The data structure that contains the set of all encoder roots. 4 ITU-T V.44 (11/2000) 3.26 stream method: The compression of data that are transmitted continually over a link with guaranteed delivery. This method is the primary focus of ITU-T V.44. 3.27 string: An unbroken sequence of
49、two or more characters. In the encoder dictionary, it consists of a first character (root) followed by one or more string-segments. 3.28 string extension: A sequence of one or more characters by which a string is extended to create a new and longer string. 3.29 string-extension length: A binary code indicating the number of characters by which the string corresponding to the immediately preceding codeword is extended. 3.30 string-segment: A section of a string that corresponds to a particular codeword in the encoder node-tree. 3.31 string collection: T
